Unraveling the major Ulva species driving local green tides through molecular analyses: spatiotemporal patterns along the Korean coast

Unraveling the major Ulva species driving local green tides through molecular analyses: spatiotemporal patterns along the Korean coast | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Unraveling the major Ulva species driving local green tides through molecular analyses: spatiotemporal patterns along the Korean coast Hye Jin Park, Seo Yeon Byeon, Sang Rul Park, Young Baek Son, Ji Hyoun Kang, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8022902/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 13 You are reading this latest preprint version Abstract Green tides – massive proliferations of green macroalgae ( Ulva spp.) – have increasingly occurred worldwide in recent years, driven by accelerating climate change and anthropogenic nutrient inputs. These blooms disrupt coastal ecosystems, leading to biodiversity loss and economic damage. In Korea, green tides have persisted on Jeju Island since the 2000s, and have also been sporadically reported on the southern mainland coasts. However, the specific Ulva species responsible for these blooms remain largely unknown. Here, we investigated Ulva community structure and relative frequencies from 46 sites (966 specimens) on Jeju Island and the southern coasts, using chloroplast tuf A gene-based phylogenetic analysis, complemented by additional nuclear 5s rDNA marker. We found considerable differences in Ulva community composition between Jeju Island and the southern coasts, along with pronounced seasonal variation. On Jeju Island, nine Ulva species were found, with Ulva ohnoi and Ulva australis dominant, whereas 10 species were observed with U. australis and Ulva linza prevailed on the southern coasts. The presence of nonindigenous Ulva species highlights the need for continuous monitoring to track their spread and biomass growth. Our results provide essential genetic insights to support effective management of green tide events in Korean coastal ecosystems. Biological sciences/Ecology Earth and environmental sciences/Ecology Earth and environmental sciences/Environmental sciences Earth and environmental sciences/Ocean sciences Climate change DNA barcoding Jeju Island Nutrient input Seaweed tide Ulva species Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Due to the recent acceleration of climate change, macroalgal blooms have been occurring more frequently in many coastal regions worldwide 1 , 2 , 3 , 4 . Since the 1990s, large-scale coastal macroalgal outbreaks have become almost routine events 2 , 4 . The terms, green tide and golden tide, collectively referred to as seaweed tide (or macroalgal bloom) were coined to describe seawater that appears as a ‘green’ or ‘brown’ carpet (or paint) due to masses of floating seaweeds. These phenomena are primarily caused by green macroalgae (Chlorophyta) and brown macroalgae (Phaeophyceae), both of which have high growth and proliferation rates associated with their unique life history traits 5 , 6 . Consequently, green and golden tides have become critical ecological and environmental issues worldwide, although the key ecological drivers underlying these blooms remain poorly understood 7 . Green tides are predominantly formed by the genus Ulva including the former Enteromorpha , whereas golden tides are caused by Sargassum species. In the northwestern Pacific, golden tides are known to be exclusively involved with Sargassum horneri 8 , 9 . Coastal areas such as estuaries, harbors and bays, where seawater is partially enclosed by natural or artificial barriers, are more prone to green macroalgal blooms (i.e. green tides). The rise in green macroalgal biomass in these regions is largely attributed to human-driven factors, such as nutrient enrichment, eutrophication and pollution 7 . Large-scale green tide events caused by rapid proliferation of Ulva species can severely impact coastal ecosystem and local economy 2 , 3 , 4 , 7 , . Previous studies have suggested that a rise in inorganic nutrients (e.g. dissolved inorganic nitrogen) and coastal eutrophication are major ecological and environmental drivers of green tide formation 7 , 10 . The increase in algal biomass associated with green tides is perhaps linked to nutrient loading, especially nitrate enrichment resulting from human activities. Understanding the possible causes and environmental factors underlying green tide formation is essential for developing mitigation and management strategies to reduce their negative impacts 1 , 2 , 4 . The genus Ulva is divided into two major morphological types – foliose and tubular. The tubular form was previously classified as the genus Enteromorpha but was later synonymized with Ulva based on genetic evidence 11 . Ulva species exhibit two distinct growth forms, such as attached or free-floating thalli. Because of their high level of phenotypic plasticity in morphology, which varies with habitat environmental conditions, accurate species identification based solely on morphology is extremely difficult 12 , 13 , 14 . For example, among species inhabiting the southern coasts of Korea and Jeju Island (located off the southernmost region of the Korean Peninsula), Ulva australis (synonym; Ulva pertusa ) and Ulva ohnoi exhibit similar thallus morphologies, making them difficult or impossible to distinguish morphologically. Therefore, discrepancies likely exist between reported and actual species numbers, and Ulva species diversity may be underestimated due to phenotypic plasticity and incomplete molecular dataset 15 , 16 . For Ulva species with simple thallus structures and lacking differentiated reproductive organs, morphological species identification is particularly challenging 17 , 18 . Molecular marker-based studies on green tides are necessary for identifying the major Ulva species responsible for these large-scale outbreaks. In the northwestern Pacific, encompassing the coastal regions of China, Japan, and Korea, molecular analyses have been widely applied to investigate the green tide phenomena. The chloroplast DNA (cpDNA) tuf A (elongation factor Tu) and the internal transcribed spacer (ITS) region of nuclear ribosomal DNA (nuDNA) are among the most commonly used genetic markers for studying Ulva diversity 15 , 19 . These markers have become popular and have significantly contributed to our understanding of genetic diversity and phylogenetic relationships among Ulva species. Moreover, the nuclear 5S ribosomal DNA (5S rDNA) marker has shown great potential for inferring interspecies phylogeny, particularly within the Ulva linza - procera - prolifera (LPP) complex or LPP clade 20 , 21 , 22 , 23 , 24 . The chloroplast rbc L (ribulose-1,5-bisphosphate carboxylase) has also been employed for DNA barcoding of Ulva species 19 . Among these markers, tuf A generally provides higher resolution for species identification compared to ITS 15 . However, species delimitation within the LPP clade remains unresolved. Therefore, in this study, we combined tuf A and5S rDNA analyses to identify Ulva species 23 , 24 . In Korean waters, local green tide has been observed year-round, particularly along the northeastern coast of Jeju Island since the 2000s 15 , 25 . More recently, green tides have also occurred sporadically along the southern coasts 26 . Nevertheless, the Ulva species primarily responsible for these blooms remain largely unidentified (but see 15 ). Furthermore, potential geographic and seasonal variation in Ulva community structure, species diversity and distributional patterns remain poorly understood. In the present study, we aimed to identify Ulva species primarily responsible for green macroalgal blooms along the Korean coast. Specifically, we assessed Ulva community structure, species composition and diversity, and seasonal variation on Jeju Island and the southern coasts based on molecular phylogenetic analyses. By applying combined analysis of tuf A and 5S rDNA, we determined the dominant Ulva species associated with green tides. The objectives of this study were to (1) identify the primary Ulva species contributing to local green tide events on Jeju Island and the southern coasts of Korea; (2) investigate geographic variation in Ulva community structure and species composition and diversity between the two regions, with a focus on spatial differences in species distribution; (3) examine temporal (seasonal) variation in Ulva community structure. This study provides a comprehensive understanding of the major Ulva species driving green tides along the Korean coast and demonstrates how Ulva species composition changes across spatial and temporal scales. Methods Sample collection and pretreatment This study was conducted on a total of 966 specimens at 46 locations along the southern coastal areas of the Korean Peninsula (the southern coasts hereafter) and the coastline of Jeju Island, which is located ~ 150–200 km off the southern coast of the mainland, from November 2019 to February 2021 (Fig. 1 , Tables S1, 2). Six hundred-twelve specimens from 31 sites and three hundred fifty-four specimens from 15 sites were used for Jeju Island and the southern coasts, respectively for genetic analysis. These sites were selected as the study sites because local green tides have been frequently observed there. Samples were collected on a seasonal basis (spring, summer, autumn, and winter), and information on latitude/longitude for the sampling sites is given in Tables S1, 2. More than five individuals were collected per morphotype of Ulva spp. at each of the sampling sites (Fig. 2 ) and all specimens were collected at 2–3 meter intervals of each other within sites. The sampling distances were kept across sites in Jeju Island and the southern coasts. These sampling schemes would allow to avoid the inadvertent collection of the same individuals 15 . In cases where the sampling areas were geographically separated by a coastal embankment, the inland site was designated as ‘in’ and the coastal site as ‘outside’. The collected samples were transported to the laboratory while maintaining the temperature below 4°C, and all samples were photographed before DNA extraction. After photographing, the samples were rinsed several times with freshwater and foliose tissue samples were completely dried in a dry oven (Jeio Tech, Korea) at 60°C for at least 24 hours. Completely dried samples were powdered using TissueLyserII (Qiagen, USA). The pulverized powder tissue sample was stored with silica gel in a 2.0 ml tube and a Desiccator Cabinet (Kastech, Korea) with an automatic dehumidification function until genetic analysis. Genetic analysis was performed at least for 2–3 individuals per morphotype for each sampling site. All the 966 samples were sequenced for tuf A gene and a subset of 119 individuals belonging to the LPP clade, as identified by tuf A, were further analyzed. However, only 105 out of the 119 samples were successfully sequenced for 5S rDNA. Genomic DNA extraction, PCR and sequencing Genomic DNA (gDNA) was extracted using i-genomic Plant DNA Extraction Mini Kit (Intron Biotechnology, Korea) according to manufacturer’s protocol. The concentration of the extracted gDNA was measured using a NanoDrop UV/VIS spectrophotometer (Thermo Fisher Scientific, USA). The cpDNA tuf A gene 19 and nuDNA 5S rDNA 20 were amplified by polymerase chain reaction (PCR). The tuf A gene was amplified with the published primers, TufGF4 (5’-GGNGCNGCNCAAATGGAYGG-3’) and TufAR (5’-CCTTCNCGAATMGCRAAWCGC-3’) 19,27 and 5S rDNA region was amplified with the primers, 5S-F (5’-GGTTGGGCAGGATTAGTA-3’) and 5S-R (5’-AGGCTTAAGTTGCGAGTT-3’) 20 . PCR reaction was performed in a total reaction volume of 15 µl with 10×Dream Taq Green buffer (Thermo Fisher Scientific) 1.5 µl, 2.5 mM dNTPs (Bio Basic Inc., Canada) 1.5 µl, 10 pmol forward/reverse primers 0.5 µl, 0.2 units of Taq DNA polymerase (Thermo Fisher Scientific) 0.1 µl, template gDNA (~ 20 ng/µl), and 9.9 µl sterile distilled water using a 2720 thermal cycler (Applied Biosystems, USA). PCR amplification was performed by 35 cycles of initial denaturation at 94°C for 4 min, denaturation at 94°C for 1 min, annealing at 45°C (for tuf A) ~ 50°C (5S rDNA) for 30–45 sec, and extension at 72°C for 1 min, followed by a final extension reaction at 72°C for 7 min. The PCR products were visualized by electrophoresis on a 2% agarose gel and purified with enzymatically Exonuclease I and Shrimp Alkaline Phosphatase (New England BioLabs, USA). Sequencing was performed with ABI PRISM 3730xl automated DNA sequencer (Applied Biosystems). For 5S rDNA region, gel extraction/excision process was undertaken prior to DNA sequencing. The 5S rDNA sequences were used for identifying the species belonging the LPP clade 20 , 22 , 23 , 24 . The resulting DNA fragments were visualized by UV transillumination and analyzed using Gel doc 1000 UV Fluorescent Gel Documentation System-PC (Biorad, USA). After identifying the shortest DNA fragment (about 200–300 bp) in each PCR product of the 5S rDNA spacer region, PCR yielding band was excised from the gel and purified using MinElute Gel Extraction Kit (Qiagen) 20 , 22 . Molecular phylogenetic analysis The 966 tuf A DNA sequences obtained were edited using Geneious prime ver. 2021.0.3 28 and aligned using Clustal Omega ver. 1.2.2 29 . For molecular-based species identification of Ulva , 55 haplotypes of 17 species plus three unidentified haplotypes, which were previously determined by tuf A phylogenetic analysis 15 , were used as references in this study (Table S1 ). Two Blidingia spp. were used as outgroup (GenBank accession numbers: MK992087, HQ610240). When monophyletic clades were formed in the phylogenetic tree, we defined those as particular “the species” 15 . When the samples did not belong to any particular clade, they were then defined as “unidentified”. Aligned DNA sequences best fitted the JC (Jukes-Cantor) model based on jModel-test ver. 2.1.7 30 . Neighbor-joining (NJ) analysis was performed using MEGA ver. 7.0 31 using 1,000 repetitions (bootstrap) with the JC model. Maximum likelihood (ML) analysis was also performed using PhyML ver. 3.1 32 with 1,000 bootstrap samplings. To determine species within the LPP clade, additional phylogenetic analysis of the 5S rDNA was conducted on 105 individuals (25 from Jeju Island and 80 from the southern coasts) out of 119 specimens that were assigned to the LPP clade based on the tuf A-based phylogenetic analysis. More specifically, the 105 samples identified as U. procera or U. prolifera (103 U. procera and two U. prolifera ) in tuf A-phylogeny were reanalyzed using the 5S rDNA marker with NJ method to distinguish U. prolifera and U. linza . Previously determined 25 haplotype sequences were used as a reference for this analysis 14 . The total sequence length, including gaps between sequences (24 ~ 125 bp), was 352 bp, and the minimum sequence length of 227 bp. We further performed model-based algorithmic species delimitation analysis of ABGD (Automatic Barcode Gap Discovery) 33 to corroborate the results of our molecular phylogeny-based species delimitation. ABGD analyses were conducted via the web-interface ( https://wwwabi.snv.jussieu.fr/public/abgd/abgdweb.html ) with default settings with the exception of the JC model and two relative gap widths (X) (X = 1.0, 1.5) applied 15 . The prior intraspecific diversity (P) was set to range from 0.001 to 0.1. The relative frequency of each Ulva species was calculated as the proportion of individuals assigned to each species out of the total number of genetically identified green algal specimens in each region (Jeju Island and the southern coasts). Species identification was based on both tuf A and 5S rDNA analyses. Community analysis The species composition ratio of Ulva at each site was calculated based on relative abundance data. Prior to analysis, the data were square-root transformed, and Bray–Curtis similarity indices were computed to assess differences in community composition between Jeju Island and the southern coasts. Non-metric multidimensional scaling (nMDS 34 ) was used to visualize spatial patterns in community structure. Statistical significance of spatial differences between the two regions was tested using analysis of similarities (ANOSIM). All analyses and visualizations were conducted using PRIMER-e v7 (PRIMER-E Ltd., Plymouth, UK). In addition, we tested for significant difference in life-form type (floating vs. benthic [attached] thalli) between the two dominant Ulva species ( U. australis and U. ohnoi ) in Jeju Island. Differences in the proportions of benthic and floating individuals between the two species ( U. australis [N = 129] and U. ohnoi [N = 209]) were evaluated using a chi-square test, and a preference index was subsequently calculated following the method Ivlev’s 35 and modified by Jacobs’s 36 . Results Model-based species delimitation The number of Ulva species identified by ABGD based on tuf A was consistent with the number of species groups (clusters) inferred from phylogenetic analyses. In the ABGD results, the number of species groups varied depending on the values of P and X. The number of groups showing the minimum difference (i.e. 1.0) between the initial partition (IP) and recursive partition (RP) was 14 (IP) and 15 (RP) when X = 1.0 (P = 0.0077), whereas the numbers decreased to 10 (IP) and 11 (RP), respectively when X = 1.5 (P = 0.0077). After excluding Blidingia spp. and unidentified taxa, the number of Ulva groups was reduced (Table 1 ; Table S4). Table 1 The number of groups (clusters) obtained from the ABGD analysis using the chloroplast ( tuf A) marker for Ulva samples from Jeju Island and the southern coasts. Species identified by phylogenetic analyses (NJ, ML) ABGD (based on tuf A) X = 1.0 X = 1.5 P = 0.0077 P = 0.0129 P = 0.0077 P = 0.0129 U. australis (= U. pertusa) 2 2 2 2 U. ohnoi 1 1 1 1 U. lactuca 0 0 0 0 U. californica 1 1 1 1 U. laetevirens ( U. rigida ) 1 0 0 0 U. flexuosa 0 0 0 0 U. arasakii 1 1 1 1 U. compressa 1 1 1 1 LPP sp. 1 ( U. procera ) 2 1 0 0 LPP sp. 2 ( U. prolifera ) 1 1 0 0 Unidentified 5 5 5 5 Total number of groups (clusters) 15 groups 13 groups 11 groups 11groups Total number of groups (clusters) of Ulva specimens analyzed in this study 10 groups 8 groups 6 group 6 group Given the minimum difference between IP and RP (one group), the number of Ulva species groups based on tuf A was estimated as ten (when P = 0.0077, X = 1.0) (Table 1 ). The results also showed that increasing X from 1.0 to 1.5 resulted in fewer species groups across all P values, indicating that the ABGD results were sensitive to parameter selection. For example, when X = 1.0, 15 groups were detected ( P = 0.0077), but this number decreased to 11 at X = 1.5 (under the same P value). Some species groups, such as U. ohnoi , U. californica and U. compressa were consistently identified across different parameter settings, whereas U. prolifera and U. procera showed variable groupings. Overall, although the number of Ulva species groups identified by ABGD was dependent on P and X values, the results were largely congruent with those of the phylogenetic analyses. Ten distinct Ulva species groups were thus recognized based on tufA (P = 0.0077, X = 1.0; Table 1 ; Table S4). Species identification based on tufA-based phylogenetic analysis The tuf A phylogeny showed that in 612 specimens from Jeju Island, 266 individuals (43.46%) were identified as U. ohnoi , 179 (29.25%) as U. australis (= U. pertusa ), 40 (6.54%) Ulva lactuca , 31 (5.07%) Ulva laetevirens (= Ulva rigida ), 28 (4.58%) belonging to the LPP sp. 1 ( Ulva procera ), 12 (1.96%) Ulva flexuosa , 7 (1.14%) Ulva compressa , 2 (0.33%) Ulva arasakii , and 8 (1.31%) unidentified. Blidingia spp. (8 specimens) and Gayralia sp. (1 specimen) were also identified, other than the genus Ulva (Fig. 3 ). By comparison, for a total of 354 specimens from the southern coasts, 123 individuals (34.75%) were determined as U. australis , 89 (25.14%) LPP sp. 1 ( U. procera ) and 2 (0.56%) LPP sp. 2 ( Ulva prolifera ), 31 (8.76%) U. ohnoi , 27 (7.63%) U. laetevirens , 18 (5.08%) Ulva californica , 17 (4.80%) U. flexuosa , 7 (1.98%) U. lactuca , 7 (1.98%) U. compressa , 4 (1.13%) U. arasakii , and 28 (7.91%) unidentified. Only a single specimen was identified as Blidingia sp. (Fig. 4 ). The phylogenetic tree encompassing the entire dataset from Jeju Island and the southern coasts is provided in Fig. S1 . Spatial variation in Ulva community structure between Jeju Island and the southern coasts The relative frequencies, distributional patterns, species composition and diversity of the entire Ulva communities were compared between Jeju Island and southern coasts. By excluding unidentified specimens, 9 Ulva species were identified for Jeju Island, whereas 10 species were found for the southern coasts, based on tuf A phylogenies (Table 2 ; Fig. 5 ). Table 2 Comparisons of relative frequencies of Ulva species communities between Jeju Island and the southern coasts, based on tuf A-based phylogenetic analysis. Species Jeju Island southern coasts sample Relative frequency (%) sample Relative frequency (%) U. australis (= U. pertusa ) 179 29.25 123 34.75 U. ohnoi 266 43.46 31 8.76 U. lactuca 40 6.54 7 1.98 U. californica 31 5.07 18 5.08 U. laetevirens ( U. rigida ) 30 4.90 27 7.63 U. flexuosa 12 1.96 17 4.80 U. arasakii 2 0.33 4 1.13 U. compressa 7 1.14 7 1.98 LPP sp. 1 ( U. procera ) 28 4.58 89 25.14 LPP sp. 2 ( U. prolifera ) - - 2 0.56 Unidentified 8 1.31 28 7.91 Blidingia sp. 8 1.31 1 0.28 Gayralia sp. 1 0.16 - - Total 612 354 For the relative frequencies of Ulva species in Jeju Island, two species, U. ohnoi (43.46%) and U. australis (29.25%), were the most predominant across all the geographic and seasonal samples (Table 2 ; Figs. 5 , 6 ). Besides, the Jeju Island- Ulva community was composed of U. lactuca (6.54%), U. californica (5.07%), U. laetevirens ( U. rigida ) (4.90%), LPP clade species ( U. procera , 4.58%), U. flexuosa (1.96%), U. compressa (1.14%) and U. arasakii (0.33%), in this order (Table 2 ; Fig. 5 ). In addition to Ulva species, other species belonging to class Ulvophyceae, such as Blidingia (N = 8) and Gayralia (N = 1) spp. were also detected. By comparison, on the southern coasts Ulva- community was comprised of U. australis (34.75%), LPP clade species ( U. procera , 25.14%; U. prolifera , 0.56%), U. ohnoi (8.76%), U. lactuca (1.98%), U. californica (5.08%), U. laetevirens ( U. rigida ) (7.63%), U. flexuosa (4.80%), U. compressa (1.98%) and U. arasakii (1.13%) in this order (Table 2 ; Fig. 5 ). In addition to Ulva species, other species belonging to class Ulvophyceae, such as Blidingia (N = 1) were also detected. Bray–Curtis similarity analysis revealed a significant spatial difference in Ulva community structure between Jeju Island and the southern coasts (ANOSIM, r = 0.334, p = 0.001). In the nMDS ordination, samples from Jeju Island and the southern coasts clearly formed separate clusters, indicating distinct spatial segregation in Ulva community composition (Fig. 6 ). A few sites from the southern coasts overlapped with the Jeju Island cluster [S7 (Yegye 3), S9 (Sulcheon), and S15 (Baekya)], suggesting partial similarity in species composition between certain locations of the two regions. Differences in life forms between U. ohnoi and U. australis were also apparent, with U. ohnoi exhibiting a higher proportion of floating thalli than U. australis on Jeju Island (Fig. S2). A chi-square analysis revealed a significant difference in the proportions of benthic and floating individuals between the two species (χ² = 16.51, p < 0.001). The within-species preference index (PI), calculated as (Floating − Benthic)/(Floating + Benthic), indicated that U. ohnoi had a strong tendency toward the floating form (PI = 0.48), whereas U. australis displayed nearly equal proportions of the two forms (PI = 0.04). When standardized using Ivlev’s 35 and Jacobs’s 36 electivity indices, U. ohnoi still showed a positive bias toward the floating form (E = 0.06; D = 0.20), while U. australis exhibited a weak preference for the benthic form (E = − 0.12; D = − 0.28). Seasonal variation in Ulva community structure in Jeju Island and the southern coasts Seasonal variation in the Ulva community structure, species composition and relative frequencies was examined for both geographic regions (Fig. 7 ). For Jeju Island, a total of 11 species were identified, of which 9 species belonged to the genus Ulva and the other two species were composed of Blidingia spp. and Gayralia spp. A majority of the sites showed that two species, U. ohnoi (35–59.2%) and U. australis (6.4–37.7%) were predominant all year round, regardless of the season (Table 2 ; Fig. 7 ). An average relative frequency of U. ohnoi and U. australis were 43.46%, 29.25%, respectively. While U. ohnoi exhibited a peak in relative frequency (59.2%) in autumn, U. australis showed the lowest frequency (6.4%) (Table S5). In addition, U. lactuca increased its frequency in autumn (28%) relative to other seasons (2.9% in summer and zero in winter and spring) (Fig. 7 ; Table S5). The other Ulva species also showed some seasonal variation in their frequencies. Ulva californica ranged from 1.6% to 11.18% (the highest frequency in spring), U. laetevirens 2.5%–9.87% (the highest in spring), Ulva procera (LPP sp. 1) 0.57%– 12.57% (the highest in winter), and Ulva flexuosa 0.80%–3.75% (the highest in winter). Ulva arasakii was only detected in spring, constituting 1.32% of the samples. Ulva compressa appeared only in summer and winter, with a relative frequency ranging from 1.71% to 2.5% (Fig. 7 ; Table S5). Similar to Jeju Island, on the southern coasts Ulva community structure also changed according to the season. Ulva australis (15.52–45.05%) and U. procera (LPP sp. 1) (7.69–43.02%) were the most dominant species, although U. procera (LPP sp. 1) was not present at all in autumn. Ulva ohnoi was low in its frequency (1.10–3.36%) except for during autumn with the highest peak (43.1%) (Fig. 7 ; Table S6). Ulva laetevirens indicated relatively stable frequencies across seasons (4.4%–12.61%) and U. flexuosa showed a higher frequency in summer (13.19%) compared to the other seasons. Ulva arasakii exhibited a relative frequency of 3.49% during winter, which decreased to 0.84% in spring. Ulva compressa only occurred in spring (5.88%). Ulva prolifera (LPP sp. 2) was present (1.68%) only in spring, although this species was not detected at all in Jeju Island (Fig. 7 ; Table S6). These findings suggest that there was considerable seasonal variation in the species composition, distribution and relative frequencies between the two geographic regions. The 5S rDNA analysis for Ulva species belonging to the LPP clade We found that U. procera formed an independent clade as U. linza ( N = 90), while the remaining U. procera specimens grouped within the U. prolifera clade ( N = 13), along with two unidentified samples (Fig. 8 ). Within the U. prolifera group, two individuals identified by the tuf A marker were classified into U. linza and U. prolifera clades, respectively. Samples initially identified as U. procera by tuf A were re-classified into U. linza and U. prolifera groups, except for two individuals (U3453, U3895) that did not cluster with either group (Fig. 8 ). Among the 25 samples from Jeju Island, 23 were identified as U. linza and 2 as U. prolifera . On the southern coasts, 67 individuals were identified as U. linza , 11 as U. prolifera , and 2 remained unidentified (Fig. 8 ). All southern coast sites contained members of the LPP clade except for Baekya in Yeosu (Fig. 1 ; Table S2). In contrast, on Jeju Island, the LPP clade-species were detected only at four sites, including the Jongdal, inside Hanlim harbor, Hyeopjae, and inside Sinchang 1 (Fig. 1 ; Table S1 ). Discussion Based on the exhaustive phylogenetic analysis on the spatial and seasonal samples of a total of 966 Ulva specimens from 46 sites along the coastlines of Jeju Island and the southern coasts of Korea, we find considerable differences in the species composition, diversity, spatial distribution and relative frequencies of the green tide forming Ulva communities between the two regions. In Jeju Island, the most significantly contributing species to local green tides turned out to be U. ohnoi and U. australis , which is consistent with the previous findings 14 , 15 . The dominance of these species in Jeju Island can be partially attributed to relatively higher sea surface temperature (SST; 14.76–15.83°C) in this region (Fig. S3), which might provide favorable growth conditions for subtropical species like U. ohnoi . On the other hand, in the southern coasts of the mainland, U. australis and LPP clade species ( U. linza ) are predominant in green tide forming Ulva communities throughout the year. The higher concentrations of nitrogen compounds and chemical oxygen demand (COD) in the southern coasts create a nutrient-rich environment with a high nitrogen availability that supports the growth of these species (Fig. S3), although the green tide algal biomass is much lower relative to Jeju Island 25 . Although the ecological implications of these differences in environmental parameters remain unclear, these findings warrant further investigation to clarify relationships between environmental differences and Ulva community composition, species distribution and also magnitude of green tides. Ulva ohnoi , the most common species in Jeju Island, is known to have a subtropical or tropical origin 37 . It was first identified and taxonomically classified in 2004 in Japan 37 and is considered one of the major species responsible for green tides in several regions including Japan 38 , 39 . Moreover, it has been reported as a widely distributed species in various areas, including South Africa 40 . Because molecular studies of Ulva species in Korean waters – particularly around Jeju Island – remain limited, it is unclear whether U. ohnoi observed along the Korean coasts in this study represents a native population or was introduced from Japan or other subtropical regions. Additional analyses using existing sequence datasets from GenBank database could help to determine whether this species was introduced and, if so, infer its potential origin. To our knowledge, this study provides the first report of U. ohnoi occurrence along the southern coasts of the Korean Peninsula. Nevertheless, when and how this subtropical or tropical U. ohnoi became established on the southern coasts remain to be elucidated. We also find differences in species composition, distribution and relative frequencies in both coastal regions according to the season. For Jeju Island, the primary contributors to the year-round local green tides were U. ohnoi and U. australis , which tend to dominate the Ulva community regardless of the season, except U. australis was replaced with U. lactuca in autumn. The two species, U. ohnoi and U. australis , are known to be remarkably tolerant to high temperatures 39 , which may contribute to sustaining large amount of green macroalgal biomass even in summer/autumn. On the other hand, for the southern coasts, U. ohnoi shows a rapid increase in frequency only during autumn. It has been reported that U. australis shows a high growth rate in spring and early summer but tends to decline sharply in August 41 . It would therefore be conceivable that the relative frequency of U. ohnoi increased rapidly in autumn due to the decrease in U. australis . Still, U. ohnoi was rarely seen in other seasons (but spring), and U. linza (LPP sp. 1) species occurred in every season except for autumn. The dominance of U. linza on the southern coasts may be attributed to several factors. This species is well-known for its physiological adaptability and tolerance to varying environmental conditions, which may confer a competitive advantage in this region 42 . Furthermore, U. linza has been reported to withstand fluctuations in temperature and salinity 42 , likely contributing to its year-round presence along the southern coasts. Ulva australis is a widespread intertidal species that occurs along the entire coastline of Japan 43 . It is known to cause green tide outbreaks in temperate regions of southern and western Japan, along the Pacific coast of central Japan 37 , 44 . This species also maintains relatively high biomass during winter 26 , 43 and forms extensive green tides in various parts of the world, including the northwestern Atlantic coast of the Iberian Peninsula (Galicia, Spain) 45 and the northern coasts of China 46 . According to Hanyuda and Kawai 47 , Ulva pertusa Areschoug, 1851 and Ulva australis Kjellman, 1897 were suggested as synonym, which was supported by our previous phylogenetic analyses demonstrating the monophyly of these taxa 14 , 15 . This species is among the most common causes of green tides in Korea and Japan, with its dominance particularly well documented in Jeju Island from 2015 to 2020 14 . A recent study suggests that U. ohnoi has the potential to undergo explosive growth in high-temperature marine ecosystems driven by ocean warming and acidification 48 . This subtropical to tropical species exhibits a high growth rate even under elevated summer water temperatures 44 . As one of the most abundant and widely distributed Ulva species, U. ohnoi is expected to cause green tides more frequently and severely under warmer environmental conditions 39 , 49 . Temperature plays a crucial role in promoting photosynthesis and growth of U. ohnoi , as higher temperatures enhance the growth of both young and adult thalli by increasing metabolic activity, particularly during summer. According to Korea Oceanographic Data Center from the National Institute of Fisheries Science (NIFS, Korea; https://www.nifs.go.kr/kodc/soo_list.kodc ) in Korea, a rise in sea surface temperature (SST) was evident for the last five years (from 2015 to 2020 in April for Jeju Island). In 2015, the average temperature across the four stations was 14.70°C (± 1.42°C SD). In 2020, the average temperature increased to 15.61°C (± 1.51°C SD). These values clearly demonstrate a noticeable increase in sea surface temperature from 2015 to 2020, with all stations showing an upward trend. This temperature increase is noteworthy as it creates favorable growth conditions for U. ohnoi 14 , 50 . U. ohnoi primarily grows attached to rocky substrates, but detached floating fronds can also proliferate vegetatively by rapidly absorbing nutrients. The observed high proportions of floating-thalli form of U. ohnoi (Fig. S2) can provide evidence supporting this hypothesis. This trait contributes to frequent green tide outbreaks in coastal areas 44 , 25 . In Japan, U. ohnoi is widely distributed on the southwest coast, where the sea temperature is warm, whereas U. australis is predominant on the northeast coast 49 . Nonindigenous species of origins of Europe include U. procera (LPP sp1.; U.linza ), U. flexuosa and U. californica . They are known to be introduced through various maritime transport vehicles 51 . Ulva californica , a species native to the Pacific Coast of North America, has recently been reported to be introduced to Europe, including Ireland and the United Kingdom, the Mediterranean, Oceania, and also Asia by hull for maritime transportation 12 , 52 , 53 . This species was first recorded from California (type locality: La Jolla, Collins et al., 1899: no. 611) and has never been reported from the Mediterranean until 2012 52 . It has physiological and ecological capabilities that enable to survive and grow even in harsh environmental conditions such as a lack of light for more than 10 months 54 . Ulva californica was shown to cause green tide events in Chile and California, USA 54 . Notably, although U. californica was present in other areas, a particular site (Population ID 29) shows only U. californica . This finding highlights the potential role of maritime transport and harbor activities as pathways for introduction of non-native species like U. californica . Such unique occurrences underscore the importance of monitoring harbor environments where nonindigenous species may establish and proliferate after successful colonization. In addition, U. linza (LPP sp. 1) and U. flexuosa are often reported as the potentially problematic species causing macroalgal blooms, and there is also a possibility of causing green tides in Korea in the future 51 , 55 , 56 . Fortunately, nonindigenous and introduced Ulva species of origins of Europe or United States, such as U. californica and U. laetevirens (= U. rigida ) seem not significantly contribute to the current occurrences of local green tides on the Korean coast. Nevertheless, continuous observation and monitoring are required, as these species have been identified as major contributors to green tide events in various parts of the world 57 . Ulva laetevirens ( = U. rigida ) was first collected in Australia in 1854 and has been reported in several Mediterranean countries since the late 1990s 58 . This species exhibits an optimal growth rate at temperatures between 12°C and 23°C 57 . During summer, its growth declines when temperatures exceed these optimal values 10 , 57 as ambient temperature is a critical factor affecting its development and growth. Consequently, the relative frequency of U. laetevirens on the southern coasts remained relatively stable or even slightly higher during mild temperature periods such as spring and autumn, compared with summer and winter. Ulva prolifera , one of the three species ( U. linza and U. procera being the other) belonging to the LPP clade, is notorious for causing massive macroalgae blooms, green tide events in the Yellow Sea of China 21 , 24 , 59 . While U. linza and U. compressa are widely distributed in the Yellow Sea, U. prolifera occurs predominantly along the east coast of China 60 . In the present study, both U. linza (N = 67; 83.75%) and U. prolifera (N = 11; 13.75%) were identified in the southern coast region based on 5S rDNA analysis. On Jeju Island, however, only U. linza (N = 23; 92%) and U. prolifera (N = 2; 8%) were detected. Additionally, U. compressa was recorded in both Jeju Island and the southern coasts. These findings suggest that U. prolifera observed along the southern coasts of Korea may have been introduced from the east coast of China 61 . Further research is required to verify this hypothesis and to determine whether U. prolifera populations in Korean waters originated through recent introductions or natural dispersal 60 . Based on 5S rDNA analysis, U. linza occurs in both Jeju Island and the southern coasts. On Jeju Island, it occurred at four of 31 sites (12.90% of all sites), with relative abundances ranging from 3.23% (inside Hallim harbor and inside Sinchang 1) to 58.06% (Jongdal). On the southern coasts, U. linza was present at 14 of 15 sites, ranging from 1.49% (Sagok) to 14.92% (Deokho). Moreover, species belonging to the LPP clade were widely distributed along the entire southern coastal region, whereas their occurrence on Jeju Island was limited to four sites: inside Hallim Harbor, inside Sinchang 1, Jongdal, and Hyeopjae. These findings provide important insights into the species composition and distributional patterns of Ulva communities in the studied coastal regions. For the LPP clade, previous studies have shown that neither tuf A nor ITS markers can reliably differentiate among the three species ( Ulva linza-procera-prolifera ) 20 . Therefore, in this study, the 5S rDNA marker was analyzed to achieve more accurate species identification within the LPP clade 20 , 22 . The analysis targeted the specimens classified as members of the LPP clade based on tuf A-derived phylogenetic results. The 5S rDNA spacer region, which has been demonstrated to resolve interspecific relationships within Ulva was used here for unambiguous species identification within the LPP clade 23 , 24 , 62 , 63 . Further research, including specimens from the type locality of U. prolifera , is required to clarify its taxonomic status 20 . The present findings suggest phylogenetic divergence within the taxa previously designated as U. procera , indicating the existence of two distinct groups (Fig. 8 ). A comprehensive follow-up investigation, particularly detailed taxonomic reevaluation, will therefore be essential to avoid species misidentification. The results of this study suggest that Ulva species diversity in Korean waters may be higher than previously recognized 15 . Our genetic analyses revealed a greater extent of hidden diversity with Ulva communities in Jeju Island and the southern coasts of Korea, indicating that the actual number of species has likely been underestimated. In particular, specimens previously identified as U. procera was re-identified as U. prolifera and U. linza based on 5S rDNA analysis, highlighting that the limited resolution of certain markers may lead to an underestimation of Ulva species diversity. For example, although U. prolifera and U. linza are genetically distinct, earlier morphological studies might have misidentified one as the other due to phenotypic plasticity and overlapping morphological features. These findings emphasize the importance of accurate species identification using molecular approaches to avoid taxonomic confusion and improve our understanding of Ulva biodiversity. Identifying the dominant Ulva species responsible for local green tide events and elucidating their physiological and ecological characteristics are critical steps toward developing effective management strategies to reduce bloom occurrences. In this study, we applied model-based species delimitation in conjunction with molecular phylogenetic analyses to enhance the precision of species identification. The resulting molecular data provide a valuable baseline for understanding the genetic structure and community composition of green tide–forming Ulva species along the coasts of Jeju Island and the southern coasts. Conclusion This study provides an in-depth assessment of spatial and seasonal variations in Ulva species composition and diversity along the coasts of Jeju Island and the southern coasts of Korea, highlighting their contributions to green tide formation. Phylogenetic analyses using tufA and 5S rDNA markers revealed distinct Ulva community structures between the two regions. U. ohnoi and U. australis predominated on Jeju Island, whereas U. linza and U. australis were dominant along the southern coasts. These patterns suggest that environmental factors—such as temperature, nutrient availability, and species-specific ecological traits—drive regional and seasonal differences in Ulva communities. The dominance of U. ohnoi on Jeju Island highlights its potential to expand under warming conditions, increasing the risk of more frequent and severe green tides. Likewise, the prevalence of U. linza on the southern coasts indicates its resilience to environmental fluctuations. The detection of nonindigenous species, including U. californica , U. procera (LPP sp. 1; U. linza ), and U. flexuosa , underscores the need for continued genetic monitoring of Ulva populations. Our findings provide essential baseline data for developing targeted management strategies to mitigate the ecological and economic impacts of green tides in Korean coastal waters. Future research should investigate the ecological and physiological characteristics of dominant Ulva species and their responses to changing environmental conditions, and also their interactions with other coexisting species within the ecosystem. Declarations CRediT authorship contribution statement Hye Jin Park : Writing – original draft, Visualization, Methodology, Formal analysis, Investigation, Data curation. Seo Yeon Byeon : Writing – original draft, Formal analysis, Methodology, Investigation. Sang Rul Park : Conceptualization, Methodology, Writing – review & editing. Young Baek Son : Visualization, Writing – review & editing. Ji Hyoun Kang : Formal analysis, Conceptualization, Methodology, Writing – review & editing. Hyuk Je Lee : Formal analysis, Conceptualization, Supervision, Methodology, Writing – review & editing, Project administration. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We would like to thank members of Estuarine and Coastal Ecology Laboratory in Department of Marine Life Sciences at Jeju National University and the Molecular Ecology and Evolution Laboratory in Department of Biological Sciences at Sangji University, for their assistance in field sample collection for this study. Funding This study was supported by Korea Institute of Marine Science and Technology Promotion (KIMST) funded to the Ministry of Oceans and Fisheries, Korea (RS-2025-02304432; RS-2025-02304428). This research was also supported by the Regional Innovation System & Education (RISE) program through the Gangwon RISE Center, funded by the Ministry of Education (MOE) and the Gangwon State (G.S.), Republic of Korea (2025-RISE-10-005). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi: xxxx Data availability Data will be made available on request. References Ye, N. et al. Green tides’ are overwhelming the coastline of our blue planet: taking the world’s largest example. Ecol. Res. 26 , 477–485. 10.1007/s11284-011-0821-8 (2011). Smetacek, V. & Zingone, A. Green and golden seaweed tides on the rise. Nature 504 , 84–88. 10.1038/nature12860 (2013). Xiao, Y., Liu, R., Kim, K., Zhang, J. & Cui, T. A random forest-based algorithm to distinguish Ulva prolifera and Sargassum from multispectral satellite images. IEEE Trans. Geosci. Remote Sens. 60 , 1–15. 10.1109/TGRS.2021.3071154 (2021). Ren, C. G. et al. The ever-lasting green tides: What can we do? Heliyon 10, (2024). 10.1016/j.heliyon.2024.e25220 Choi, S. K. et al. Population dynamics of the ‘golden tides’ seaweed, Sargassum horneri , on the southwestern coast of Korea: The extent and formation of golden tides. Sustainability 12 , 2903. 10.3390/su12072903 (2020). Choi, S. K. et al. Changes in the dynamics and nutrient budget of a macroalgal community exposed to land-based fish farm discharge off Jeju Island, Korea. Sustainability 13 , 11793. 10.3390/su132111793 (2021). Wang, H., Wang, G. & Gu, W. Macroalgal blooms caused by marine nutrient changes resulting from human activities. J. Appl. Ecol. 57 , 766–776. 10.1111/1365-2664.13587 (2020). Byeon, S. Y. et al. The origin and population genetic structure of the ‘golden tide’seaweeds, Sargassum horneri , in Korean waters. Sci. Rep. 9 , 7757. 10.1038/s41598-019-44170-x (2019). Byeon, S. Y. et al. Comparative analysis of sequence polymorphism in complete organelle genomes of the ‘Golden Tide’seaweed Sargassum horneri between Korean and Chinese forms. Sustainability 12 , 7280. 10.3390/su12187280 (2020). Gladyshev, M. & Gubelit, Y. I. Green tides: new consequences of the eutrophication of natural waters (invited review). Contemp. Probl. Ecol. 12 , 109–125. 10.1134/S1995425519020057 (2019). Hayden, H. S. et al. Linnaeus was right all along: Ulva and Enteromorpha are not distinct genera. Eur. J. Phycol. 38 , 277–294. 10.1080/1364253031000136321 (2003). Loughnane, C. J., McIvor, L. M., Rindi, F., Stengel, D. B. & Guiry, M. D. Morphology, rbc L phylogeny and distribution of distromatic Ulva (Ulvophyceae, Chlorophyta) in Ireland and southern Britain. Phycologia 47 , 416–429. 10.2216/PH07-61.1 (2008). Hofmann, L. C., Nettleton, J. C., Neefus, C. D. & Mathieson, A. C. Cryptic diversity of Ulva (Ulvales, Chlorophyta) in the Great Bay Estuarine System (Atlantic USA): introduced and indigenous distromatic species. Eur. J. Phycol. 45 , 230–239. 10.1080/09670261003746201 (2010). Park, H. J., Byeon, S. Y., Park, S. R. & Lee, H. J. Temporal variation in the community structure of green tide forming macroalgae (Chlorophyta; genus Ulva ) on the coast of Jeju Island, Korea based on DNA barcoding. Korean J. Environ. Biology . 40 , 464–476. 10.11626/KJEB.2022.40.4.464 (2022). Kang, J. H. et al. Species composition, diversity, and distribution of the genus Ulva along the coast of Jeju Island, Korea based on molecular phylogenetic analysis. PLoS One . 14 , e0219958. 10.1371/journal.pone.0219958 (2019). Fort, A. et al. Exhaustive reanalysis of barcode sequences from public repositories highlights ongoing misidentifications and impacts taxa diversity and distribution. Mol. Ecol. Resour. 22 , 86–101. 10.1111/1755-0998.13453 (2022). Blomster, J., Maggs, C. A., Stanhope, M. J. & Molecular and morphological analysis of Enteromorpha intestinalis and E. compressa (Chlorophyta) in the British Isles. J. Phycol. 34, 319–340, (1998). 10.1046/j.1529-8817.1998.340319.x Hayden, H. S. & Waaland, J. R. A molecular systematic study of Ulva (Ulvaceae, Ulvales) from the northeast Pacific. Phycologia 43 , 364–382. 10.2216/i0031-8884-43-4-364.1 (2004). Saunders, G. W. & Kucera, H. An evaluation of rbc L, tuf A, UPA, LSU and ITS as DNA barcode markers for the marine green macroalgae. Cryptogamie Algologie . 31 , 487–528 (2010). Shimada, S., Yokoyama, N., Arai, S. & Hiraoka, M. Phylogeography of the genus Ulva (Ulvophyceae, Chlorophyta), with special reference to the Japanese freshwater and brackish taxa. J. Appl. Phycol. 20 , 979–989. 10.1007/s10811-007-9296-y (2008). Zhang, X. et al. Settlement of vegetative fragments of Ulva prolifera confirmed as an important seed source for succession of a large-scale green tide bloom. Limnol. Oceanogr. 56 , 233–242. 10.4319/lo.2011.56.1.0233 (2011). Kawashima, Y., Akasaki, T., Matsumoto, Y., Yamazaki, Y. & Shimada, S. Species identification of imported and Japanese commercial green algal products based on phylogenetic analyses using the nrITS2 and 5S rDNA spacer regions. Fish. Sci. 79 , 521–529. 10.1007/s12562-013-0610-0 (2013). Cui, J. et al. Taxonomic reassessment of Ulva prolifera (Ulvophyceae, Chlorophyta) based on specimens from the type locality and Yellow Sea green tides. Phycologia 57 , 692–704. 10.2216/17-139.1 (2018). Xia, Z. et al. Distribution of Ulva prolifera , the dominant species in green tides along the Jiangsu Province coast in the Southern Yellow Sea, China. J. Sea Res. 196 , 102436. 10.1016/j.seares.2023.102436 (2023). Park, J., Shin, S., Chae, J., Lee, H. & Mo, Y. Development of the Methods for Controlling and Managing the Marine Ecosystem Disturbing and Harmful Organisms. Marine Ecology Division, Ministry of Oceans and Fisheries. Sejong, Korea (2022). Park, S. R. Seasonal patterns and recruitment dynamics of green tide-forming Ulva species along the intertidal rocky shores of the southern coast of Korea. Ocean. Sci. J. 49 , 383–390. 10.1007/s12601-014-0035-4 (2014). Famà, P., Wysor, B., Kooistra, W. H. & Zuccarello, G. C. Molecular phylogeny of the genus Caulerpa (Caulerpales, Chlorophyta) inferred from chloroplast tuf A gene1. J. Phycol. 38 , 1040–1050. 10.1046/j.1529-8817.2002.t01-1-01237.x (2002). Kearse, M. et al. Geneious Basic: an integrated and extendable desktop software platform for the organization and analysis of sequence data. Bioinformatics 28 , 1647–1649. 10.1093/bioinformatics/bts199 (2012). Sievers, F. et al. Fast, scalable generation of high-quality protein multiple sequence alignments using Clustal Omega. Mol. Syst. Biol. 7 10.1038/msb.2011.75 (2011). Darriba, D., Taboada, G. L., Doallo, R. & Posada, D. jModelTest 2: more models, new heuristics and parallel computing. Nat. Methods . 9 , 772. 10.1038/nmeth.2109 (2012). Tamura, K. & Nei, M. Estimation of the number of nucleotide substitutions in the control region of mitochondrial DNA in humans and chimpanzees. Mol. Biol. Evol. 10 , 512–526. 10.1093/oxfordjournals.molbev.a040023 (1993). Guindon, S. & Gascuel, O. A simple, fast, and accurate algorithm to estimate large phylogenies by maximum likelihood. Syst. Biol. 52 , 696–704. 10.1080/10635150390235520 (2003). Puillandre, N., Lambert, A., Brouillet, S. & Achaz, G. A. B. G. D. Automatic Barcode Gap Discovery for primary species delimitation. Mol. Ecol. 21 , 1864–1877. 10.1111/j.1365-294X.2011.05239.x (2012). Clarke, K. & Gorley, R. Getting started with PRIMER v7. PRIMER-e: plymouth, plymouth marine laboratory 20 (2015). Ivlev, V. S. Experimental ecology of the feeding of fishes (Yale University Press, 1961). Jacobs, J. Quantitative measurement of food selection: A modification of the forage ratio and Ivlev's electivity index. Oecologia 14 , 413–417. 10.1007/BF00384581 (1974). Hiraoka, M., Ohno, M., Kawaguchi, S. & Yoshida, G. Crossing test among floating Ulva thalli forminggreen tide'in Japan. Hydrobiologia 512 , 239–245. 10.1023/B:HYDR.0000020332.12641.a2 (2004). Kirkendale, L., Saunders, G. W. & Winberg, P. A. Molecular Survey of Ulva (Chlorophyta) in Temperate Australia Reveals Enhanced Levels of Cosmopolitanism. J. Phycol. 49 , 69–81. 10.1111/jpy.12016 (2013). Zanolla, M., Carmona, R., Kawai, H., Stengel, D. B. & Altamirano, M. Role of thermal photosynthetic plasticity in the dispersal and settlement of two global green tide formers: Ulva pertusa and U. ohnoi . Mar. Biol. 166 , 123. 10.1007/s00227-019-3578-1 (2019). Bachoo, T., Bolton, J. J., Macey, B. M., Kandjengo, L. & Reddy, M. M. Resolving the identity of commercially cultivated Ulva (Ulvaceae, Chlorophyta) in integrated seaweed-abalone aquaculture farms in South Africa. J. Phycol. 59 , 1272–1283. 10.1111/jpy.13391 (2023). Kim, K. Y. et al. Physiological ecology and seasonality of Ulva pertusa on a temperate rocky shore. Phycologia 43 , 483–492. 10.2216/i0031-8884-43-4-483.1 (2004). Kim, J. H., Kang, E. J., Park, M. G., Lee, B. G. & Kim, K. Y. Effects of temperature and irradiance on photosynthesis and growth of a green-tide-forming species ( Ulva linza ) in the Yellow Sea. J. Appl. Phycol. 23 , 421–432. 10.1007/s10811-010-9590-y (2011). Yoshida, G., Uchimura, M. & Hiraoka, M. Persistent occurrence of floating Ulva green tide in Hiroshima Bay, Japan: seasonal succession and growth patterns of Ulva pertusa and Ulva spp.(Chlorophyta. Ulvales) Hydrobiologia . 758 , 223–233. 10.1007/s10750-015-2292-3 (2015). Hiraoka, M., Shimada, S., Uenosono, M. & Masuda, M. A new green-tide‐forming alga, Ulva ohnoi Hiraoka et Shimada sp. nov.(Ulvales, Ulvophyceae) from Japan. Phycol. Res. 52 , 17–29. 10.1111/j.1440-1835.2003.00321.x (2004). López, S. B., Fernández, I. B., Lozano, R. B. & Ugarte, J. C. Is the cryptic alien seaweed Ulva pertusa (Ulvales, Chlorophyta) widely distributed along European Atlantic coasts? Bot. Mar. 50 10.1515/BOT.2007.030 (2007). Xing, Q. et al. A novel approach of monitoring Ulva pertusa green tide on the basis of UAV and deep learning. Water 15 , 3080. 10.3390/w15173080 (2023). Hanyuda, T. & Kawai, H. Genetic examination of the type specimen of Ulva australis suggests that it was introduced to Australia. Phycol. Res. 66 , 238–241. 10.1111/pre.12222 (2018). Kang, E. J. et al. Evaluating bloom potential of the green-tide forming alga Ulva ohnoi under ocean acidification and warming. Sci. Total Environ. 769 , 144443. 10.1016/j.scitotenv.2020.144443 (2021). Nakamura, M. et al. Photosynthesis and growth of Ulva ohnoi and Ulva pertusa (Ulvophyceae) under high light and high temperature conditions, and implications for green tide in Japan. Phycol. Res. 68 , 152–160. 10.1111/pre.12410 (2020). Korea Oceanographic Data Center (KODC). N. I. o. F. S. N. Sea surface temperature data (2015 and 2020) for Jeju Island, Korea , (2022). https://www.nifs.go.kr/kodc/soo_list.kodc Mineur, F., Johnson, M. P. & Maggs, C. A. Macroalgal introductions by hull fouling on recreational vessels: seaweeds and sailors. Environ. Manage. 42 , 667–676. 10.1007/s00267-008-9185-4 (2008). Wolf, M. A., Sciuto, K., Andreoli, C. & Moro, I. Ulva (Chlorophyta, Ulvales) Biodiversity in the North Adriatic Sea (Mediterranean, Italy): Cryptic Species and New Introductions. J. Phycol. 48 , 1510–1521. 10.1111/jpy.12005 (2012). Steinhagen, S., Karez, R. & Weinberger, F. Cryptic, alien and lost species: molecular diversity of Ulva sensu lato along the German coasts of the North and Baltic Seas. Eur. J. Phycol. 54 , 466–483. 10.1080/09670262.2019.1597925 (2019). Santelices, B. Banks of microscopic forms and survival to darkness of propagules and microscopic stages of macroalgae. (2002). Flagella, M. M., Verlaque, M., Soria, A. & Buia, M. C. Macroalgal survival in ballast water tanks. Mar. Pollut Bull. 54 , 1395–1401. 10.1016/j.marpolbul.2007.05.015 (2007). Flagella, M. M., Andreakis, N., Hiraoka, M., Verlaque, M. & Buia, M. C. Identification of cryptic Ulva species (Chlorophyta, Ulvales) transported by ballast water. J. Biol. Res. 13 , 47 (2010). de Casabianca, M. L., Barthelemy, N., Serrano, O. & Sfriso, A. Growth rate of Ulva rigida in different Mediterranean eutrophicated sites. Bioresour Technol. 82 , 27–31. 10.1016/s0960-8524(01)00155-9 (2002). Mao, Y., Kim, J. K., Wilson, R. & Yarish, C. The appearance of Ulva laetevirens (Ulvophyceae, Chlorophyta) in the northeast coast of the United States of America. J. Ocean. Univ. China . 13 , 865–870. 10.1007/s11802-014-2419-6 (2014). Wu, H. et al. Physiological acclimation of Ulva prolifera to seasonal environmental factors drives green tides in the Yellow Sea. Mar. Environ. Res. 179 , 105695. 10.1016/j.marenvres.2022.105695 (2022). Kang, E. J., Kim, J. H., Kim, K., Choi, H. G. & Kim, K. Y. Re-evaluation of green tide-forming species in the Yellow Sea. Algae 29, 267–277, (2014). 10.4490/algae.2014.29.4.267 Son, Y. B., Choi, B. J., Kim, Y. H. & Park, Y. G. Tracing floating green algae blooms in the Yellow Sea and the East China Sea using GOCI satellite data and Lagrangian transport simulations. Remote Sens. Environ. 156 , 21–33. 10.1016/j.rse.2014.09.024 (2015). Yotsukura, N., Kawai, T., Motomura, T. & Ichimura, T. Tandem 5S ribosomal RNA genes and the spacer region sequences of three Japanese Laminaria species. J. Appl. Phycol. 14 , 233–239. 10.1023/A:1021166218681 (2002). Yotsukura, N., Kawai, T., Kawashima, S., Ebata, H. & Ichimura, T. Nucleotide sequence diversity of the 5S rDNA spacer in the simple blade kelp genera Laminaria , Cymathaere and Kjellmaniella (Laminariales, Phaeophyceae) from northern Japan. Phycol. Res. 54 , 269–279. 10.1111/j.1440-1835.2006.00434.x (2006). Marine Environment Information System (MEIS). Marine environment observation data: 2021 raw data. Korea Marine Environment Management Corporation. (2021). Available at: http://meis.go.kr . (accessed 30 October 2025). Additional Declarations No competing interests reported. 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14:06:02","extension":"html","order_by":29,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":212187,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/4ad6c583532f86a435085165.html"},{"id":96915291,"identity":"3d6dcd04-ed7b-423c-9a19-99865dd795a4","added_by":"auto","created_at":"2025-11-27 14:07:04","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":780585,"visible":true,"origin":"","legend":"\u003cp\u003eMap showing collection sites for \u003cem\u003eUlva \u003c/em\u003especies along the coast of Jeju Island and the southern coasts of Korea. Thirty-one and 15 sampling locations on Jeju Island and the southern coasts, respectively are indicated. \u003cem\u003eUlva\u003c/em\u003e specimens were collected on a seasonal basis and thus they were sampled for every site four times.\u003c/p\u003e","description":"","filename":"Fig1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/7190d90707c67c209da9b74f.jpg"},{"id":96746955,"identity":"3707de73-d18b-4b34-b7ce-29f88d091032","added_by":"auto","created_at":"2025-11-25 16:10:18","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":948732,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative morphological features of \u003cem\u003eUlva \u003c/em\u003especies identified based on \u003cem\u003etuf\u003c/em\u003eA phylogenetic analysis (\u003csup\u003e14\u003c/sup\u003e; this study). The scale bar is 15 cm [a. \u003cem\u003eU. ohnoi\u003c/em\u003e; b\u003cem\u003e. U. arasakii\u003c/em\u003e; c. \u003cem\u003eU. californica\u003c/em\u003e; d. \u003cem\u003eU. australis\u003c/em\u003e (=\u003cem\u003eU. pertusa\u003c/em\u003e); e. LPP clade species (\u003cem\u003eU. procera\u003c/em\u003e); f. \u003cem\u003eU. laetevirens\u003c/em\u003e (=\u003cem\u003eU. rigida\u003c/em\u003e); g. \u003cem\u003eU. flexuosa\u003c/em\u003e;\u003cem\u003e \u003c/em\u003eh.\u003cem\u003e U. lactuca\u003c/em\u003e;\u003cem\u003e \u003c/em\u003ei.\u003cem\u003e U. compressa\u003c/em\u003e;\u003cem\u003e \u003c/em\u003ej–k.\u003cem\u003e \u003c/em\u003eLPP clade species (\u003cem\u003eU. prolifera\u003c/em\u003e).].\u003c/p\u003e","description":"","filename":"Fig2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/01ada3c26df73f90267a2cb4.jpg"},{"id":96746957,"identity":"1ad2cfd1-3afa-4e72-b35e-ee64807c5379","added_by":"auto","created_at":"2025-11-25 16:10:18","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":411444,"visible":true,"origin":"","legend":"\u003cp\u003eNeighbor-joining (NJ) phylogeny based on 669 \u003cem\u003etuf\u003c/em\u003eA sequences (612 specimens of Jeju Island plus 55 previously determined haplotype sequences and two unidentified haplotypes) of \u003cem\u003eUlva\u003c/em\u003e and two sequences of \u003cem\u003eBlidingia\u003c/em\u003e species as outgroup\u003csup\u003e15\u003c/sup\u003e.Reference sequences were obtained from GenBank and used for phylogenetic analysis of species identification. A large number of specimens found within a particular clade are marked as “Group” with the total number of specimens [e.g., Group 1 (N=40)]. Numbers on the nodes indicate bootstrap values for maximum likelihood (ML) and neighbor-joining (NJ), respectively.\u003c/p\u003e","description":"","filename":"Fig3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/46b573c0727b119b6aa73760.jpg"},{"id":96914903,"identity":"5c8bab2d-b52a-4a45-a92a-32cdb5348d4e","added_by":"auto","created_at":"2025-11-27 14:06:34","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":415245,"visible":true,"origin":"","legend":"\u003cp\u003eNeighbor-joining (NJ) phylogeny based on 411 \u003cem\u003etuf\u003c/em\u003eA sequences (354 specimens of the southern coasts plus 55 previously determined haplotype sequences and two unidentified haplotypes) of \u003cem\u003eUlva\u003c/em\u003e and two sequences of \u003cem\u003eBlidingia\u003c/em\u003e species\u003csup\u003e14\u003c/sup\u003e.Reference sequences were obtained from GenBank and used for phylogenetic analysis of species identification. A large number of specimens found within a particular clade are marked as “Group” with the total number of specimens [e.g., Group 1 (N=13)]. Numbers on the nodes indicate bootstrap values for maximum likelihood (ML) and neighbor-joining (NJ), respectively.\u003c/p\u003e","description":"","filename":"Fig4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/689b7423defa90166753458f.jpg"},{"id":96746961,"identity":"997f4910-d2ba-4501-b059-2137288fc0b6","added_by":"auto","created_at":"2025-11-25 16:10:18","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":403719,"visible":true,"origin":"","legend":"\u003cp\u003eComparisons of relative frequencies of \u003cem\u003eUlva\u003c/em\u003e species between \u003cem\u003eUlva\u003c/em\u003e-communities from Jeju Island and the southern coasts. Jeju Island; samplings conducted from November 2019 to February 2021, the southern coasts; from January 2021 to October 2021. LPP sp. 1 was identified as \u003cem\u003eU. procera\u003c/em\u003e, while LPP sp. 2 was identified as \u003cem\u003eU. prolifera\u003c/em\u003e,\u003cem\u003e \u003c/em\u003ebased on \u003cem\u003etuf\u003c/em\u003eA analysis.\u003c/p\u003e","description":"","filename":"Fig5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/529f1e0d9b3ebde991fd5e42.jpg"},{"id":96913502,"identity":"37a6bff9-f4c6-48d3-a3af-4eb372cba0c4","added_by":"auto","created_at":"2025-11-27 14:02:19","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":339507,"visible":true,"origin":"","legend":"\u003cp\u003eResults of community similarity analysis of \u003cem\u003eUlva\u003c/em\u003e species composition for Jeju Island and southern coasts. (A) Results of heat map analysis. (B) Results of non-metric multidimensional scaling (nMDS) plot analysis.\u003c/p\u003e","description":"","filename":"Fig6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/67987f59c76f673671313272.jpg"},{"id":96914165,"identity":"17e4aafb-5743-4c33-9a2e-fc3aecb20b6d","added_by":"auto","created_at":"2025-11-27 14:05:33","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":531563,"visible":true,"origin":"","legend":"\u003cp\u003eSeasonal changes in relative frequency (%) of \u003cem\u003eUlva\u003c/em\u003e species in green tide-forming assemblages in Jeju Island and the southern coasts across four seasons (spring, summer, autumn, and winter). Data were collected during field surveys conducted in four different seasons. (A) Stacked bar charts showing the relative frequency of each species; species names are listed on the right. (B) Line graphs illustrating seasonal abundance (% frequency) trends for each species in Jeju Island and the southern coasts.\u003c/p\u003e","description":"","filename":"Fig7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/db53ef519a94e502efac87ac.jpg"},{"id":96746970,"identity":"64a2cd33-2c4a-4448-9725-27fc96bfc4c9","added_by":"auto","created_at":"2025-11-25 16:10:18","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":279978,"visible":true,"origin":"","legend":"\u003cp\u003eUnrooted neighbor-joining (NJ) tree of 5S rDNA spacer region of the LPP complex. Bootstrap values of ≥ 50 % for 1,000 replicates are given at each node. Groups were defined when the same single lineage was formed.\u003c/p\u003e","description":"","filename":"Fig8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/76fbc468acac0a3c9c392654.jpg"},{"id":97135373,"identity":"e577d2e8-c83f-4bcf-8c99-5562a42856fb","added_by":"auto","created_at":"2025-12-01 09:40:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5539966,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/6390e9f1-c514-446d-8bac-66de40bb2473.pdf"},{"id":96914534,"identity":"dad9df99-a731-4f31-9641-4041e5aea49f","added_by":"auto","created_at":"2025-11-27 14:06:01","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":642107,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementarymaterialsSicentificReports.docx","url":"https://assets-eu.researchsquare.com/files/rs-8022902/v1/90ae91a322735ddbe4b7ee39.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Unraveling the major Ulva species driving local green tides through molecular analyses: spatiotemporal patterns along the Korean coast","fulltext":[{"header":"Introduction","content":"\u003cp\u003eDue to the recent acceleration of climate change, macroalgal blooms have been occurring more frequently in many coastal regions worldwide\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Since the 1990s, large-scale coastal macroalgal outbreaks have become almost routine events\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. The terms, green tide and golden tide, collectively referred to as seaweed tide (or macroalgal bloom) were coined to describe seawater that appears as a \u0026lsquo;green\u0026rsquo; or \u0026lsquo;brown\u0026rsquo; carpet (or paint) due to masses of floating seaweeds. These phenomena are primarily caused by green macroalgae (Chlorophyta) and brown macroalgae (Phaeophyceae), both of which have high growth and proliferation rates associated with their unique life history traits\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Consequently, green and golden tides have become critical ecological and environmental issues worldwide, although the key ecological drivers underlying these blooms remain poorly understood\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Green tides are predominantly formed by the genus \u003cem\u003eUlva\u003c/em\u003e including the former \u003cem\u003eEnteromorpha\u003c/em\u003e, whereas golden tides are caused by \u003cem\u003eSargassum\u003c/em\u003e species. In the northwestern Pacific, golden tides are known to be exclusively involved with \u003cem\u003eSargassum horneri\u003c/em\u003e\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eCoastal areas such as estuaries, harbors and bays, where seawater is partially enclosed by natural or artificial barriers, are more prone to green macroalgal blooms (i.e. green tides). The rise in green macroalgal biomass in these regions is largely attributed to human-driven factors, such as nutrient enrichment, eutrophication and pollution\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Large-scale green tide events caused by rapid proliferation of \u003cem\u003eUlva\u003c/em\u003e species can severely impact coastal ecosystem and local economy\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003c/sup\u003e. Previous studies have suggested that a rise in inorganic nutrients (e.g. dissolved inorganic nitrogen) and coastal eutrophication are major ecological and environmental drivers of green tide formation\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. The increase in algal biomass associated with green tides is perhaps linked to nutrient loading, especially nitrate enrichment resulting from human activities. Understanding the possible causes and environmental factors underlying green tide formation is essential for developing mitigation and management strategies to reduce their negative impacts\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe genus \u003cem\u003eUlva\u003c/em\u003e is divided into two major morphological types \u0026ndash; foliose and tubular. The tubular form was previously classified as the genus \u003cem\u003eEnteromorpha\u003c/em\u003e but was later synonymized with \u003cem\u003eUlva\u003c/em\u003e based on genetic evidence\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. \u003cem\u003eUlva\u003c/em\u003e species exhibit two distinct growth forms, such as attached or free-floating thalli. Because of their high level of phenotypic plasticity in morphology, which varies with habitat environmental conditions, accurate species identification based solely on morphology is extremely difficult\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. For example, among species inhabiting the southern coasts of Korea and Jeju Island (located off the southernmost region of the Korean Peninsula), \u003cem\u003eUlva australis\u003c/em\u003e (synonym; \u003cem\u003eUlva pertusa\u003c/em\u003e) and \u003cem\u003eUlva ohnoi\u003c/em\u003e exhibit similar thallus morphologies, making them difficult or impossible to distinguish morphologically. Therefore, discrepancies likely exist between reported and actual species numbers, and \u003cem\u003eUlva\u003c/em\u003e species diversity may be underestimated due to phenotypic plasticity and incomplete molecular dataset\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. For \u003cem\u003eUlva\u003c/em\u003e species with simple thallus structures and lacking differentiated reproductive organs, morphological species identification is particularly challenging\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eMolecular marker-based studies on green tides are necessary for identifying the major \u003cem\u003eUlva\u003c/em\u003e species responsible for these large-scale outbreaks. In the northwestern Pacific, encompassing the coastal regions of China, Japan, and Korea, molecular analyses have been widely applied to investigate the green tide phenomena. The chloroplast DNA (cpDNA) \u003cem\u003etuf\u003c/em\u003eA (elongation factor Tu) and the internal transcribed spacer (ITS) region of nuclear ribosomal DNA (nuDNA) are among the most commonly used genetic markers for studying \u003cem\u003eUlva\u003c/em\u003e diversity\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. These markers have become popular and have significantly contributed to our understanding of genetic diversity and phylogenetic relationships among \u003cem\u003eUlva\u003c/em\u003e species. Moreover, the nuclear 5S ribosomal DNA (5S rDNA) marker has shown great potential for inferring interspecies phylogeny, particularly within the \u003cem\u003eUlva linza\u003c/em\u003e-\u003cem\u003eprocera\u003c/em\u003e-\u003cem\u003eprolifera\u003c/em\u003e (LPP) complex or LPP clade\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. The chloroplast \u003cem\u003erbc\u003c/em\u003eL (ribulose-1,5-bisphosphate carboxylase) has also been employed for DNA barcoding of \u003cem\u003eUlva\u003c/em\u003e species\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Among these markers, \u003cem\u003etuf\u003c/em\u003eA generally provides higher resolution for species identification compared to ITS\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. However, species delimitation within the LPP clade remains unresolved. Therefore, in this study, we combined \u003cem\u003etuf\u003c/em\u003eA and5S rDNA analyses to identify \u003cem\u003eUlva\u003c/em\u003e species\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn Korean waters, local green tide has been observed year-round, particularly along the northeastern coast of Jeju Island since the 2000s\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. More recently, green tides have also occurred sporadically along the southern coasts\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. Nevertheless, the \u003cem\u003eUlva\u003c/em\u003e species primarily responsible for these blooms remain largely unidentified (but see\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e). Furthermore, potential geographic and seasonal variation in \u003cem\u003eUlva\u003c/em\u003e community structure, species diversity and distributional patterns remain poorly understood.\u003c/p\u003e\u003cp\u003eIn the present study, we aimed to identify \u003cem\u003eUlva\u003c/em\u003e species primarily responsible for green macroalgal blooms along the Korean coast. Specifically, we assessed \u003cem\u003eUlva\u003c/em\u003e community structure, species composition and diversity, and seasonal variation on Jeju Island and the southern coasts based on molecular phylogenetic analyses. By applying combined analysis of \u003cem\u003etuf\u003c/em\u003eA and 5S rDNA, we determined the dominant \u003cem\u003eUlva\u003c/em\u003e species associated with green tides. The objectives of this study were to (1) identify the primary \u003cem\u003eUlva\u003c/em\u003e species contributing to local green tide events on Jeju Island and the southern coasts of Korea; (2) investigate geographic variation in \u003cem\u003eUlva\u003c/em\u003e community structure and species composition and diversity between the two regions, with a focus on spatial differences in species distribution; (3) examine temporal (seasonal) variation in \u003cem\u003eUlva\u003c/em\u003e community structure. This study provides a comprehensive understanding of the major \u003cem\u003eUlva\u003c/em\u003e species driving green tides along the Korean coast and demonstrates how \u003cem\u003eUlva\u003c/em\u003e species composition changes across spatial and temporal scales.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eSample collection and pretreatment\u003c/h2\u003e\u003cp\u003eThis study was conducted on a total of 966 specimens at 46 locations along the southern coastal areas of the Korean Peninsula (the southern coasts hereafter) and the coastline of Jeju Island, which is located\u0026thinsp;~\u0026thinsp;150\u0026ndash;200 km off the southern coast of the mainland, from November 2019 to February 2021 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Tables S1, 2). Six hundred-twelve specimens from 31 sites and three hundred fifty-four specimens from 15 sites were used for Jeju Island and the southern coasts, respectively for genetic analysis. These sites were selected as the study sites because local green tides have been frequently observed there. Samples were collected on a seasonal basis (spring, summer, autumn, and winter), and information on latitude/longitude for the sampling sites is given in Tables S1, 2. More than five individuals were collected per morphotype of \u003cem\u003eUlva\u003c/em\u003e spp. at each of the sampling sites (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and all specimens were collected at 2\u0026ndash;3 meter intervals of each other within sites. The sampling distances were kept across sites in Jeju Island and the southern coasts. These sampling schemes would allow to avoid the inadvertent collection of the same individuals\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. In cases where the sampling areas were geographically separated by a coastal embankment, the inland site was designated as \u0026lsquo;in\u0026rsquo; and the coastal site as \u0026lsquo;outside\u0026rsquo;.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe collected samples were transported to the laboratory while maintaining the temperature below 4\u0026deg;C, and all samples were photographed before DNA extraction. After photographing, the samples were rinsed several times with freshwater and foliose tissue samples were completely dried in a dry oven (Jeio Tech, Korea) at 60\u0026deg;C for at least 24 hours. Completely dried samples were powdered using TissueLyserII (Qiagen, USA). The pulverized powder tissue sample was stored with silica gel in a 2.0 ml tube and a Desiccator Cabinet (Kastech, Korea) with an automatic dehumidification function until genetic analysis. Genetic analysis was performed at least for 2\u0026ndash;3 individuals per morphotype for each sampling site. All the 966 samples were sequenced for \u003cem\u003etuf\u003c/em\u003eA gene and a subset of 119 individuals belonging to the LPP clade, as identified by \u003cem\u003etuf\u003c/em\u003eA, were further analyzed. However, only 105 out of the 119 samples were successfully sequenced for 5S rDNA.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eGenomic DNA extraction, PCR and sequencing\u003c/h3\u003e\n\u003cp\u003e Genomic DNA (gDNA) was extracted using i-genomic Plant DNA Extraction Mini Kit (Intron Biotechnology, Korea) according to manufacturer\u0026rsquo;s protocol. The concentration of the extracted gDNA was measured using a NanoDrop UV/VIS spectrophotometer (Thermo Fisher Scientific, USA). The cpDNA \u003cem\u003etuf\u003c/em\u003eA gene\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e and nuDNA 5S rDNA\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e were amplified by polymerase chain reaction (PCR). The \u003cem\u003etuf\u003c/em\u003eA gene was amplified with the published primers, TufGF4 (5\u0026rsquo;-GGNGCNGCNCAAATGGAYGG-3\u0026rsquo;) and TufAR (5\u0026rsquo;-CCTTCNCGAATMGCRAAWCGC-3\u0026rsquo;)\u003csup\u003e19,27\u003c/sup\u003e and 5S rDNA region was amplified with the primers, 5S-F (5\u0026rsquo;-GGTTGGGCAGGATTAGTA-3\u0026rsquo;) and 5S-R (5\u0026rsquo;-AGGCTTAAGTTGCGAGTT-3\u0026rsquo;)\u003csup\u003e20\u003c/sup\u003e. PCR reaction was performed in a total reaction volume of 15 \u0026micro;l with 10\u0026times;Dream Taq Green buffer (Thermo Fisher Scientific) 1.5 \u0026micro;l, 2.5 mM dNTPs (Bio Basic Inc., Canada) 1.5 \u0026micro;l, 10 pmol forward/reverse primers 0.5 \u0026micro;l, 0.2 units of Taq DNA polymerase (Thermo Fisher Scientific) 0.1 \u0026micro;l, template gDNA (~\u0026thinsp;20 ng/\u0026micro;l), and 9.9 \u0026micro;l sterile distilled water using a 2720 thermal cycler (Applied Biosystems, USA). PCR amplification was performed by 35 cycles of initial denaturation at 94\u0026deg;C for 4 min, denaturation at 94\u0026deg;C for 1 min, annealing at 45\u0026deg;C (for \u003cem\u003etuf\u003c/em\u003eA)\u0026thinsp;~\u0026thinsp;50\u0026deg;C (5S rDNA) for 30\u0026ndash;45 sec, and extension at 72\u0026deg;C for 1 min, followed by a final extension reaction at 72\u0026deg;C for 7 min. The PCR products were visualized by electrophoresis on a 2% agarose gel and purified with enzymatically Exonuclease I and Shrimp Alkaline Phosphatase (New England BioLabs, USA). Sequencing was performed with ABI PRISM 3730xl automated DNA sequencer (Applied Biosystems).\u003c/p\u003e\u003cp\u003eFor 5S rDNA region, gel extraction/excision process was undertaken prior to DNA sequencing. The 5S rDNA sequences were used for identifying the species belonging the LPP clade\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. The resulting DNA fragments were visualized by UV transillumination and analyzed using Gel doc 1000 UV Fluorescent Gel Documentation System-PC (Biorad, USA). After identifying the shortest DNA fragment (about 200\u0026ndash;300 bp) in each PCR product of the 5S rDNA spacer region, PCR yielding band was excised from the gel and purified using MinElute Gel Extraction Kit (Qiagen)\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eMolecular phylogenetic analysis\u003c/h3\u003e\n\u003cp\u003eThe 966 \u003cem\u003etuf\u003c/em\u003eA DNA sequences obtained were edited using Geneious prime ver. 2021.0.3\u003csup\u003e28\u003c/sup\u003e and aligned using Clustal Omega ver. 1.2.2\u003csup\u003e29\u003c/sup\u003e. For molecular-based species identification of \u003cem\u003eUlva\u003c/em\u003e, 55 haplotypes of 17 species plus three unidentified haplotypes, which were previously determined by \u003cem\u003etuf\u003c/em\u003eA phylogenetic analysis\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, were used as references in this study (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Two \u003cem\u003eBlidingia\u003c/em\u003e spp. were used as outgroup (GenBank accession numbers: MK992087, HQ610240). When monophyletic clades were formed in the phylogenetic tree, we defined those as particular \u0026ldquo;the species\u0026rdquo;\u003csup\u003e15\u003c/sup\u003e. When the samples did not belong to any particular clade, they were then defined as \u0026ldquo;unidentified\u0026rdquo;. Aligned DNA sequences best fitted the JC (Jukes-Cantor) model based on jModel-test ver. 2.1.7\u003csup\u003e30\u003c/sup\u003e. Neighbor-joining (NJ) analysis was performed using MEGA ver. 7.0\u003csup\u003e31\u003c/sup\u003e using 1,000 repetitions (bootstrap) with the JC model. Maximum likelihood (ML) analysis was also performed using PhyML ver. 3.1\u003csup\u003e32\u003c/sup\u003e with 1,000 bootstrap samplings.\u003c/p\u003e\u003cp\u003eTo determine species within the LPP clade, additional phylogenetic analysis of the 5S rDNA was conducted on 105 individuals (25 from Jeju Island and 80 from the southern coasts) out of 119 specimens that were assigned to the LPP clade based on the \u003cem\u003etuf\u003c/em\u003eA-based phylogenetic analysis. More specifically, the 105 samples identified as \u003cem\u003eU. procera\u003c/em\u003e or \u003cem\u003eU. prolifera\u003c/em\u003e (103 \u003cem\u003eU. procera\u003c/em\u003e and two \u003cem\u003eU. prolifera\u003c/em\u003e) in \u003cem\u003etuf\u003c/em\u003eA-phylogeny were reanalyzed using the 5S rDNA marker with NJ method to distinguish \u003cem\u003eU. prolifera\u003c/em\u003e and \u003cem\u003eU. linza\u003c/em\u003e. Previously determined 25 haplotype sequences were used as a reference for this analysis\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. The total sequence length, including gaps between sequences (24\u0026thinsp;~\u0026thinsp;125 bp), was 352 bp, and the minimum sequence length of 227 bp.\u003c/p\u003e\u003cp\u003eWe further performed model-based algorithmic species delimitation analysis of ABGD (Automatic Barcode Gap Discovery)\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e to corroborate the results of our molecular phylogeny-based species delimitation. ABGD analyses were conducted via the web-interface (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://wwwabi.snv.jussieu.fr/public/abgd/abgdweb.html\u003c/span\u003e\u003cspan address=\"https://wwwabi.snv.jussieu.fr/public/abgd/abgdweb.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) with default settings with the exception of the JC model and two relative gap widths (X) (X\u0026thinsp;=\u0026thinsp;1.0, 1.5) applied\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. The prior intraspecific diversity (P) was set to range from 0.001 to 0.1. The relative frequency of each \u003cem\u003eUlva\u003c/em\u003e species was calculated as the proportion of individuals assigned to each species out of the total number of genetically identified green algal specimens in each region (Jeju Island and the southern coasts). Species identification was based on both \u003cem\u003etuf\u003c/em\u003eA and 5S rDNA analyses.\u003c/p\u003e\n\u003ch3\u003eCommunity analysis\u003c/h3\u003e\n\u003cp\u003eThe species composition ratio of \u003cem\u003eUlva\u003c/em\u003e at each site was calculated based on relative abundance data. Prior to analysis, the data were square-root transformed, and Bray\u0026ndash;Curtis similarity indices were computed to assess differences in community composition between Jeju Island and the southern coasts. Non-metric multidimensional scaling (nMDS\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e) was used to visualize spatial patterns in community structure. Statistical significance of spatial differences between the two regions was tested using analysis of similarities (ANOSIM). All analyses and visualizations were conducted using PRIMER-e v7 (PRIMER-E Ltd., Plymouth, UK).\u003c/p\u003e\u003cp\u003eIn addition, we tested for significant difference in life-form type (floating vs. benthic [attached] thalli) between the two dominant \u003cem\u003eUlva\u003c/em\u003e species (\u003cem\u003eU. australis\u003c/em\u003e and \u003cem\u003eU. ohnoi\u003c/em\u003e) in Jeju Island. Differences in the proportions of benthic and floating individuals between the two species (\u003cem\u003eU. australis\u003c/em\u003e [N\u0026thinsp;=\u0026thinsp;129] and \u003cem\u003eU. ohnoi\u003c/em\u003e [N\u0026thinsp;=\u0026thinsp;209]) were evaluated using a chi-square test, and a preference index was subsequently calculated following the method Ivlev\u0026rsquo;s\u003csup\u003e35\u003c/sup\u003e and modified by Jacobs\u0026rsquo;s\u003csup\u003e36\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eModel-based species delimitation\u003c/h2\u003e\u003cp\u003eThe number of \u003cem\u003eUlva\u003c/em\u003e species identified by ABGD based on \u003cem\u003etuf\u003c/em\u003eA was consistent with the number of species groups (clusters) inferred from phylogenetic analyses. In the ABGD results, the number of species groups varied depending on the values of P and X. The number of groups showing the minimum difference (i.e. 1.0) between the initial partition (IP) and recursive partition (RP) was 14 (IP) and 15 (RP) when X\u0026thinsp;=\u0026thinsp;1.0 (P\u0026thinsp;=\u0026thinsp;0.0077), whereas the numbers decreased to 10 (IP) and 11 (RP), respectively when X\u0026thinsp;=\u0026thinsp;1.5 (P\u0026thinsp;=\u0026thinsp;0.0077). After excluding \u003cem\u003eBlidingia\u003c/em\u003e spp. and unidentified taxa, the number of \u003cem\u003eUlva\u003c/em\u003e groups was reduced (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Table S4).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThe number of groups (clusters) obtained from the ABGD analysis using the chloroplast (\u003cem\u003etuf\u003c/em\u003eA) marker for \u003cem\u003eUlva\u003c/em\u003e samples from Jeju Island and the southern coasts.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u003cp\u003eSpecies identified by phylogenetic analyses (NJ, ML)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e\u003cp\u003eABGD (based on \u003cem\u003etuf\u003c/em\u003eA)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eX\u0026thinsp;=\u0026thinsp;1.0\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003eX\u0026thinsp;=\u0026thinsp;1.5\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0077\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0129\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0077\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0129\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. australis (=\u0026thinsp;U. pertusa)\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. ohnoi\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. lactuca\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. californica\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. laetevirens\u003c/em\u003e (\u003cem\u003eU. rigida\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. flexuosa\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. arasakii\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. compressa\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLPP sp. 1 (\u003cem\u003eU. procera\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLPP sp. 2 (\u003cem\u003eU. prolifera\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eUnidentified\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal number of groups (clusters)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e15 groups\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e13 groups\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e11 groups\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e11groups\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eTotal number of groups (clusters) of\u003c/b\u003e \u003cb\u003eUlva\u003c/b\u003e \u003cb\u003especimens analyzed in this study\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cb\u003e10 groups\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003e8 groups\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003e6 group\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003e6 group\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eGiven the minimum difference between IP and RP (one group), the number of \u003cem\u003eUlva\u003c/em\u003e species groups based on \u003cem\u003etuf\u003c/em\u003eA was estimated as ten (when \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0077, X\u0026thinsp;=\u0026thinsp;1.0) (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The results also showed that increasing X from 1.0 to 1.5 resulted in fewer species groups across all \u003cem\u003eP\u003c/em\u003e values, indicating that the ABGD results were sensitive to parameter selection. For example, when X\u0026thinsp;=\u0026thinsp;1.0, 15 groups were detected (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0077), but this number decreased to 11 at X\u0026thinsp;=\u0026thinsp;1.5 (under the same \u003cem\u003eP\u003c/em\u003e value). Some species groups, such as \u003cem\u003eU. ohnoi\u003c/em\u003e, \u003cem\u003eU. californica\u003c/em\u003e and \u003cem\u003eU. compressa\u003c/em\u003e were consistently identified across different parameter settings, whereas \u003cem\u003eU. prolifera\u003c/em\u003e and \u003cem\u003eU. procera\u003c/em\u003e showed variable groupings. Overall, although the number of \u003cem\u003eUlva\u003c/em\u003e species groups identified by ABGD was dependent on P and X values, the results were largely congruent with those of the phylogenetic analyses. Ten distinct \u003cem\u003eUlva\u003c/em\u003e species groups were thus recognized based on \u003cem\u003etufA\u003c/em\u003e (P\u0026thinsp;=\u0026thinsp;0.0077, X\u0026thinsp;=\u0026thinsp;1.0; Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Table S4).\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eSpecies identification based on tufA-based phylogenetic analysis\u003c/h3\u003e\n\u003cp\u003eThe \u003cem\u003etuf\u003c/em\u003eA phylogeny showed that in 612 specimens from Jeju Island, 266 individuals (43.46%) were identified as \u003cem\u003eU. ohnoi\u003c/em\u003e, 179 (29.25%) as \u003cem\u003eU. australis\u003c/em\u003e (=\u0026thinsp;\u003cem\u003eU. pertusa\u003c/em\u003e), 40 (6.54%) \u003cem\u003eUlva lactuca\u003c/em\u003e, 31 (5.07%) \u003cem\u003eUlva laetevirens\u003c/em\u003e (=\u0026thinsp;\u003cem\u003eUlva rigida\u003c/em\u003e), 28 (4.58%) belonging to the LPP sp. 1 (\u003cem\u003eUlva procera\u003c/em\u003e), 12 (1.96%) \u003cem\u003eUlva flexuosa\u003c/em\u003e, 7 (1.14%) \u003cem\u003eUlva compressa\u003c/em\u003e, 2 (0.33%) \u003cem\u003eUlva arasakii\u003c/em\u003e, and 8 (1.31%) unidentified. \u003cem\u003eBlidingia\u003c/em\u003e spp. (8 specimens) and \u003cem\u003eGayralia\u003c/em\u003e sp. (1 specimen) were also identified, other than the genus \u003cem\u003eUlva\u003c/em\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBy comparison, for a total of 354 specimens from the southern coasts, 123 individuals (34.75%) were determined as \u003cem\u003eU. australis\u003c/em\u003e, 89 (25.14%) LPP sp. 1 (\u003cem\u003eU. procera\u003c/em\u003e) and 2 (0.56%) LPP sp. 2 (\u003cem\u003eUlva prolifera\u003c/em\u003e), 31 (8.76%) \u003cem\u003eU. ohnoi\u003c/em\u003e, 27 (7.63%) \u003cem\u003eU. laetevirens\u003c/em\u003e, 18 (5.08%) \u003cem\u003eUlva californica\u003c/em\u003e, 17 (4.80%) \u003cem\u003eU. flexuosa\u003c/em\u003e, 7 (1.98%) \u003cem\u003eU. lactuca\u003c/em\u003e, 7 (1.98%) \u003cem\u003eU. compressa\u003c/em\u003e, 4 (1.13%) \u003cem\u003eU. arasakii\u003c/em\u003e, and 28 (7.91%) unidentified. Only a single specimen was identified as \u003cem\u003eBlidingia\u003c/em\u003e sp. (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). The phylogenetic tree encompassing the entire dataset from Jeju Island and the southern coasts is provided in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003e\u003c/h3\u003e\n\u003cdiv class=\"Heading\"\u003e\u003cb\u003eSpatial variation in Ulva community structure between Jeju Island and the southern coasts\u003c/b\u003e\u003c/div\u003e\u003cp\u003eThe relative frequencies, distributional patterns, species composition and diversity of the entire \u003cem\u003eUlva\u003c/em\u003e communities were compared between Jeju Island and southern coasts. By excluding unidentified specimens, 9 \u003cem\u003eUlva\u003c/em\u003e species were identified for Jeju Island, whereas 10 species were found for the southern coasts, based on \u003cem\u003etuf\u003c/em\u003eA phylogenies (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eComparisons of relative frequencies of \u003cem\u003eUlva\u003c/em\u003e species communities between Jeju Island and the southern coasts, based on \u003cem\u003etuf\u003c/em\u003eA-based phylogenetic analysis.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eSpecies\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eJeju Island\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003esouthern coasts\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003esample\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRelative frequency (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003esample\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eRelative frequency (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. australis\u003c/em\u003e (=\u0026thinsp;\u003cem\u003eU. pertusa\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e179\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e29.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e123\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e34.75\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. ohnoi\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e266\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e43.46\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e8.76\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. lactuca\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e6.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.98\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. californica\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5.07\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5.08\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. laetevirens\u003c/em\u003e (\u003cem\u003eU. rigida\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e4.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e7.63\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. flexuosa\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.96\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e4.80\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. arasakii\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.33\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.13\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eU. compressa\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.98\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLPP sp. 1 (\u003cem\u003eU. procera\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e4.58\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e25.14\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLPP sp. 2 (\u003cem\u003eU. prolifera\u003c/em\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.56\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eUnidentified\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e7.91\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eBlidingia\u003c/em\u003e sp.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.31\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.28\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eGayralia\u003c/em\u003e sp.\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e612\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e354\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFor the relative frequencies of \u003cem\u003eUlva\u003c/em\u003e species in Jeju Island, two species, \u003cem\u003eU. ohnoi\u003c/em\u003e (43.46%) and \u003cem\u003eU. australis\u003c/em\u003e (29.25%), were the most predominant across all the geographic and seasonal samples (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Besides, the Jeju Island-\u003cem\u003eUlva\u003c/em\u003e community was composed of \u003cem\u003eU. lactuca\u003c/em\u003e (6.54%), \u003cem\u003eU. californica\u003c/em\u003e (5.07%), \u003cem\u003eU. laetevirens\u003c/em\u003e (\u003cem\u003eU. rigida\u003c/em\u003e) (4.90%), LPP clade species (\u003cem\u003eU. procera\u003c/em\u003e, 4.58%), \u003cem\u003eU. flexuosa\u003c/em\u003e (1.96%), \u003cem\u003eU. compressa\u003c/em\u003e (1.14%) and \u003cem\u003eU. arasakii\u003c/em\u003e (0.33%), in this order (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). In addition to \u003cem\u003eUlva\u003c/em\u003e species, other species belonging to class Ulvophyceae, such as \u003cem\u003eBlidingia\u003c/em\u003e (N\u0026thinsp;=\u0026thinsp;8) and \u003cem\u003eGayralia\u003c/em\u003e (N\u0026thinsp;=\u0026thinsp;1) spp. were also detected. By comparison, on the southern coasts \u003cem\u003eUlva-\u003c/em\u003ecommunity was comprised of \u003cem\u003eU. australis\u003c/em\u003e (34.75%), LPP clade species (\u003cem\u003eU. procera\u003c/em\u003e, 25.14%; \u003cem\u003eU. prolifera\u003c/em\u003e, 0.56%), \u003cem\u003eU. ohnoi\u003c/em\u003e (8.76%), \u003cem\u003eU. lactuca\u003c/em\u003e (1.98%), \u003cem\u003eU. californica\u003c/em\u003e (5.08%), \u003cem\u003eU. laetevirens\u003c/em\u003e (\u003cem\u003eU. rigida\u003c/em\u003e) (7.63%), \u003cem\u003eU. flexuosa\u003c/em\u003e (4.80%), \u003cem\u003eU. compressa\u003c/em\u003e (1.98%) and \u003cem\u003eU. arasakii\u003c/em\u003e (1.13%) in this order (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). In addition to \u003cem\u003eUlva\u003c/em\u003e species, other species belonging to class Ulvophyceae, such as \u003cem\u003eBlidingia\u003c/em\u003e (N\u0026thinsp;=\u0026thinsp;1) were also detected.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBray\u0026ndash;Curtis similarity analysis revealed a significant spatial difference in \u003cem\u003eUlva\u003c/em\u003e community structure between Jeju Island and the southern coasts (ANOSIM, r\u0026thinsp;=\u0026thinsp;0.334, p\u0026thinsp;=\u0026thinsp;0.001). In the nMDS ordination, samples from Jeju Island and the southern coasts clearly formed separate clusters, indicating distinct spatial segregation in \u003cem\u003eUlva\u003c/em\u003e community composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). A few sites from the southern coasts overlapped with the Jeju Island cluster [S7 (Yegye 3), S9 (Sulcheon), and S15 (Baekya)], suggesting partial similarity in species composition between certain locations of the two regions.\u003c/p\u003e\u003cp\u003eDifferences in life forms between \u003cem\u003eU. ohnoi\u003c/em\u003e and \u003cem\u003eU. australis\u003c/em\u003e were also apparent, with \u003cem\u003eU. ohnoi\u003c/em\u003e exhibiting a higher proportion of floating thalli than \u003cem\u003eU. australis\u003c/em\u003e on Jeju Island (Fig. S2). A chi-square analysis revealed a significant difference in the proportions of benthic and floating individuals between the two species (χ\u0026sup2; = 16.51, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). The within-species preference index (PI), calculated as (Floating\u0026thinsp;\u0026minus;\u0026thinsp;Benthic)/(Floating\u0026thinsp;+\u0026thinsp;Benthic), indicated that \u003cem\u003eU. ohnoi\u003c/em\u003e had a strong tendency toward the floating form (PI\u0026thinsp;=\u0026thinsp;0.48), whereas \u003cem\u003eU. australis\u003c/em\u003e displayed nearly equal proportions of the two forms (PI\u0026thinsp;=\u0026thinsp;0.04). When standardized using Ivlev\u0026rsquo;s\u003csup\u003e35\u003c/sup\u003e and Jacobs\u0026rsquo;s\u003csup\u003e36\u003c/sup\u003e electivity indices, \u003cem\u003eU. ohnoi\u003c/em\u003e still showed a positive bias toward the floating form (E\u0026thinsp;=\u0026thinsp;0.06; D\u0026thinsp;=\u0026thinsp;0.20), while \u003cem\u003eU. australis\u003c/em\u003e exhibited a weak preference for the benthic form (E\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;0.12; D\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;0.28).\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e\u003cb\u003eSeasonal variation in Ulva community structure in Jeju Island and the southern coasts\u003c/b\u003e\u003c/h2\u003e\u003cp\u003eSeasonal variation in the \u003cem\u003eUlva\u003c/em\u003e community structure, species composition and relative frequencies was examined for both geographic regions (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). For Jeju Island, a total of 11 species were identified, of which 9 species belonged to the genus \u003cem\u003eUlva\u003c/em\u003e and the other two species were composed of \u003cem\u003eBlidingia\u003c/em\u003e spp. and \u003cem\u003eGayralia\u003c/em\u003e spp. A majority of the sites showed that two species, \u003cem\u003eU. ohnoi\u003c/em\u003e (35\u0026ndash;59.2%) and \u003cem\u003eU. australis\u003c/em\u003e (6.4\u0026ndash;37.7%) were predominant all year round, regardless of the season (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). An average relative frequency of \u003cem\u003eU. ohnoi\u003c/em\u003e and \u003cem\u003eU. australis\u003c/em\u003e were 43.46%, 29.25%, respectively. While \u003cem\u003eU. ohnoi\u003c/em\u003e exhibited a peak in relative frequency (59.2%) in autumn, \u003cem\u003eU. australis\u003c/em\u003e showed the lowest frequency (6.4%) (Table S5). In addition, \u003cem\u003eU. lactuca\u003c/em\u003e increased its frequency in autumn (28%) relative to other seasons (2.9% in summer and zero in winter and spring) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e; Table S5). The other \u003cem\u003eUlva\u003c/em\u003e species also showed some seasonal variation in their frequencies. \u003cem\u003eUlva californica\u003c/em\u003e ranged from 1.6% to 11.18% (the highest frequency in spring), \u003cem\u003eU. laetevirens\u003c/em\u003e 2.5%\u0026ndash;9.87% (the highest in spring), \u003cem\u003eUlva procera\u003c/em\u003e (LPP sp. 1) 0.57%\u0026ndash; 12.57% (the highest in winter), and \u003cem\u003eUlva flexuosa\u003c/em\u003e 0.80%\u0026ndash;3.75% (the highest in winter). \u003cem\u003eUlva arasakii\u003c/em\u003e was only detected in spring, constituting 1.32% of the samples. \u003cem\u003eUlva compressa\u003c/em\u003e appeared only in summer and winter, with a relative frequency ranging from 1.71% to 2.5% (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e; Table S5).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSimilar to Jeju Island, on the southern coasts \u003cem\u003eUlva\u003c/em\u003e community structure also changed according to the season. \u003cem\u003eUlva australis\u003c/em\u003e (15.52\u0026ndash;45.05%) and \u003cem\u003eU. procera\u003c/em\u003e (LPP sp. 1) (7.69\u0026ndash;43.02%) were the most dominant species, although \u003cem\u003eU. procera\u003c/em\u003e (LPP sp. 1) was not present at all in autumn. \u003cem\u003eUlva ohnoi\u003c/em\u003e was low in its frequency (1.10\u0026ndash;3.36%) except for during autumn with the highest peak (43.1%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e; Table S6). \u003cem\u003eUlva laetevirens\u003c/em\u003e indicated relatively stable frequencies across seasons (4.4%\u0026ndash;12.61%) and \u003cem\u003eU. flexuosa\u003c/em\u003e showed a higher frequency in summer (13.19%) compared to the other seasons. \u003cem\u003eUlva arasakii\u003c/em\u003e exhibited a relative frequency of 3.49% during winter, which decreased to 0.84% in spring. \u003cem\u003eUlva compressa\u003c/em\u003e only occurred in spring (5.88%). \u003cem\u003eUlva prolifera\u003c/em\u003e (LPP sp. 2) was present (1.68%) only in spring, although this species was not detected at all in Jeju Island (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e; Table S6). These findings suggest that there was considerable seasonal variation in the species composition, distribution and relative frequencies between the two geographic regions.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003eThe 5S rDNA analysis for Ulva species belonging to the LPP clade\u003c/h2\u003e\u003cp\u003eWe found that \u003cem\u003eU. procera\u003c/em\u003e formed an independent clade as \u003cem\u003eU. linza\u003c/em\u003e (\u003cem\u003eN\u003c/em\u003e\u0026thinsp;=\u0026thinsp;90), while the remaining \u003cem\u003eU. procera\u003c/em\u003e specimens grouped within the \u003cem\u003eU. prolifera\u003c/em\u003e clade (\u003cem\u003eN\u003c/em\u003e\u0026thinsp;=\u0026thinsp;13), along with two unidentified samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Within the \u003cem\u003eU. prolifera\u003c/em\u003e group, two individuals identified by the \u003cem\u003etuf\u003c/em\u003eA marker were classified into \u003cem\u003eU. linza\u003c/em\u003e and \u003cem\u003eU. prolifera\u003c/em\u003e clades, respectively. Samples initially identified as \u003cem\u003eU. procera\u003c/em\u003e by \u003cem\u003etuf\u003c/em\u003eA were re-classified into \u003cem\u003eU. linza\u003c/em\u003e and \u003cem\u003eU. prolifera\u003c/em\u003e groups, except for two individuals (U3453, U3895) that did not cluster with either group (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Among the 25 samples from Jeju Island, 23 were identified as \u003cem\u003eU. linza\u003c/em\u003e and 2 as \u003cem\u003eU. prolifera\u003c/em\u003e. On the southern coasts, 67 individuals were identified as \u003cem\u003eU. linza\u003c/em\u003e, 11 as \u003cem\u003eU. prolifera\u003c/em\u003e, and 2 remained unidentified (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). All southern coast sites contained members of the LPP clade except for Baekya in Yeosu (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Table S2). In contrast, on Jeju Island, the LPP clade-species were detected only at four sites, including the Jongdal, inside Hanlim harbor, Hyeopjae, and inside Sinchang 1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e; Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eBased on the exhaustive phylogenetic analysis on the spatial and seasonal samples of a total of 966 \u003cem\u003eUlva\u003c/em\u003e specimens from 46 sites along the coastlines of Jeju Island and the southern coasts of Korea, we find considerable differences in the species composition, diversity, spatial distribution and relative frequencies of the green tide forming \u003cem\u003eUlva\u003c/em\u003e communities between the two regions. In Jeju Island, the most significantly contributing species to local green tides turned out to be \u003cem\u003eU. ohnoi\u003c/em\u003e and \u003cem\u003eU. australis\u003c/em\u003e, which is consistent with the previous findings\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. The dominance of these species in Jeju Island can be partially attributed to relatively higher sea surface temperature (SST; 14.76\u0026ndash;15.83\u0026deg;C) in this region (Fig. S3), which might provide favorable growth conditions for subtropical species like \u003cem\u003eU. ohnoi\u003c/em\u003e. On the other hand, in the southern coasts of the mainland, \u003cem\u003eU. australis\u003c/em\u003e and LPP clade species (\u003cem\u003eU. linza\u003c/em\u003e) are predominant in green tide forming \u003cem\u003eUlva\u003c/em\u003e communities throughout the year. The higher concentrations of nitrogen compounds and chemical oxygen demand (COD) in the southern coasts create a nutrient-rich environment with a high nitrogen availability that supports the growth of these species (Fig. S3), although the green tide algal biomass is much lower relative to Jeju Island\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Although the ecological implications of these differences in environmental parameters remain unclear, these findings warrant further investigation to clarify relationships between environmental differences and \u003cem\u003eUlva\u003c/em\u003e community composition, species distribution and also magnitude of green tides.\u003c/p\u003e\u003cp\u003e\u003cem\u003eUlva ohnoi\u003c/em\u003e, the most common species in Jeju Island, is known to have a subtropical or tropical origin\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. It was first identified and taxonomically classified in 2004 in Japan\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e and is considered one of the major species responsible for green tides in several regions including Japan\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. Moreover, it has been reported as a widely distributed species in various areas, including South Africa\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Because molecular studies of \u003cem\u003eUlva\u003c/em\u003e species in Korean waters \u0026ndash; particularly around Jeju Island \u0026ndash; remain limited, it is unclear whether \u003cem\u003eU. ohnoi\u003c/em\u003e observed along the Korean coasts in this study represents a native population or was introduced from Japan or other subtropical regions. Additional analyses using existing sequence datasets from GenBank database could help to determine whether this species was introduced and, if so, infer its potential origin. To our knowledge, this study provides the first report of \u003cem\u003eU. ohnoi\u003c/em\u003e occurrence along the southern coasts of the Korean Peninsula. Nevertheless, when and how this subtropical or tropical \u003cem\u003eU. ohnoi\u003c/em\u003e became established on the southern coasts remain to be elucidated.\u003c/p\u003e\u003cp\u003eWe also find differences in species composition, distribution and relative frequencies in both coastal regions according to the season. For Jeju Island, the primary contributors to the year-round local green tides were \u003cem\u003eU. ohnoi\u003c/em\u003e and \u003cem\u003eU. australis\u003c/em\u003e, which tend to dominate the \u003cem\u003eUlva\u003c/em\u003e community regardless of the season, except \u003cem\u003eU. australis\u003c/em\u003e was replaced with \u003cem\u003eU. lactuca\u003c/em\u003e in autumn. The two species, \u003cem\u003eU. ohnoi\u003c/em\u003e and \u003cem\u003eU. australis\u003c/em\u003e, are known to be remarkably tolerant to high temperatures\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e, which may contribute to sustaining large amount of green macroalgal biomass even in summer/autumn. On the other hand, for the southern coasts, \u003cem\u003eU. ohnoi\u003c/em\u003e shows a rapid increase in frequency only during autumn. It has been reported that \u003cem\u003eU. australis\u003c/em\u003e shows a high growth rate in spring and early summer but tends to decline sharply in August\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. It would therefore be conceivable that the relative frequency of \u003cem\u003eU. ohnoi\u003c/em\u003e increased rapidly in autumn due to the decrease in \u003cem\u003eU. australis\u003c/em\u003e. Still, \u003cem\u003eU. ohnoi\u003c/em\u003e was rarely seen in other seasons (but spring), and \u003cem\u003eU. linza\u003c/em\u003e (LPP sp. 1) species occurred in every season except for autumn.\u003c/p\u003e\u003cp\u003eThe dominance of \u003cem\u003eU. linza\u003c/em\u003e on the southern coasts may be attributed to several factors. This species is well-known for its physiological adaptability and tolerance to varying environmental conditions, which may confer a competitive advantage in this region\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. Furthermore, \u003cem\u003eU. linza\u003c/em\u003e has been reported to withstand fluctuations in temperature and salinity\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, likely contributing to its year-round presence along the southern coasts.\u003c/p\u003e\u003cp\u003e\u003cem\u003eUlva australis\u003c/em\u003e is a widespread intertidal species that occurs along the entire coastline of Japan\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. It is known to cause green tide outbreaks in temperate regions of southern and western Japan, along the Pacific coast of central Japan\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. This species also maintains relatively high biomass during winter\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e and forms extensive green tides in various parts of the world, including the northwestern Atlantic coast of the Iberian Peninsula (Galicia, Spain)\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e and the northern coasts of China\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. According to Hanyuda and Kawai\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e, \u003cem\u003eUlva pertusa\u003c/em\u003e Areschoug, 1851 and \u003cem\u003eUlva australis\u003c/em\u003e Kjellman, 1897 were suggested as synonym, which was supported by our previous phylogenetic analyses demonstrating the monophyly of these taxa\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. This species is among the most common causes of green tides in Korea and Japan, with its dominance particularly well documented in Jeju Island from 2015 to 2020\u003csup\u003e14\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eA recent study suggests that \u003cem\u003eU. ohnoi\u003c/em\u003e has the potential to undergo explosive growth in high-temperature marine ecosystems driven by ocean warming and acidification\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. This subtropical to tropical species exhibits a high growth rate even under elevated summer water temperatures\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. As one of the most abundant and widely distributed \u003cem\u003eUlva\u003c/em\u003e species, \u003cem\u003eU. ohnoi\u003c/em\u003e is expected to cause green tides more frequently and severely under warmer environmental conditions\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. Temperature plays a crucial role in promoting photosynthesis and growth of \u003cem\u003eU. ohnoi\u003c/em\u003e, as higher temperatures enhance the growth of both young and adult thalli by increasing metabolic activity, particularly during summer. According to Korea Oceanographic Data Center from the National Institute of Fisheries Science (NIFS, Korea; \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.nifs.go.kr/kodc/soo_list.kodc\u003c/span\u003e\u003cspan address=\"https://www.nifs.go.kr/kodc/soo_list.kodc\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) in Korea, a rise in sea surface temperature (SST) was evident for the last five years (from 2015 to 2020 in April for Jeju Island). In 2015, the average temperature across the four stations was 14.70\u0026deg;C (\u0026plusmn;\u0026thinsp;1.42\u0026deg;C SD). In 2020, the average temperature increased to 15.61\u0026deg;C (\u0026plusmn;\u0026thinsp;1.51\u0026deg;C SD). These values clearly demonstrate a noticeable increase in sea surface temperature from 2015 to 2020, with all stations showing an upward trend. This temperature increase is noteworthy as it creates favorable growth conditions for \u003cem\u003eU. ohnoi\u003c/em\u003e\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003cem\u003eU. ohnoi\u003c/em\u003e primarily grows attached to rocky substrates, but detached floating fronds can also proliferate vegetatively by rapidly absorbing nutrients. The observed high proportions of floating-thalli form of \u003cem\u003eU. ohnoi\u003c/em\u003e (Fig. S2) can provide evidence supporting this hypothesis. This trait contributes to frequent green tide outbreaks in coastal areas\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. In Japan, \u003cem\u003eU. ohnoi\u003c/em\u003e is widely distributed on the southwest coast, where the sea temperature is warm, whereas \u003cem\u003eU. australis\u003c/em\u003e is predominant on the northeast coast\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eNonindigenous species of origins of Europe include \u003cem\u003eU. procera\u003c/em\u003e (LPP sp1.; \u003cem\u003eU.linza\u003c/em\u003e), \u003cem\u003eU. flexuosa\u003c/em\u003e and \u003cem\u003eU. californica\u003c/em\u003e. They are known to be introduced through various maritime transport vehicles\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. \u003cem\u003eUlva californica\u003c/em\u003e, a species native to the Pacific Coast of North America, has recently been reported to be introduced to Europe, including Ireland and the United Kingdom, the Mediterranean, Oceania, and also Asia by hull for maritime transportation\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e,\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e. This species was first recorded from California (type locality: La Jolla, Collins et al., 1899: no. 611) and has never been reported from the Mediterranean until 2012\u003csup\u003e52\u003c/sup\u003e. It has physiological and ecological capabilities that enable to survive and grow even in harsh environmental conditions such as a lack of light for more than 10 months\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. \u003cem\u003eUlva californica\u003c/em\u003e was shown to cause green tide events in Chile and California, USA\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. Notably, although \u003cem\u003eU. californica\u003c/em\u003e was present in other areas, a particular site (Population ID 29) shows only \u003cem\u003eU. californica\u003c/em\u003e. This finding highlights the potential role of maritime transport and harbor activities as pathways for introduction of non-native species like \u003cem\u003eU. californica\u003c/em\u003e. Such unique occurrences underscore the importance of monitoring harbor environments where nonindigenous species may establish and proliferate after successful colonization. In addition, \u003cem\u003eU. linza\u003c/em\u003e (LPP sp. 1) and \u003cem\u003eU. flexuosa\u003c/em\u003e are often reported as the potentially problematic species causing macroalgal blooms, and there is also a possibility of causing green tides in Korea in the future\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e,\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e,\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. Fortunately, nonindigenous and introduced \u003cem\u003eUlva\u003c/em\u003e species of origins of Europe or United States, such as \u003cem\u003eU. californica\u003c/em\u003e and \u003cem\u003eU. laetevirens\u003c/em\u003e (=\u0026thinsp;\u003cem\u003eU. rigida\u003c/em\u003e) seem not significantly contribute to the current occurrences of local green tides on the Korean coast. Nevertheless, continuous observation and monitoring are required, as these species have been identified as major contributors to green tide events in various parts of the world\u003csup\u003e\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003cem\u003eUlva laetevirens\u003c/em\u003e (\u003cem\u003e=\u0026thinsp;U. rigida\u003c/em\u003e) was first collected in Australia in 1854 and has been reported in several Mediterranean countries since the late 1990s\u003csup\u003e\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e. This species exhibits an optimal growth rate at temperatures between 12\u0026deg;C and 23\u0026deg;C\u003csup\u003e57\u003c/sup\u003e. During summer, its growth declines when temperatures exceed these optimal values\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e as ambient temperature is a critical factor affecting its development and growth. Consequently, the relative frequency of \u003cem\u003eU. laetevirens\u003c/em\u003e on the southern coasts remained relatively stable or even slightly higher during mild temperature periods such as spring and autumn, compared with summer and winter.\u003c/p\u003e\u003cp\u003e\u003cem\u003eUlva prolifera\u003c/em\u003e, one of the three species (\u003cem\u003eU. linza\u003c/em\u003e and \u003cem\u003eU. procera\u003c/em\u003e being the other) belonging to the LPP clade, is notorious for causing massive macroalgae blooms, green tide events in the Yellow Sea of China\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e. While \u003cem\u003eU. linza\u003c/em\u003e and \u003cem\u003eU. compressa\u003c/em\u003e are widely distributed in the Yellow Sea, \u003cem\u003eU. prolifera\u003c/em\u003e occurs predominantly along the east coast of China\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e. In the present study, both \u003cem\u003eU. linza\u003c/em\u003e (N\u0026thinsp;=\u0026thinsp;67; 83.75%) and \u003cem\u003eU. prolifera\u003c/em\u003e (N\u0026thinsp;=\u0026thinsp;11; 13.75%) were identified in the southern coast region based on 5S rDNA analysis. On Jeju Island, however, only \u003cem\u003eU. linza\u003c/em\u003e (N\u0026thinsp;=\u0026thinsp;23; 92%) and \u003cem\u003eU. prolifera\u003c/em\u003e (N\u0026thinsp;=\u0026thinsp;2; 8%) were detected. Additionally, \u003cem\u003eU. compressa\u003c/em\u003e was recorded in both Jeju Island and the southern coasts. These findings suggest that \u003cem\u003eU. prolifera\u003c/em\u003e observed along the southern coasts of Korea may have been introduced from the east coast of China\u003csup\u003e\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e. Further research is required to verify this hypothesis and to determine whether \u003cem\u003eU. prolifera\u003c/em\u003e populations in Korean waters originated through recent introductions or natural dispersal\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eBased on 5S rDNA analysis, \u003cem\u003eU. linza\u003c/em\u003e occurs in both Jeju Island and the southern coasts. On Jeju Island, it occurred at four of 31 sites (12.90% of all sites), with relative abundances ranging from 3.23% (inside Hallim harbor and inside Sinchang 1) to 58.06% (Jongdal). On the southern coasts, \u003cem\u003eU. linza\u003c/em\u003e was present at 14 of 15 sites, ranging from 1.49% (Sagok) to 14.92% (Deokho). Moreover, species belonging to the LPP clade were widely distributed along the entire southern coastal region, whereas their occurrence on Jeju Island was limited to four sites: inside Hallim Harbor, inside Sinchang 1, Jongdal, and Hyeopjae. These findings provide important insights into the species composition and distributional patterns of \u003cem\u003eUlva\u003c/em\u003e communities in the studied coastal regions.\u003c/p\u003e\u003cp\u003eFor the LPP clade, previous studies have shown that neither \u003cem\u003etuf\u003c/em\u003eA nor ITS markers can reliably differentiate among the three species (\u003cem\u003eUlva linza-procera-prolifera\u003c/em\u003e)\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Therefore, in this study, the 5S rDNA marker was analyzed to achieve more accurate species identification within the LPP clade\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. The analysis targeted the specimens classified as members of the LPP clade based on \u003cem\u003etuf\u003c/em\u003eA-derived phylogenetic results. The 5S rDNA spacer region, which has been demonstrated to resolve interspecific relationships within \u003cem\u003eUlva\u003c/em\u003e was used here for unambiguous species identification within the LPP clade\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e,\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e. Further research, including specimens from the type locality of \u003cem\u003eU. prolifera\u003c/em\u003e, is required to clarify its taxonomic status\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. The present findings suggest phylogenetic divergence within the taxa previously designated as \u003cem\u003eU. procera\u003c/em\u003e, indicating the existence of two distinct groups (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). A comprehensive follow-up investigation, particularly detailed taxonomic reevaluation, will therefore be essential to avoid species misidentification.\u003c/p\u003e\u003cp\u003eThe results of this study suggest that \u003cem\u003eUlva\u003c/em\u003e species diversity in Korean waters may be higher than previously recognized\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. Our genetic analyses revealed a greater extent of hidden diversity with \u003cem\u003eUlva\u003c/em\u003e communities in Jeju Island and the southern coasts of Korea, indicating that the actual number of species has likely been underestimated. In particular, specimens previously identified as \u003cem\u003eU. procera\u003c/em\u003e was re-identified as \u003cem\u003eU. prolifera\u003c/em\u003e and \u003cem\u003eU. linza\u003c/em\u003e based on 5S rDNA analysis, highlighting that the limited resolution of certain markers may lead to an underestimation of \u003cem\u003eUlva\u003c/em\u003e species diversity. For example, although \u003cem\u003eU. prolifera\u003c/em\u003e and \u003cem\u003eU. linza\u003c/em\u003e are genetically distinct, earlier morphological studies might have misidentified one as the other due to phenotypic plasticity and overlapping morphological features. These findings emphasize the importance of accurate species identification using molecular approaches to avoid taxonomic confusion and improve our understanding of \u003cem\u003eUlva\u003c/em\u003e biodiversity. Identifying the dominant \u003cem\u003eUlva\u003c/em\u003e species responsible for local green tide events and elucidating their physiological and ecological characteristics are critical steps toward developing effective management strategies to reduce bloom occurrences. In this study, we applied model-based species delimitation in conjunction with molecular phylogenetic analyses to enhance the precision of species identification. The resulting molecular data provide a valuable baseline for understanding the genetic structure and community composition of green tide\u0026ndash;forming \u003cem\u003eUlva\u003c/em\u003e species along the coasts of Jeju Island and the southern coasts.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study provides an in-depth assessment of spatial and seasonal variations in \u003cem\u003eUlva\u003c/em\u003e species composition and diversity along the coasts of Jeju Island and the southern coasts of Korea, highlighting their contributions to green tide formation. Phylogenetic analyses using \u003cem\u003etufA\u003c/em\u003e and 5S rDNA markers revealed distinct \u003cem\u003eUlva\u003c/em\u003e community structures between the two regions. \u003cem\u003eU. ohnoi\u003c/em\u003e and \u003cem\u003eU. australis\u0026nbsp;\u003c/em\u003epredominated on Jeju Island, whereas \u003cem\u003eU. linza\u003c/em\u003e and \u003cem\u003eU. australis\u0026nbsp;\u003c/em\u003ewere dominant along the southern coasts. These patterns suggest that environmental factors\u0026mdash;such as temperature, nutrient availability, and species-specific ecological traits\u0026mdash;drive regional and seasonal differences in \u003cem\u003eUlva\u003c/em\u003e communities. The dominance of \u003cem\u003eU. ohnoi\u003c/em\u003e on Jeju Island highlights its potential to expand under warming conditions, increasing the risk of more frequent and severe green tides. Likewise, the prevalence of \u003cem\u003eU. linza\u003c/em\u003e on the southern coasts indicates its resilience to environmental fluctuations. The detection of nonindigenous species, including \u003cem\u003eU. californica\u003c/em\u003e, \u003cem\u003eU. procera\u003c/em\u003e (LPP sp. 1; \u003cem\u003eU. linza\u003c/em\u003e), and \u003cem\u003eU. flexuosa\u003c/em\u003e, underscores the need for continued genetic monitoring of \u003cem\u003eUlva\u003c/em\u003e populations. Our findings provide essential baseline data for developing targeted management strategies to mitigate the ecological and economic impacts of green tides in Korean coastal waters. Future research should investigate the ecological and physiological characteristics of dominant \u003cem\u003eUlva\u003c/em\u003e species and their responses to changing environmental conditions, and also their interactions with other coexisting species within the ecosystem.\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eHye Jin Park\u003c/strong\u003e: Writing \u0026ndash; original draft, Visualization, Methodology, Formal analysis, Investigation, Data curation. \u003cstrong\u003eSeo Yeon Byeon\u003c/strong\u003e: Writing \u0026ndash; original draft, Formal analysis, Methodology, Investigation.\u003cstrong\u003e\u0026nbsp;Sang Rul Park\u003c/strong\u003e: Conceptualization, Methodology, Writing \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eYoung Baek Son\u003c/strong\u003e: Visualization, Writing \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eJi Hyoun Kang\u003c/strong\u003e: Formal analysis, Conceptualization, Methodology, Writing \u0026ndash; review \u0026amp; editing. \u003cstrong\u003eHyuk Je Lee\u003c/strong\u003e: Formal analysis, Conceptualization, Supervision, Methodology, Writing \u0026ndash; review \u0026amp; editing, Project administration.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Competing Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe would like to thank members of Estuarine and Coastal Ecology Laboratory in Department of Marine Life Sciences at Jeju National University and the Molecular Ecology and Evolution Laboratory in Department of Biological Sciences at Sangji University, for their assistance in field sample collection for this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by Korea Institute of Marine Science and Technology Promotion (KIMST) funded to the Ministry of Oceans and Fisheries, Korea (RS-2025-02304432; RS-2025-02304428). This research was also supported by the Regional Innovation System \u0026amp; Education (RISE) program through the Gangwon RISE Center, funded by the Ministry of Education (MOE) and the Gangwon State (G.S.), Republic of Korea (2025-RISE-10-005).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary material associated with this article can be found, in the online version, at doi: xxxx\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eYe, N. et al. Green tides\u0026rsquo; are overwhelming the coastline of our blue planet: taking the world\u0026rsquo;s largest example. \u003cem\u003eEcol. Res.\u003c/em\u003e \u003cb\u003e26\u003c/b\u003e, 477\u0026ndash;485. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s11284-011-0821-8\u003c/span\u003e\u003cspan address=\"10.1007/s11284-011-0821-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2011).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSmetacek, V. \u0026amp; Zingone, A. Green and golden seaweed tides on the rise. \u003cem\u003eNature\u003c/em\u003e \u003cb\u003e504\u003c/b\u003e, 84\u0026ndash;88. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1038/nature12860\u003c/span\u003e\u003cspan address=\"10.1038/nature12860\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2013).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXiao, Y., Liu, R., Kim, K., Zhang, J. \u0026amp; Cui, T. A random forest-based algorithm to distinguish \u003cem\u003eUlva prolifera\u003c/em\u003e and Sargassum from multispectral satellite images. \u003cem\u003eIEEE Trans. Geosci. Remote Sens.\u003c/em\u003e \u003cb\u003e60\u003c/b\u003e, 1\u0026ndash;15. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1109/TGRS.2021.3071154\u003c/span\u003e\u003cspan address=\"10.1109/TGRS.2021.3071154\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2021).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRen, C. G. et al. The ever-lasting green tides: What can we do? \u003cem\u003eHeliyon\u003c/em\u003e 10, (2024). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.heliyon.2024.e25220\u003c/span\u003e\u003cspan address=\"10.1016/j.heliyon.2024.e25220\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChoi, S. K. et al. Population dynamics of the \u0026lsquo;golden tides\u0026rsquo; seaweed, \u003cem\u003eSargassum horneri\u003c/em\u003e, on the southwestern coast of Korea: The extent and formation of golden tides. \u003cem\u003eSustainability\u003c/em\u003e \u003cb\u003e12\u003c/b\u003e, 2903. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/su12072903\u003c/span\u003e\u003cspan address=\"10.3390/su12072903\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2020).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eChoi, S. K. et al. Changes in the dynamics and nutrient budget of a macroalgal community exposed to land-based fish farm discharge off Jeju Island, Korea. \u003cem\u003eSustainability\u003c/em\u003e \u003cb\u003e13\u003c/b\u003e, 11793. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/su132111793\u003c/span\u003e\u003cspan address=\"10.3390/su132111793\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2021).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWang, H., Wang, G. \u0026amp; Gu, W. Macroalgal blooms caused by marine nutrient changes resulting from human activities. \u003cem\u003eJ. Appl. Ecol.\u003c/em\u003e \u003cb\u003e57\u003c/b\u003e, 766\u0026ndash;776. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/1365-2664.13587\u003c/span\u003e\u003cspan address=\"10.1111/1365-2664.13587\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2020).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eByeon, S. Y. et al. The origin and population genetic structure of the \u0026lsquo;golden tide\u0026rsquo;seaweeds, \u003cem\u003eSargassum horneri\u003c/em\u003e, in Korean waters. \u003cem\u003eSci. Rep.\u003c/em\u003e \u003cb\u003e9\u003c/b\u003e, 7757. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1038/s41598-019-44170-x\u003c/span\u003e\u003cspan address=\"10.1038/s41598-019-44170-x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eByeon, S. Y. et al. Comparative analysis of sequence polymorphism in complete organelle genomes of the \u0026lsquo;Golden Tide\u0026rsquo;seaweed \u003cem\u003eSargassum horneri\u003c/em\u003e between Korean and Chinese forms. \u003cem\u003eSustainability\u003c/em\u003e \u003cb\u003e12\u003c/b\u003e, 7280. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/su12187280\u003c/span\u003e\u003cspan address=\"10.3390/su12187280\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2020).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGladyshev, M. \u0026amp; Gubelit, Y. I. Green tides: new consequences of the eutrophication of natural waters (invited review). \u003cem\u003eContemp. Probl. Ecol.\u003c/em\u003e \u003cb\u003e12\u003c/b\u003e, 109\u0026ndash;125. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1134/S1995425519020057\u003c/span\u003e\u003cspan address=\"10.1134/S1995425519020057\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHayden, H. S. et al. Linnaeus was right all along: \u003cem\u003eUlva\u003c/em\u003e and \u003cem\u003eEnteromorpha\u003c/em\u003e are not distinct genera. \u003cem\u003eEur. J. Phycol.\u003c/em\u003e \u003cb\u003e38\u003c/b\u003e, 277\u0026ndash;294. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1080/1364253031000136321\u003c/span\u003e\u003cspan address=\"10.1080/1364253031000136321\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2003).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLoughnane, C. J., McIvor, L. M., Rindi, F., Stengel, D. B. \u0026amp; Guiry, M. D. Morphology, \u003cem\u003erbc\u003c/em\u003eL phylogeny and distribution of distromatic \u003cem\u003eUlva\u003c/em\u003e (Ulvophyceae, Chlorophyta) in Ireland and southern Britain. \u003cem\u003ePhycologia\u003c/em\u003e \u003cb\u003e47\u003c/b\u003e, 416\u0026ndash;429. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2216/PH07-61.1\u003c/span\u003e\u003cspan address=\"10.2216/PH07-61.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2008).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHofmann, L. C., Nettleton, J. C., Neefus, C. D. \u0026amp; Mathieson, A. C. Cryptic diversity of \u003cem\u003eUlva\u003c/em\u003e (Ulvales, Chlorophyta) in the Great Bay Estuarine System (Atlantic USA): introduced and indigenous distromatic species. \u003cem\u003eEur. J. Phycol.\u003c/em\u003e \u003cb\u003e45\u003c/b\u003e, 230\u0026ndash;239. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1080/09670261003746201\u003c/span\u003e\u003cspan address=\"10.1080/09670261003746201\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2010).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePark, H. J., Byeon, S. Y., Park, S. R. \u0026amp; Lee, H. J. Temporal variation in the community structure of green tide forming macroalgae (Chlorophyta; genus \u003cem\u003eUlva\u003c/em\u003e) on the coast of Jeju Island, Korea based on DNA barcoding. \u003cem\u003eKorean J. Environ. Biology\u003c/em\u003e. \u003cb\u003e40\u003c/b\u003e, 464\u0026ndash;476. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.11626/KJEB.2022.40.4.464\u003c/span\u003e\u003cspan address=\"10.11626/KJEB.2022.40.4.464\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKang, J. H. et al. Species composition, diversity, and distribution of the genus \u003cem\u003eUlva\u003c/em\u003e along the coast of Jeju Island, Korea based on molecular phylogenetic analysis. \u003cem\u003ePLoS One\u003c/em\u003e. \u003cb\u003e14\u003c/b\u003e, e0219958. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1371/journal.pone.0219958\u003c/span\u003e\u003cspan address=\"10.1371/journal.pone.0219958\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFort, A. et al. Exhaustive reanalysis of barcode sequences from public repositories highlights ongoing misidentifications and impacts taxa diversity and distribution. \u003cem\u003eMol. Ecol. Resour.\u003c/em\u003e \u003cb\u003e22\u003c/b\u003e, 86\u0026ndash;101. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/1755-0998.13453\u003c/span\u003e\u003cspan address=\"10.1111/1755-0998.13453\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBlomster, J., Maggs, C. A., Stanhope, M. J. \u0026amp; Molecular and morphological analysis of \u003cem\u003eEnteromorpha intestinalis\u003c/em\u003e and \u003cem\u003eE. compressa\u003c/em\u003e (Chlorophyta) in the British Isles. \u003cem\u003eJ. Phycol.\u003c/em\u003e 34, 319\u0026ndash;340, (1998). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1046/j.1529-8817.1998.340319.x\u003c/span\u003e\u003cspan address=\"10.1046/j.1529-8817.1998.340319.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHayden, H. S. \u0026amp; Waaland, J. R. A molecular systematic study of \u003cem\u003eUlva\u003c/em\u003e (Ulvaceae, Ulvales) from the northeast Pacific. \u003cem\u003ePhycologia\u003c/em\u003e \u003cb\u003e43\u003c/b\u003e, 364\u0026ndash;382. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2216/i0031-8884-43-4-364.1\u003c/span\u003e\u003cspan address=\"10.2216/i0031-8884-43-4-364.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2004).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSaunders, G. W. \u0026amp; Kucera, H. An evaluation of \u003cem\u003erbc\u003c/em\u003eL, \u003cem\u003etuf\u003c/em\u003eA, UPA, LSU and ITS as DNA barcode markers for the marine green macroalgae. \u003cem\u003eCryptogamie Algologie\u003c/em\u003e. \u003cb\u003e31\u003c/b\u003e, 487\u0026ndash;528 (2010).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShimada, S., Yokoyama, N., Arai, S. \u0026amp; Hiraoka, M. Phylogeography of the genus \u003cem\u003eUlva\u003c/em\u003e (Ulvophyceae, Chlorophyta), with special reference to the Japanese freshwater and brackish taxa. \u003cem\u003eJ. Appl. Phycol.\u003c/em\u003e \u003cb\u003e20\u003c/b\u003e, 979\u0026ndash;989. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s10811-007-9296-y\u003c/span\u003e\u003cspan address=\"10.1007/s10811-007-9296-y\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2008).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZhang, X. et al. Settlement of vegetative fragments of \u003cem\u003eUlva prolifera\u003c/em\u003e confirmed as an important seed source for succession of a large-scale green tide bloom. \u003cem\u003eLimnol. Oceanogr.\u003c/em\u003e \u003cb\u003e56\u003c/b\u003e, 233\u0026ndash;242. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.4319/lo.2011.56.1.0233\u003c/span\u003e\u003cspan address=\"10.4319/lo.2011.56.1.0233\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2011).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKawashima, Y., Akasaki, T., Matsumoto, Y., Yamazaki, Y. \u0026amp; Shimada, S. Species identification of imported and Japanese commercial green algal products based on phylogenetic analyses using the nrITS2 and 5S rDNA spacer regions. \u003cem\u003eFish. Sci.\u003c/em\u003e \u003cb\u003e79\u003c/b\u003e, 521\u0026ndash;529. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s12562-013-0610-0\u003c/span\u003e\u003cspan address=\"10.1007/s12562-013-0610-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2013).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCui, J. et al. Taxonomic reassessment of \u003cem\u003eUlva prolifera\u003c/em\u003e (Ulvophyceae, Chlorophyta) based on specimens from the type locality and Yellow Sea green tides. \u003cem\u003ePhycologia\u003c/em\u003e \u003cb\u003e57\u003c/b\u003e, 692\u0026ndash;704. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2216/17-139.1\u003c/span\u003e\u003cspan address=\"10.2216/17-139.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2018).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXia, Z. et al. Distribution of \u003cem\u003eUlva prolifera\u003c/em\u003e, the dominant species in green tides along the Jiangsu Province coast in the Southern Yellow Sea, China. \u003cem\u003eJ. Sea Res.\u003c/em\u003e \u003cb\u003e196\u003c/b\u003e, 102436. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.seares.2023.102436\u003c/span\u003e\u003cspan address=\"10.1016/j.seares.2023.102436\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePark, J., Shin, S., Chae, J., Lee, H. \u0026amp; Mo, Y. Development of the Methods for Controlling and Managing the Marine Ecosystem Disturbing and Harmful Organisms. \u003cem\u003eMarine Ecology Division, Ministry of Oceans and Fisheries. Sejong, Korea\u003c/em\u003e (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePark, S. R. Seasonal patterns and recruitment dynamics of green tide-forming \u003cem\u003eUlva\u003c/em\u003e species along the intertidal rocky shores of the southern coast of Korea. \u003cem\u003eOcean. Sci. J.\u003c/em\u003e \u003cb\u003e49\u003c/b\u003e, 383\u0026ndash;390. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s12601-014-0035-4\u003c/span\u003e\u003cspan address=\"10.1007/s12601-014-0035-4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2014).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFam\u0026agrave;, P., Wysor, B., Kooistra, W. H. \u0026amp; Zuccarello, G. C. Molecular phylogeny of the genus \u003cem\u003eCaulerpa\u003c/em\u003e (Caulerpales, Chlorophyta) inferred from chloroplast \u003cem\u003etuf\u003c/em\u003eA gene1. \u003cem\u003eJ. Phycol.\u003c/em\u003e \u003cb\u003e38\u003c/b\u003e, 1040\u0026ndash;1050. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1046/j.1529-8817.2002.t01-1-01237.x\u003c/span\u003e\u003cspan address=\"10.1046/j.1529-8817.2002.t01-1-01237.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2002).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKearse, M. et al. Geneious Basic: an integrated and extendable desktop software platform for the organization and analysis of sequence data. \u003cem\u003eBioinformatics\u003c/em\u003e \u003cb\u003e28\u003c/b\u003e, 1647\u0026ndash;1649. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/bioinformatics/bts199\u003c/span\u003e\u003cspan address=\"10.1093/bioinformatics/bts199\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2012).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSievers, F. et al. Fast, scalable generation of high-quality protein multiple sequence alignments using Clustal Omega. \u003cem\u003eMol. Syst. Biol.\u003c/em\u003e \u003cb\u003e7\u003c/b\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1038/msb.2011.75\u003c/span\u003e\u003cspan address=\"10.1038/msb.2011.75\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2011).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDarriba, D., Taboada, G. L., Doallo, R. \u0026amp; Posada, D. jModelTest 2: more models, new heuristics and parallel computing. \u003cem\u003eNat. Methods\u003c/em\u003e. \u003cb\u003e9\u003c/b\u003e, 772. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1038/nmeth.2109\u003c/span\u003e\u003cspan address=\"10.1038/nmeth.2109\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2012).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eTamura, K. \u0026amp; Nei, M. Estimation of the number of nucleotide substitutions in the control region of mitochondrial DNA in humans and chimpanzees. \u003cem\u003eMol. Biol. Evol.\u003c/em\u003e \u003cb\u003e10\u003c/b\u003e, 512\u0026ndash;526. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1093/oxfordjournals.molbev.a040023\u003c/span\u003e\u003cspan address=\"10.1093/oxfordjournals.molbev.a040023\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (1993).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGuindon, S. \u0026amp; Gascuel, O. A simple, fast, and accurate algorithm to estimate large phylogenies by maximum likelihood. \u003cem\u003eSyst. Biol.\u003c/em\u003e \u003cb\u003e52\u003c/b\u003e, 696\u0026ndash;704. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1080/10635150390235520\u003c/span\u003e\u003cspan address=\"10.1080/10635150390235520\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2003).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePuillandre, N., Lambert, A., Brouillet, S. \u0026amp; Achaz, G. A. B. G. D. Automatic Barcode Gap Discovery for primary species delimitation. \u003cem\u003eMol. Ecol.\u003c/em\u003e \u003cb\u003e21\u003c/b\u003e, 1864\u0026ndash;1877. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/j.1365-294X.2011.05239.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1365-294X.2011.05239.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2012).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eClarke, K. \u0026amp; Gorley, R. Getting started with PRIMER v7. \u003cem\u003ePRIMER-e: plymouth, plymouth marine laboratory\u003c/em\u003e 20 (2015).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eIvlev, V. S. \u003cem\u003eExperimental ecology of the feeding of fishes\u003c/em\u003e (Yale University Press, 1961).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eJacobs, J. Quantitative measurement of food selection: A modification of the forage ratio and Ivlev's electivity index. \u003cem\u003eOecologia\u003c/em\u003e \u003cb\u003e14\u003c/b\u003e, 413\u0026ndash;417. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/BF00384581\u003c/span\u003e\u003cspan address=\"10.1007/BF00384581\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (1974).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHiraoka, M., Ohno, M., Kawaguchi, S. \u0026amp; Yoshida, G. Crossing test among floating \u003cem\u003eUlva\u003c/em\u003e thalli forminggreen tide'in Japan. \u003cem\u003eHydrobiologia\u003c/em\u003e \u003cb\u003e512\u003c/b\u003e, 239\u0026ndash;245. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1023/B:HYDR.0000020332.12641.a2\u003c/span\u003e\u003cspan address=\"10.1023/B:HYDR.0000020332.12641.a2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2004).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKirkendale, L., Saunders, G. W. \u0026amp; Winberg, P. A. Molecular Survey of \u003cem\u003eUlva\u003c/em\u003e (Chlorophyta) in Temperate Australia Reveals Enhanced Levels of Cosmopolitanism. \u003cem\u003eJ. Phycol.\u003c/em\u003e \u003cb\u003e49\u003c/b\u003e, 69\u0026ndash;81. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/jpy.12016\u003c/span\u003e\u003cspan address=\"10.1111/jpy.12016\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2013).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZanolla, M., Carmona, R., Kawai, H., Stengel, D. B. \u0026amp; Altamirano, M. Role of thermal photosynthetic plasticity in the dispersal and settlement of two global green tide formers: \u003cem\u003eUlva pertusa\u003c/em\u003e and \u003cem\u003eU. ohnoi\u003c/em\u003e. \u003cem\u003eMar. Biol.\u003c/em\u003e \u003cb\u003e166\u003c/b\u003e, 123. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s00227-019-3578-1\u003c/span\u003e\u003cspan address=\"10.1007/s00227-019-3578-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eBachoo, T., Bolton, J. J., Macey, B. M., Kandjengo, L. \u0026amp; Reddy, M. M. Resolving the identity of commercially cultivated Ulva (Ulvaceae, Chlorophyta) in integrated seaweed-abalone aquaculture farms in South Africa. \u003cem\u003eJ. Phycol.\u003c/em\u003e \u003cb\u003e59\u003c/b\u003e, 1272\u0026ndash;1283. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/jpy.13391\u003c/span\u003e\u003cspan address=\"10.1111/jpy.13391\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKim, K. Y. et al. Physiological ecology and seasonality of \u003cem\u003eUlva pertusa\u003c/em\u003e on a temperate rocky shore. \u003cem\u003ePhycologia\u003c/em\u003e \u003cb\u003e43\u003c/b\u003e, 483\u0026ndash;492. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.2216/i0031-8884-43-4-483.1\u003c/span\u003e\u003cspan address=\"10.2216/i0031-8884-43-4-483.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2004).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKim, J. H., Kang, E. J., Park, M. G., Lee, B. G. \u0026amp; Kim, K. Y. Effects of temperature and irradiance on photosynthesis and growth of a green-tide-forming species (\u003cem\u003eUlva linza\u003c/em\u003e) in the Yellow Sea. \u003cem\u003eJ. Appl. Phycol.\u003c/em\u003e \u003cb\u003e23\u003c/b\u003e, 421\u0026ndash;432. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s10811-010-9590-y\u003c/span\u003e\u003cspan address=\"10.1007/s10811-010-9590-y\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2011).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYoshida, G., Uchimura, M. \u0026amp; Hiraoka, M. Persistent occurrence of floating \u003cem\u003eUlva\u003c/em\u003e green tide in Hiroshima Bay, Japan: seasonal succession and growth patterns of \u003cem\u003eUlva pertusa\u003c/em\u003e and \u003cem\u003eUlva\u003c/em\u003e spp.(Chlorophyta. \u003cem\u003eUlvales) Hydrobiologia\u003c/em\u003e. \u003cb\u003e758\u003c/b\u003e, 223\u0026ndash;233. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s10750-015-2292-3\u003c/span\u003e\u003cspan address=\"10.1007/s10750-015-2292-3\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2015).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHiraoka, M., Shimada, S., Uenosono, M. \u0026amp; Masuda, M. A new green-tide‐forming alga, \u003cem\u003eUlva ohnoi\u003c/em\u003e Hiraoka et Shimada sp. nov.(Ulvales, Ulvophyceae) from Japan. \u003cem\u003ePhycol. Res.\u003c/em\u003e \u003cb\u003e52\u003c/b\u003e, 17\u0026ndash;29. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/j.1440-1835.2003.00321.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1440-1835.2003.00321.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2004).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eL\u0026oacute;pez, S. B., Fern\u0026aacute;ndez, I. B., Lozano, R. B. \u0026amp; Ugarte, J. C. Is the cryptic alien seaweed \u003cem\u003eUlva pertusa\u003c/em\u003e (Ulvales, Chlorophyta) widely distributed along European Atlantic coasts? \u003cem\u003eBot. Mar.\u003c/em\u003e \u003cb\u003e50\u003c/b\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1515/BOT.2007.030\u003c/span\u003e\u003cspan address=\"10.1515/BOT.2007.030\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2007).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eXing, Q. et al. A novel approach of monitoring \u003cem\u003eUlva pertusa\u003c/em\u003e green tide on the basis of UAV and deep learning. \u003cem\u003eWater\u003c/em\u003e \u003cb\u003e15\u003c/b\u003e, 3080. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/w15173080\u003c/span\u003e\u003cspan address=\"10.3390/w15173080\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2023).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHanyuda, T. \u0026amp; Kawai, H. Genetic examination of the type specimen of \u003cem\u003eUlva australis\u003c/em\u003e suggests that it was introduced to Australia. \u003cem\u003ePhycol. Res.\u003c/em\u003e \u003cb\u003e66\u003c/b\u003e, 238\u0026ndash;241. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/pre.12222\u003c/span\u003e\u003cspan address=\"10.1111/pre.12222\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2018).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKang, E. J. et al. Evaluating bloom potential of the green-tide forming alga Ulva ohnoi under ocean acidification and warming. \u003cem\u003eSci. Total Environ.\u003c/em\u003e \u003cb\u003e769\u003c/b\u003e, 144443. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.scitotenv.2020.144443\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2020.144443\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2021).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNakamura, M. et al. Photosynthesis and growth of \u003cem\u003eUlva ohnoi\u003c/em\u003e and \u003cem\u003eUlva pertusa\u003c/em\u003e (Ulvophyceae) under high light and high temperature conditions, and implications for green tide in Japan. \u003cem\u003ePhycol. Res.\u003c/em\u003e \u003cb\u003e68\u003c/b\u003e, 152\u0026ndash;160. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/pre.12410\u003c/span\u003e\u003cspan address=\"10.1111/pre.12410\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2020).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKorea Oceanographic Data Center (KODC). N. I. o. F. S. N. \u003cem\u003eSea surface temperature data (2015 and 2020) for Jeju Island, Korea\u003c/em\u003e, (2022). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.nifs.go.kr/kodc/soo_list.kodc\u003c/span\u003e\u003cspan address=\"https://www.nifs.go.kr/kodc/soo_list.kodc\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMineur, F., Johnson, M. P. \u0026amp; Maggs, C. A. Macroalgal introductions by hull fouling on recreational vessels: seaweeds and sailors. \u003cem\u003eEnviron. Manage.\u003c/em\u003e \u003cb\u003e42\u003c/b\u003e, 667\u0026ndash;676. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s00267-008-9185-4\u003c/span\u003e\u003cspan address=\"10.1007/s00267-008-9185-4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2008).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWolf, M. A., Sciuto, K., Andreoli, C. \u0026amp; Moro, I. \u003cem\u003eUlva\u003c/em\u003e (Chlorophyta, Ulvales) Biodiversity in the North Adriatic Sea (Mediterranean, Italy): Cryptic Species and New Introductions. \u003cem\u003eJ. Phycol.\u003c/em\u003e \u003cb\u003e48\u003c/b\u003e, 1510\u0026ndash;1521. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/jpy.12005\u003c/span\u003e\u003cspan address=\"10.1111/jpy.12005\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2012).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSteinhagen, S., Karez, R. \u0026amp; Weinberger, F. Cryptic, alien and lost species: molecular diversity of \u003cem\u003eUlva sensu lato\u003c/em\u003e along the German coasts of the North and Baltic Seas. \u003cem\u003eEur. J. Phycol.\u003c/em\u003e \u003cb\u003e54\u003c/b\u003e, 466\u0026ndash;483. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1080/09670262.2019.1597925\u003c/span\u003e\u003cspan address=\"10.1080/09670262.2019.1597925\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2019).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSantelices, B. Banks of microscopic forms and survival to darkness of propagules and microscopic stages of macroalgae. (2002).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFlagella, M. M., Verlaque, M., Soria, A. \u0026amp; Buia, M. C. Macroalgal survival in ballast water tanks. \u003cem\u003eMar. Pollut Bull.\u003c/em\u003e \u003cb\u003e54\u003c/b\u003e, 1395\u0026ndash;1401. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.marpolbul.2007.05.015\u003c/span\u003e\u003cspan address=\"10.1016/j.marpolbul.2007.05.015\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2007).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eFlagella, M. M., Andreakis, N., Hiraoka, M., Verlaque, M. \u0026amp; Buia, M. C. Identification of cryptic \u003cem\u003eUlva\u003c/em\u003e species (Chlorophyta, Ulvales) transported by ballast water. \u003cem\u003eJ. Biol. Res.\u003c/em\u003e \u003cb\u003e13\u003c/b\u003e, 47 (2010).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ede Casabianca, M. L., Barthelemy, N., Serrano, O. \u0026amp; Sfriso, A. Growth rate of \u003cem\u003eUlva rigida\u003c/em\u003e in different Mediterranean eutrophicated sites. \u003cem\u003eBioresour Technol.\u003c/em\u003e \u003cb\u003e82\u003c/b\u003e, 27\u0026ndash;31. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/s0960-8524(01)00155-9\u003c/span\u003e\u003cspan address=\"10.1016/s0960-8524(01)00155-9\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2002).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMao, Y., Kim, J. K., Wilson, R. \u0026amp; Yarish, C. The appearance of \u003cem\u003eUlva laetevirens\u003c/em\u003e (Ulvophyceae, Chlorophyta) in the northeast coast of the United States of America. \u003cem\u003eJ. Ocean. Univ. China\u003c/em\u003e. \u003cb\u003e13\u003c/b\u003e, 865\u0026ndash;870. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1007/s11802-014-2419-6\u003c/span\u003e\u003cspan address=\"10.1007/s11802-014-2419-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2014).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eWu, H. et al. Physiological acclimation of \u003cem\u003eUlva prolifera\u003c/em\u003e to seasonal environmental factors drives green tides in the Yellow Sea. \u003cem\u003eMar. Environ. Res.\u003c/em\u003e \u003cb\u003e179\u003c/b\u003e, 105695. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.marenvres.2022.105695\u003c/span\u003e\u003cspan address=\"10.1016/j.marenvres.2022.105695\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2022).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKang, E. J., Kim, J. H., Kim, K., Choi, H. G. \u0026amp; Kim, K. Y. Re-evaluation of green tide-forming species in the Yellow Sea. \u003cem\u003eAlgae\u003c/em\u003e 29, 267\u0026ndash;277, (2014). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.4490/algae.2014.29.4.267\u003c/span\u003e\u003cspan address=\"10.4490/algae.2014.29.4.267\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eSon, Y. B., Choi, B. J., Kim, Y. H. \u0026amp; Park, Y. G. Tracing floating green algae blooms in the Yellow Sea and the East China Sea using GOCI satellite data and Lagrangian transport simulations. \u003cem\u003eRemote Sens. Environ.\u003c/em\u003e \u003cb\u003e156\u003c/b\u003e, 21\u0026ndash;33. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.rse.2014.09.024\u003c/span\u003e\u003cspan address=\"10.1016/j.rse.2014.09.024\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2015).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYotsukura, N., Kawai, T., Motomura, T. \u0026amp; Ichimura, T. Tandem 5S ribosomal RNA genes and the spacer region sequences of three Japanese Laminaria species. \u003cem\u003eJ. Appl. Phycol.\u003c/em\u003e \u003cb\u003e14\u003c/b\u003e, 233\u0026ndash;239. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1023/A:1021166218681\u003c/span\u003e\u003cspan address=\"10.1023/A:1021166218681\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2002).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYotsukura, N., Kawai, T., Kawashima, S., Ebata, H. \u0026amp; Ichimura, T. Nucleotide sequence diversity of the 5S rDNA spacer in the simple blade kelp genera \u003cem\u003eLaminaria\u003c/em\u003e, \u003cem\u003eCymathaere\u003c/em\u003e and \u003cem\u003eKjellmaniella\u003c/em\u003e (Laminariales, Phaeophyceae) from northern Japan. \u003cem\u003ePhycol. Res.\u003c/em\u003e \u003cb\u003e54\u003c/b\u003e, 269\u0026ndash;279. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1111/j.1440-1835.2006.00434.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1440-1835.2006.00434.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2006).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eMarine Environment Information System (MEIS). Marine environment observation data: 2021 raw data. Korea Marine Environment Management Corporation. (2021). Available at: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://meis.go.kr\u003c/span\u003e\u003cspan address=\"http://meis.go.kr\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. (accessed 30 October 2025).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Climate change, DNA barcoding, Jeju Island, Nutrient input, Seaweed tide, Ulva species","lastPublishedDoi":"10.21203/rs.3.rs-8022902/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8022902/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGreen tides \u0026ndash; massive proliferations of green macroalgae (\u003cem\u003eUlva\u003c/em\u003e spp.) \u0026ndash; have increasingly occurred worldwide in recent years, driven by accelerating climate change and anthropogenic nutrient inputs. These blooms disrupt coastal ecosystems, leading to biodiversity loss and economic damage. In Korea, green tides have persisted on Jeju Island since the 2000s, and have also been sporadically reported on the southern mainland coasts. However, the specific \u003cem\u003eUlva\u003c/em\u003e species responsible for these blooms remain largely unknown. Here, we investigated \u003cem\u003eUlva\u003c/em\u003e community structure and relative frequencies from 46 sites (966 specimens) on Jeju Island and the southern coasts, using chloroplast \u003cem\u003etuf\u003c/em\u003eA gene-based phylogenetic analysis, complemented by additional nuclear 5s rDNA marker. We found considerable differences in \u003cem\u003eUlva\u003c/em\u003e community composition between Jeju Island and the southern coasts, along with pronounced seasonal variation. On Jeju Island, nine \u003cem\u003eUlva\u003c/em\u003e species were found, with \u003cem\u003eUlva ohnoi\u003c/em\u003e and \u003cem\u003eUlva australis\u003c/em\u003e dominant, whereas 10 species were observed with \u003cem\u003eU. australis\u003c/em\u003e and \u003cem\u003eUlva linza\u003c/em\u003e prevailed on the southern coasts. The presence of nonindigenous \u003cem\u003eUlva\u003c/em\u003e species highlights the need for continuous monitoring to track their spread and biomass growth. Our results provide essential genetic insights to support effective management of green tide events in Korean coastal ecosystems.\u003c/p\u003e","manuscriptTitle":"Unraveling the major Ulva species driving local green tides through molecular analyses: spatiotemporal patterns along the Korean coast","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-25 16:10:13","doi":"10.21203/rs.3.rs-8022902/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-04T12:41:19+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-04T10:16:43+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-02T04:47:56+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-29T04:37:26+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"336461011157300020161862664170140950198","date":"2025-11-14T11:19:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"230345880948581064452549166145377192469","date":"2025-11-12T17:44:26+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"34338422535435997742631002988801207187","date":"2025-11-12T07:02:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"217140045510887946401881677903786387458","date":"2025-11-12T06:56:45+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-12T06:48:16+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-11-10T13:18:43+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-05T09:20:37+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-05T09:19:57+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-11-03T23:08:10+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a85693ea-9409-4dd9-9117-67acfe4b3dac","owner":[],"postedDate":"November 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":57993403,"name":"Biological sciences/Ecology"},{"id":57993404,"name":"Earth and environmental sciences/Ecology"},{"id":57993405,"name":"Earth and environmental sciences/Environmental sciences"},{"id":57993406,"name":"Earth and environmental sciences/Ocean sciences"}],"tags":[],"updatedAt":"2026-04-20T07:11:46+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-25 16:10:13","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8022902","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8022902","identity":"rs-8022902","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}