Seawater temperature and tidal action as modulators of Ulva spp. micropropagules density in a eutrophicated macrotidal coastal system

Seawater temperature and tidal action as modulators of Ulva spp. micropropagules density in a eutrophicated macrotidal coastal system | 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 Research Article Seawater temperature and tidal action as modulators of Ulva spp. micropropagules density in a eutrophicated macrotidal coastal system Agustina Mariel Caniguan, Maria Eugenia Becherucci, Marianela Gastaldi, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5320006/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 10 Mar, 2025 Read the published version in Estuaries and Coasts → Version 1 posted 5 You are reading this latest preprint version Abstract Micropropagules are the reproductive and dispersal means of macroalgae, often linked to green macroalgal blooms in eutrophicated coastal areas. In San Antonio Bay (North Patagonia, Argentina), increased nutrients have led to Ulva spp. blooms in spring and early summer, coinciding with high Ulva spp. micropropagules (MPU) density at low tide. This study aimed to describe the variation in MPU densities throughout a year and in a tidal cycle and their relationship with environmental variables. For this, MPU density, macroalgal biomass, weight of mature and immature thalli, and seawater physical and chemical variables were determined: 1) monthly for a year at low tide, 2) during a tidal cycle at one-hour intervals covering the low tide period (approx. five hours), and 3) at different depths in the water column during daytime and nighttime high tides. Maximum MPU density (33983±9553 cel ml -1 ) occurred in February, while macroalgal biomass peaked in December. MPU density, seawater temperature, salinity, chlorophyll-a, and nutrients increased during low tide but decreased at high tide, with no evidence of vertical stratification. MPU density was positively associated with seawater temperature during low tide and throughout the year. We conclude that MPU variation is associated with seawater temperature annually and with tidal action daily. High MPU densities during summer raise chlorophyll and dissolved oxygen levels, while tidal flow dilutes and exports MPU. These results provide insights into the dynamics of the dispersal phase of an opportunistic and globally distributed green algal genus for the first time. green tides macroalgal biomass eutrophication propagules coastal area Patagonia Argentina Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Ulva Linnaeus 1753, is a cosmopolitan genus of green algae with species inhabiting all oceans and estuaries of the world. Species of Ulva are characterised by their opportunistic nature, including a simple thallus structure (sheet-like or tubular) and rapid nutrient uptake and growth rates, which favour its success in a wide variety of environments (Fong et al. 1996). These green macroalgae dominate coastal areas bearing wide variations in salinity, seawater temperature, and available light or nutrients (Chen et al. 2019 ; Bews et al. 2021 ). Regarding the latter, a high contribution of anthropogenic nitrogen often causes uncontrolled growth, forming blooms (Teichberg et al. 2010 ), and in extreme conditions, they can break away and float from the mats, developing green tides (Fletcher 1996 , Ding et al. 2009 ; Ye et al. 2011 ; Liu et al. 2013 ). An increase in the eutrophication of coastal areas is expected in a near future scenario, mainly enhanced by anthropogenic pressure. This, combined with the effect of climate change (i.e. elevated temperature and carbon dioxide concentration), would increase the frequency and magnitude of macroalgal blooms (including green tides) globally, which in turn may result in habitat degradations and a more physiologically stressful environment at the community level (Worm et al. 1999 ; Le Luherne et al. 2016 ; Wåhlström et al. 2020 ; Ji & Gao 2021 ). Ulva has both sexual and asexual reproduction through planktonic micropropagules, including biflagellate gametes and quadriflagellated zoospores, respectively, which are released into the environment and serve as dispersal agents (Smith 1947 ; Hoxmark 1975 ; Phillips 1990 ). Studies on macroalgal blooms have been focused primarily on the macroscopic phases of this macroalgal life cycle, with less emphasis on micropropagules, probably due to the difficulty of quantifying them using traditional methods. In fact, in previous studies, the abundance of MPU was recorded only indirectly by culturing cumulative plankton samples and counting emerging seedlings (Hoffman & Camus 1989 ; Schories 1995 ; Granhag et al. 2007 ; Heydt et al. 2012 ; Huo et al. 2014 ; Li et al. 2014 ; Liu et al. 2012 ; Han et al. 2019 ; Xiaoxiang et al. 2020 ). The reproduction of Ulva is strongly controlled by physical conditions, such as photoperiod, temperature, light intensity, and water movement, among others (Balar & Mantri 2020 ). For instance, the release of gametes increases with increasing daylight hours (Lüning et al. 2008 ). Light intensity affects the distribution of MPU in the water column with gametes (positively phototactic) in more superficial waters and zoospores in deeper waters (negatively phototactic; Jones & Babb 1968 ). Warm seawater temperature favours the release of Ulva gametes and zoospores and their germination (Nordby 1977 ; Song et al. 2014 ). Given the limited mobility of the MPU, it is postulated that water movement would dilute the concentration of gametes, decreasing the probability of fertilization. On the contrary, calm waters increase the probability of encounters between gametes through positive phototactic and communication by pheromones (Brawley & Johnson 1992 ). A single study has shown that water motion stimulates the gametes and zoospores release in U. lactuca (Gordon & Brawley 2004 ), while gamete release could be inhibited under calmer conditions (Stratmann et al. 1996 ). San Antonio Bay is a coastal marine area in northern Argentine Patagonia. Tidal channels characterize the intertidal zone of the bay. The tidal channel near San Antonio Oeste is a eutrophic system due to the extra input of organic and inorganic nutrients from residential wastewater (Martinetto et al. 2010 ). As a consequence, there is an increase in the biomass of macroalgae, mainly of Ulva species, which form frequent green macroalgal blooms from late winter to early summer (Teichberg et al. 2010 ; Gastaldi et al. 2016 ; Becherucci et al. 2021 ). Coinciding with the time of most significant coverage of Ulva in the subtidal and intertidal zones of San Antonio Bay, Saad et al. ( 2019 ) found a high concentration of MPU during low tide in the spring months, constituting up to 95% of all planktonic cells. This finding is the first record of such high densities of macroalgal MPU. The main objective of this study is to evaluate the MPU density dynamics in the SAB. For this, we 1) describe the variation of MPU density over a year and during the tide cycle, 2) describe the variation of MPU density during the whole low tide period, and 3) contrast the distribution of MPU between a nocturnal and a diurnal high tide. Additionally, we measured physical and chemical variables in each sampling to explore its relationships with MPU variation. We hypothesize that (1) MPU production increases during the warmer months of the year due to the increased macroalgal biomass and other ambient variables such as seawater temperature and nutrients, (2) MPU density is modulated by the tides, increasing their concentration during low tide, while diluting them during high tide, and (3) MPU density is modulated by light, which increases their concentration at daytime. 2. Methods 2.1. Study site San Antonio Bay (SAB) is a semi-desert coastal system with a semi-diurnal macrotidal regime featuring average amplitudes of 6.26 m and maximum amplitudes of 9.24 m (Naval Hydrography Service, SHN). SAB exchanges a significant volume of water with the San Matías Gulf in each tidal cycle, and its semicircular shape protects against the high energy of tides and waves, allowing the formation of extensive tidal flats and channels (Carbone et al. 2014 ; Fig. 1 a). It is a hypersaline system due to the low average precipitation (250 mm per year). The average annual atmospheric temperature is 15.1°C, with extreme temperatures in July (austral winter, minimum of -7.7°C) and February (austral summer, maximum of 41.4°C, respectively). The mean annual air humidity is 57%, and the average annual wind speed is 18 km h − 1 , reaching an average of 25 km h − 1 in spring-summer, with the predominant wind direction from the west (Genchi et al. 2010 ; González et al. 2010 ). The study was carried out in the channel bordering San Antonio Oeste (40°43'37.1"S 64°56'52.6" W; Fig. 1 a). The samples were collected in the subtidal area covering 80 m 2, characterized by a low tide of 0.5 m and a high tide of up to 5.3 m deep. Since the sampling site is located inland of the bay, it remains uncovered for approximately 5 hours (Gastaldi et al. 2016 ; Supplementary Fig. 1), providing an ideal opportunity for experimentation. In the bay, both Ulva spp. tubular-shaped thalli in the intertidal and lamellar-shaped thalli in the subtidal were observed. We only worked with the lamellar form Ulva thalli (Fig. 1 b). Although in previous studies on the SAB, Ulva lactuca has been mentioned as the species responsible for the green blooms, its taxonomic identity has not yet been determined, so it may have been a complex of several Ulva species. Given that the taxonomic determination is beyond the objective of the present study, we use the term Ulva spp. to refer to the complex of species responsible for producing green blooms and micropropagules in the SAB. 2.2. Sample collection 2.2.1. Annual cycle sampling A monthly sampling was performed from October 2021 to September 2022 to evaluate the temporal variation of the density of MPU. Seawater samples were collected in triplicate in 250 ml bottles during the low tide and neap tides (Service of Naval Hydrography, SHN). The samples were fixed with formaldehyde to a final concentration of 0.4% and stored in a cool, dark place until analysis. For counting (expressed in cel ml − 1 ), the Utermöhl ( 1958 ) method was applied, using sedimentation chambers in an inverted microscope (Fig. 2 a). According to Venrick ( 1978 ), the counting error was estimated in a random number of fields, with a maximum error of 20%. Macroalgae biomass data were collected at the same time as the MPU sampling. Twenty quadrats (25x25 cm) were randomly thrown into the field in 200 m 2 , within which all Ulva spp. individuals were collected. The samples were transported cold, and in the laboratory, they were dried in an oven at 60 ºC for > 72 hours. The dry weight (g m − 2 ) was determined by the gravimetric method. Additionally, 30 thalli were randomly collected in the same area and stored under cold and humid conditions until processing to determine the weight of mature and immature thalli. To determine the state of maturity, the thalli were thawed and subsequently inspected with a stereoscope to identify the reproductive structures (sporangia/gametangia), which were visualized as a dark spot of a cellular layer capable of detaching or depigmentation at the margins (Fig. 2 b,d). After inspection, the thalli were dried in an oven at 60°C for > 72 hours, and the biomass was determined by the gravimetric method (in terms of dry weight, in g). The in situ seawater temperature was recorded with a digital thermometer, and water samples were collected for subsequent laboratory analysis. Salinity, pH, and dissolved oxygen (DO, mg l − 1 ) were measured immediately (< 1 h of taken) with an Atlas Scientific multiparameter probe (kits EC-10, 103P, and 103DX). To determine chlorophyll-a (Chla-a) and suspended solids, water samples were filtered through 45 mm glass fiber filters (0.7 µm pore size). The extraction of Chla-a was carried out by incubating the filters with 96% ethanol for 12 hours, and the measurements were taken at absorbances of 665 and 750 nm with a Persee T7S spectrophotometer, applying a correction for phaeopigments by adding HCL 1N. The concentration was expressed in µg l − 1 following Marker et al. ( 1980 ). The filters were dried in an oven at 60°C to obtain the suspended solids, and the total concentration was obtained by applying the gravimetric method (APHA 1998). The inorganic fraction was calculated after the combustion of the filters at 500°C in a muffle for 3 hours (the concentrations were expressed in mg l − 1 ). Chromophoric dissolved organic matter (CDOM, mg l − 1 ) was measured from the filtered water by UV-visible spectral analysis (200–800 nm) with spectrophotometer at an absorbance of 350 nm according to Helms et al. ( 2008 ) and nutrients (nitrate, nitrite, ammonium y phosphate, mg l − 1 ) using standard colorimetric methods with a spectrophotometer according to standardized protocols (Strickland & Parsons 1972). 2.2.2. Tide cycle sampling Two different samplings were carried out to evaluate the variation of MPU density during a tidal cycle at the peak of MPU (December; Saad et al. 2019 ). The first was carried out during the five-hour period of diurnal low tide. To do so, 250 ml of water was collected in bottles every hour from 7:40 a.m. to 12:40 p.m. The second sampling was conducted during two consecutive high tides, one at day and the other at night. Water samples were collected from the water column at four depths (0 m, 1.5 m, 3 m, and 4.5 m) using a 5 L Van Dorn bottle. The water characteristics (temperature, salinity, pH, and other reported parameters) were also measured and determined during low tide and at each depth during high tide samplings (daytime and nighttime). These measurements followed the protocols described in the "Annual cycle sampling" section. 2.3. Statistical analysis All statistical analyses were performed with R program version 3.6.1 (R Core Team 2021 ). Differences in MPU density between months, hours, and high tides were determined using the Bonferroni adjusted 'emmeans' function from the emmeans package (Lenth et al. 2019 ) with a negative binomial error distribution and log link function determined from a Generalized Linear Model with a 95% confidence interval (GLM: function 'glm.nb', package MASS; Ripley et al. 2013 ; Supplementary Tables 1). Differences in macroalgal biomass and mature thalli's dry weight were determined similarly but with a gamma error distribution and log link function (Supplementary Tables 2). To evaluate the effect of environment variables on MPU density, we used a generalized linear mixed model with the distribution mentioned above (GLMM, function 'glmmTMB', glmmTMB package; Brooks et al. 2023 ) and with the “months”, “hours” and “depth” as random factors according to models accounted the annual variations, low tide and day tide variations respectively. Possible proxy variables of the MPU, such as total and organic suspended solids, DO, and Chl-a, were discarded, leaving the following predictor variables available: inorganic suspended solids, chromophoric dissolved organic matter, temperature, salinity, nitrate, nitrite, ammonium and phosphate for the construction of the models. Due to possible correlations between variables, Pearson rank correlation coefficients were calculated for all environmental variables (Supplementary Table 3). Of the correlated ones, only one of the variables was selected to the extent possible. Model assumptions were verified by plotting the residuals against the fitted values ​​of the global model and by a QQ plot using the DHARMa (Hartig & Harting 2022) and performance (Lüdecke et al. 2021 ) packages. The models (GLMM) were evaluated with information theory procedures (Burnham & Anderson 1998 ). The Akaike information criterion corrected for small sample size (AICc) was calculated for each model. Model comparison was performed by evaluating the DAICc, defined as the difference between the lowest AICc value (i.e., the best of the suitable models) and the AICc of all other models, as well as the AICc weight. The AICc weight of a model (AICc Weight) indicates the relative probability that the specific model is the best of all the models. In cases where the AICc weight of the most appropriate model did not exceed a value of 0.7, we evaluated the support of the predictor variables. Parameter estimates were calculated using model-averaged parameter estimates based on the AICc weights of all candidate models, and we also calculated the 95% confidence interval limits of the parameter estimates. 3. Results 3.1. Annual cycle MPU density varied between months (GLM: LRS = 1924.9, p < 0.05, n = 36). MPU density was maximum in February (33983 ± 9553 cel ml − 1 ) and higher in November, December, and January (8048 ± 654 cel ml − 1 , 5699 ± 528 cel ml − 1 , 2922 ± 448 cel ml − 1 respectively) than the cold months (from March to October, 239 ± 285 cel ml − 1 ; Fig. 3 a). Macroalgae biomass varied between months (GLM: LRS = 139.7, p < 0.05, n = 240), being higher in warm months (November to April, 53.4 ± 38.4 g m − 2 ) than cold months (May to October, 21.5 ± 14.4 g m − 2 ; Fig. 3 b). The dry weight of mature thalli varied between months (GLM: LRS = 31.2, p < 0.05, n = 98). Only December (7.59 ± 3.23 g) was higher than May and June (2.32 ± 3.24 g; Fig. 3 c), while in the remaining months, it was constant (4.40 ± 4.13 g). The biomass of mature thalli was greater than that of immature thalli in all months (GLM: LRS = 62.8, p < 0.05, n = 347). The temperature, salinity, dissolved oxygen, and pH exhibited marked seasonal fluctuations, with higher values during the warm months (October to March; 25.36 ± 2.32°C, 34.76 ± 0.67; 17.33 ± 2.96 mg l − 1 , 8.67, respectively) and lower values during the cold months (April to September; 14.03 ± 2.85°C, 31.91 ± 0.29, 13.05 ± 2.58 mg l − 1 , 7.79, respectively; Supplementary Table 4.I; Fig. 4 a,d). Chl-a described two peaks that coincided with those of MPU. In November, the Chl-a concentration reached 18.9 ± 1.4 µg l − 1 , and in February, it peaked at 44.3 ± 12.5 µg l − 1 . Throughout the remaining months, the Chl-a values remained below 12.9 µg l − 1 (Fig. 4 e). CDOM revealed higher values during January and February (0.78 ± 0.19 and 0.58 ± 0.16 µg l − 1 , respectively), coinciding with the maximum MPU density (Fig. 4 f). The organic suspended solids showed two maxima, one in December of 9.33 ± 0.58 mg l − 1 , which coincides with the maximum macroalgal biomass, and one in February of 8.0 ± 1.0 mg l − 1 , which coincided with the maximum of MPU; the remaining months presented values lower than 6.34 mg l − 1 (Fig. 4 g). For nutrients, nitrate concentration was maximum and nitrite minimum, both with constant values ​​throughout the year (average concentration 2.910 ± 0.797 mg l − 1 and 0.065 ± 0.046 mg l − 1 , respectively). Ammonium and phosphate had intermediate concentrations with peaks in December (1.540 ± 0.409 mg l − 1 and 2.010 ± 0.095 mg l − 1 , respectively) that coincided with the first peak of MPU density and the maximum of macroalgal biomass (Fig. 4 h). Regarding the role of environmental variables in MPU annual variation, the global model (MPU ~ temperature + nitrate + ammonium + phosphate + ISS + (1|month)) accounted for a dispersion value of 0.83. Model validation for all dependent variables tested indicated normality and homogeneity of residuals. The best model describing the MPU variation included temperature as an explanatory variable (Table 1 , 2 ). The best model accounted for 97% of the variation. The MPU increased exponentially with temperature starting at 25°C, reaching the maximum value at 28.9°C (Fig. 5 ). Table 1 Summary of model-selection results for models explaining variations in MPU during low tide with temperature (T), inorganic suspended solid (ISS), and the nutrients nitrate (N), ammonium (A), and phosphate (P). Models are listed in decreasing order of importance. Associated hypothesis Candidate models df AICc ΔAICc Weight AICc MPU varied only by T T 4 538.9 0.00 0.53 MPU varied by T and ISS T + ISS 5 539.4 0.50 0.41 MPU varied by T and nutrients T + N + A + P 7 545.0 6.07 0.025 Neither of the variables is associated with the MPU Null model 3 545.6 6.69 0.019 MPU varied by ISS ISS 4 546.7 7.75 0.011 Global model T + ISS + N + A + P 8 547.4 8.49 0.008 MPU varied by nutrients N + A + P 6 550.8 11.90 0.001 MPU varied by ISS and nutrients ISS + N + A + P 7 553.4 14.45 0.00 Table 2 Parameter estimates, standard errors (ES) and 95% confidence interval limits for explanatory variables describing variation in MPU during low tide. Explanatory variables excluding zero are in bold. Explanatory variable Estimate SE Confidence interval Lower Upper Intercept 1.92 1.63 -1.37 5.16 Temperature 0.25 0.06 0.12 0.39 ISS -0.06 0.04 -0.14 0.02 3.2. Tidal cycle 3.2.1. Low tide The density of MPU increased during the low tide period (GLM: LRS = 94.88; p < 0.05, n = 18). Start with a minimum of 559 ± 125 cel ml − 1 at 7:40 h and reach maximum values ​​at 10:40 and 11:40 h of 8081 ± 4410 and 10511 ± 7345 cel ml − 1 , respectively. There was a significant decrease in MPU density in the last hour (12:40 h) to 2839 ± 540 cel ml − 1 (Fig. 6 ). The environmental variables also showed a pronounced increasing trend over time (Fig. 7 ; Supplementary Table 4.II). At low tide, temperature increased by 9.3 ºC, dissolved oxygen by 14.92 mg l − 1 , salinity by 2.9, and pH by 0.52 (Fig. 7 a,d). Chl-a concentrations started at 3.94 ± 0.73 and peaked at 5.63 ± 0.38 µg l − 1 (Fig. 7 e). Similarly, CDOM started at 0.197 ± 0.035 and peaked at 0.577 ± 0.030 at 11:40. Organic suspended solids began at 4 mg l − 1 in the initial hour and peaked at 9 mg l − 1 at 11:40 h, coinciding with the minimum and maximum MPU density (Fig. 7 f,g). Within nutrients, nitrate experienced the most pronounced increase (3.63 mg l − 1 ), while nitrite, ammonium and phosphate also rose but to a lesser extent (0.07, 0.62, and 0.23 mg l − 1 , respectively; Fig. 7 h). Regarding the role of environmental variables in low tide MPU variation, the global model (MPU ~ temperature + nitrate + ammonium + phosphate + ISS + (1|hours)) accounted for a dispersion value of 1.8. Model validation for all dependent variables tested indicated normality and homogeneity of residuals. When the last time is removed, the best model is temperature. When not removed, the null model (Weight AICc = 0.66) followed by temperature (Weight AICc = 0.21) were the models that best explained the MPU variation during low tide. However, according to multiple inferences, neither measured variable explains the MPU density (CI temperature = [ -0.29; 0.43]). The null model accounted for 80% of the variation. Although MPU density increases with temperature (Fig. 8 ), in the last hour of low tide, where a temperature peak is observed, MPU density decreases dramatically (observed at 12:40 h in Fig. 8 ). 3.2.2. High tide Considering daytime versus nighttime during high tide, the MPU density was lower than at low tide (78 ± 96 cel ml − 1 and 4665 ± 4678 cel ml − 1 , respectively; GLM: LRS = 139.42, p < 0.05, n = 42). The MPU density at daytime was higher than at nighttime (128 ± 115 cel ml − 1 and 25 ± 18 cel ml − 1 , respectively; GLM: LRS = 27.31, p < 0.05, n = 24; Fig. 9 ). The MPU density varied among depths in both high tides (daytime: GLM: LRS = 4.48, p < 0.05, n = 12 and nighttime: GLM: LRS = 11.34, p < 0.05, n = 12). In the daytime, the MPU density at 3 m depth was higher than at the bottom and the surface. In the nighttime the superficial MPU density was higher than at 1.5 m. Most physical and chemical variables experienced a decrease concerning low tide. Temperature decreased by 0.51 ºC, salinity by 1.33, OSS by 1.67 mg l − 1 , ISS by 2.75 mg l − 1 , Chl-a by 2.33 mg l − 1 , CDOM by 0.27 mg l − 1 , nitrate by 2.37 mg l − 1 , nitrite by 0.06 mg l − 1 , ammonium by 0.64 mg l − 1 , phosphate by 0.21 mg l − 1 (Supplementary Table 4.III). The global model (MPU ~ temperature + pH + ISS + ammonium + phosphate + (1|depth)) accounted for a dispersion value of 1.4. Model validation for all dependent variables tested indicated normality and homogeneity of residuals. Temperature (Weight AICc = 0.53) followed by temperature and ISS (Weight AICc = 0.28) were the models that best explained the MPU variation during high tide. However, according to multiple inferences, only temperature explained the MPU density (CI temperature = [ 0.89; 2.12], CI ISS = [-0.28; 0.05]; Fig. 10 ). This model accounted for 54% of the variation. 4. Discussion We found a temporal variation of MPU densities guided mainly by seawater temperature and tidal action. To our knowledge, this study represents the first comprehensive monthly monitoring of MPU density during a year and through direct counting. Although indirect methods (for example, cultivation) could help to determine the proportion of MPU that is converted into biomass, our direct counting results show a more realistic amount of MPU that circulates through the system and participates in biogeochemical cycles, mainly in summer when the density was notably high (Maggs & Callow 2003 ; Zhang et al. 2009 ). 4.1. Annual cycle Field investigations have primarily focused on recording MPU density and environmental variables during the green tide events. For example, in the Yellow Sea, a high density of MPU was recorded during the world's largest green tide event (Huo et al. 2014 ). However, our study observed a MPU density peak after the macroalgal biomass peak. Therefore, at least in our study site, a high MPU density is not necessarily related to high macroalgal biomass. It could be thought that the observed pattern results from an important bias not to include information on macroalgae biomass in the intertidal zone. However, it has been shown that the cover of Ulva spp. it does not vary between the intertidal and subtidal zones (Gastaldi et al. 2016 ). The strong association observed between MPU density and temperature and the coincidence between their maxima in February evidences the stimulating effect of high temperatures on the release of MPU. Nordby ( 1977 ) and Song et al. ( 2014 ) also found a strong dependence between these variables. On the other hand, low temperature and luminosity during the cold months have been shown to limit the release and germination of propagules, as well as the growth of seedlings (Steffensen et al. 1976, Lüning et al. 2008 ; Lyalina 2018 ). This would explain the low density of propagules and macroalgal biomass observed during the autumn and winter (with temperatures of 10.9 to 24°C). Despite this, the propagules deposited in the sediment in all seasons can survive for up to 10 months under simulated winter conditions, germinating again when conditions are favourable (Schories 1995 ; Santelices et al. 2002 ). In this sense, it is highly plausible that a substantial percentage of Ulva spp. comes from an “MPU bank” accumulated in the previous months, with February possibly contributing the most to these deposits. In this context, our findings highlight the importance of future research evaluating the effect of increasing temperature in a climate change scenario on the increase in biomass and frequency of Ulva species (Ji & Gao 2021 ). The sensitivity of MPU to variations in nutrient concentrations is greater than that of adult thalli, mainly due to phosphate limitation (Sousa et al. 2007 ). While we did not find a significant relationship between MPU and nutrients, the high concentration of nitrates in the channel is well known (Martinetto et al., 2010 , Teichberg et al. 2010 , Becherucci et al. 2021 , and this study), and therefore, these nutrients could regulate the production of MPU throughout the year in San Antonio Bay. It is essential to mention that the range of nutrient concentrations and temperatures estimated for optimal growth of Ulva thalli in previous studies are coincident with those recorded here (Dan et al. 2002 ; Wang et al. 2019 ; Wang et al. 2020 ). Therefore, although these variables could limit the production of MPU at certain times of the year (Liu et al. 2012 ), they would not limit the germination and growth of adult thalli in San Antonio Bay. The strong correlation between Chla-a and MPU density indicates that a significant portion of Chla-a is due to the MPU. This phenomenon was previously observed by Saad et al. ( 2019 ) for San Antonio Bay. Currently, no precedent shows that macroalgal micropropagules dominate (at least during low tide) practically the entire phytoplankton assemblage in coastal systems. Furthermore, given that the maximum MPU densities recorded from November to February are comparable with those found in situations of microalgae blooms (Delgado & Alcarraz 1999; Place et al. 2012 ), we can define that the MPU in San Antonio Bay are capable to produce “merophytoplankton blooms”. Additionally, the strong correlation between dissolved oxygen and MPU shows the vital contribution of MPU to dissolved oxygen concentrations in San Antonio Bay through photosynthesis. This contribution is particularly noteworthy during the time of year when biological oxygen demand increases due to the decomposition process of adult Ulva thalli (Wallace et al. 2014 ). 4.2. Tidal cycle We found strong changes in densities of MPU within the tidal cycle that are consistent with what we proposed in hypothesis 2. The increase in the density of MPU during low tide could be attributed to two phenomena. First, a biological phenomenon, characterized by a constant rate of MPU production by macroalgae. Second, a physical phenomenon suggests an accumulation of MPU from the innermost sectors of the bay and intertidal zone during the low tides. The biological phenomenon is supported by studies of cultures with Ulva species that have shown that elevated levels of temperature, radiation, and nutrients in seawater greatly facilitate the production of MPU (Nordby 1977 ; Lüning et al. 2008 ; Song et al. 2014 ). Additionally, laboratory experiments revealed that 1 cm 2 thallus fragments (single cell layer) of Ulva prolifera , can release up to 6.62 x 10 6 propagules after 48 h under optimal conditions (Zhang et al. 2013 ). The observed increase in temperature, the slight increase in nutrients, and, probably, the intense irradiation during low tide make it highly plausible that the progressive increase in MPU density is fundamentally due to a constant rate of in situ production by macroalgae. The physical phenomenon refers to the process of continuous drainage of water from higher parts of the channel during low tide, which may be enriched with MPU and contribute significantly to the MPU stock. During the tide cycle sampling, Ulva spp. covered a large portion of the seafloor of the studied channel (both subtidal and intertidal areas), exhibiting a conspicuous green macroalgal bloom previously documented (Gastaldi et al. 2016 ; Fig. 1 b). This elevated macroalgal biomass is expected to provide an additional source of MPU to our study site, supplementing the production by the local Ulva spp. The tidal action notably influenced MPU density in the SAO channel, where a strong contrast in MPU density (and environmental variables) was observed between low and high tide. During high tide, the density of MPU and the magnitude of environmental variables decreased compared to low tide due to increased water volume. This, combined with the high energy associated with the flow of water from the mouth of the San Matías Gulf, causes the MPU to experience strong dilution and advection throughout the water column, resulting in a homogeneous distribution rather than stratification as expected (Aliotta et al. 2000 ; Vara & Mazio 1983 ). Studies also support this phenomenon by showing that tidal ebb and flow currents in tidal flat channels can transport up to 100% of nanoplanktonic biomass over long distances by advection (or horizontal transport, Rusch & Huettel 2000 ). The minimum densities of MPU recorded during the first and last hour of low tide show the strong influence exerted by the speed of the ebb in the initial hour and the flow in the final hour on the horizontal displacement of the MPU (Fig. 6 ). Despite the strong dilution and washing of MPU during the tidal ebb and flow, a significant portion of MPU may settle to the seafloor during the low tide period. In a study on the rate of sinking and viability of algal propagules, it was found that 25% of the propagules of the species Ulva rigida with sizes less than 15 µ, can be deposited on the bottom in a time interval of 135 minutes in turbulent conditions (Hoffmann & Camus 1989). In this study, the duration of low tide exceeds this time, and the ranges of MPU sizes recorded were between 4 and 16 µ (Supplementary Fig. 2). 5. Conclusions The present study allowed us to identify in the interior channel of San Antonio Bay the presence of MPU and the macroalgal phase in several months of the year, with a maximum density of MPU in February, which did not coincide with the occurrence of the macroalgal bloom event of December. Water temperature is the main variable associated with MPU density. This finding has ecological relevance since the increase in temperature predicted by climate change could increase the density of MPU and, consequently, the mass and frequency of green tides. The macrotidal system of San Antonio Bay has a notable influence on the spatio-temporal dynamics of MPU, with a continuous cycle of MPU production and export throughout the tidal cycles. During tidal ebb, the water exports a large part of the MPU out of the bay, while another part is retained in subtidal pools. At low tide, the current flow decreases, increasing the temperature and the release and settling of MPU to the water. During tidal flow, the increase in the velocity and volume of water dilutes the MPU, decreasing their density at high tide relative to low tide. Declarations Ethics approval and consent to participate Not applicable. Consent for publication Not applicable. Availability of data and materials The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request. Competing interests The authors declare that there are no competing interests. Authors' contributions All authors have contributed substantially to its development given their experience in the study of plankton (JFS) and the Ulva macroalga (MAN, MEB and MG) in the area. Each has contributed to the conception or design of the work or the acquisition, analysis, or interpretation of data and has critically reviewed each version. MAC actively participated in the study design and each of the sampling and processing, analyzed and interpreted the data, wrote the manuscript and evaluated comments and revisions. Acknowledgements The authors would like to thank Maite A. Barrena, Giuliana M. Burgueño, Denis N. Landette, Patricio J. Pereyra, Mariano Rosset and Guillermo M. Svendsen for their collaboration in sampling and sample processing. The support provided by the staff of the Escuela Superior de Ciencias Marinas and the Centro de Investigación Aplicada y Transferencia Tecnológica en Recursos Marinos Almirante Storni is gratefully acknowledged. Funding This study was funded by Estímulo a las Vocaciones Científicas-Consejo Interuniversitario Nacional (EVC-CIN, Buenos Aires, Argentina) awarded to M.A.C. and Explorers Club Youth Activity Fund Rising Explorers Grant (New York, USA) to M.A.C. Also was supported by Universidad Nacional del Comahue (PIN1 o4/oo7) to J.F.S and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (PIBAA-CONICET 28720210100721CO) to J.F.S. References Aliotta, S., E.N. Schnack, F.I. Isla, and G.O. Lizasoain. 2000. Desarrollo secuencial de formas de fondo en un régimen de marea macromareal. Asociación Argentina de Sedimentología 7: 95–107. [APHA] American Public Health Association. 2012. Standard methods for the examination of water and wastewater, 22nd edn, Washington. Balar, N.B., and V.A. Mantri. 2020. 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The contribution of phytoplankton degradation to chromophoric dissolved organic matter (CDOM) in eutrophic shallow lakes: Field and experimental evidence. Water Research 43 (18): 4685-4697. https://doi.org/10.1016/j.watres.2009.07.024 Supplementary Files SupplementaryMaterial.docx Cite Share Download PDF Status: Published Journal Publication published 10 Mar, 2025 Read the published version in Estuaries and Coasts → Version 1 posted Reviewers agreed at journal 20 Nov, 2024 Reviewers invited by journal 05 Nov, 2024 Editor invited by journal 04 Nov, 2024 Editor assigned by journal 23 Oct, 2024 First submitted to journal 23 Oct, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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(ESCIMAR)","correspondingAuthor":false,"prefix":"","firstName":"Maite","middleName":"Andrea","lastName":"Narvarte","suffix":""},{"id":374413278,"identity":"9a2c2b8f-42ba-4018-bf85-9e036150bc65","order_by":4,"name":"Juan Francisco Saad","email":"","orcid":"","institution":"Centro de Investigación Aplicada y Transferencia Tecnológica en Recursos Marinos Almirante Storni (CIMAS) and Escuela Superior de Ciencias Marinas (ESCIMAR)","correspondingAuthor":false,"prefix":"","firstName":"Juan","middleName":"Francisco","lastName":"Saad","suffix":""}],"badges":[],"createdAt":"2024-10-23 15:07:51","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5320006/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5320006/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s12237-025-01517-0","type":"published","date":"2025-03-10T15:57:10+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":69339105,"identity":"6df7dc73-42c1-43b1-b4e7-46191a3dd366","added_by":"auto","created_at":"2024-11-19 10:39:41","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":172792,"visible":true,"origin":"","legend":"\u003cp\u003ea) Location of the sampling site in San Antonio Bay and b) lamellar form \u003cem\u003eUlva \u003c/em\u003ethalli in the subtidal zone\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/e338faec98409f92dc5c9fa3.png"},{"id":69339421,"identity":"53ddeecc-82c8-47dd-874d-41287ed654cc","added_by":"auto","created_at":"2024-11-19 10:47:41","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":268664,"visible":true,"origin":"","legend":"\u003cp\u003ea) MPU examined under light microscopy (400X) with rose bengal stain previously fixed in formaldehyde. b) Observation with the naked eye of sporangia/gametangia located as patches on the margin, c) or a depigmentation at the margins in a mature \u003cem\u003eUlva\u003c/em\u003espp. thallus. d) View under the optical microscope (400X) of the reproductive cells of the sporangia/gametangia, cells with MPU can be observed: I) in a state of division without complete development, II) completely developed, III) in release and, IV) completely released (empty cells)\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/b72c4a4a006aac887317b926.png"},{"id":69340359,"identity":"2a42f72b-b17d-4167-888d-0e1746cbe110","added_by":"auto","created_at":"2024-11-19 10:55:41","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":127880,"visible":true,"origin":"","legend":"\u003cp\u003eAverage values (bars) ​​and standard deviations (vertical lines) obtained monthly throughout the year at low tide. a) MPU density, b) macroalgal biomass, and c) dry weight of mature (white) and immature (gray) thalli. The letters above the bars show the result of the post-hoc test between months (in bold), only for mature thalli in c, and between mature and immature thalli for each month (in red)\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/d6f299e630520a7c26ae2ea3.png"},{"id":69339425,"identity":"e04d2d37-ec59-4cac-a41f-f92fa53d1b18","added_by":"auto","created_at":"2024-11-19 10:47:41","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":60732,"visible":true,"origin":"","legend":"\u003cp\u003eAverage values (points and bars) and standard deviations (vertical lines) ​​of environmental variables obtained monthly throughout the year at low tide. a) temperature, b) dissolved oxygen, c) salinity, d) pH, e) chlorophyll-a, f) chromophoric dissolved organic matter, g) suspended solids (inorganics in grey and organic in white), and h) nutrients (ammonium in yellow, nitrate in blue, nitrite in grey and phosphate in red)\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/c7d911f9c2fa1f1aaa4a1613.png"},{"id":69340358,"identity":"9355d704-f17d-4dd0-9d27-0ae9d4054814","added_by":"auto","created_at":"2024-11-19 10:55:41","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":24411,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between MPU density and temperature throughout the annual cycle. Dots represent the measured values, the solid line represents the predicted values from the GLMM model, and the grey area indicates the confidence interval.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/262073819b940462d2637126.png"},{"id":69339422,"identity":"a8adda77-f4b0-4c75-a8b6-1c2e9d8ac357","added_by":"auto","created_at":"2024-11-19 10:47:41","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":55861,"visible":true,"origin":"","legend":"\u003cp\u003eAverage values (bars) and standard deviations (vertical lines) of MPU density obtained hourly during the low tide period. The grey solid line represents the tide height; note the influence of the ebb and flow of the tide on the initial and final times\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/bd07eb4d506a5c88cc43d033.png"},{"id":69340360,"identity":"7cecf023-f8e4-47e0-b72f-409da086c2ea","added_by":"auto","created_at":"2024-11-19 10:55:41","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":59653,"visible":true,"origin":"","legend":"\u003cp\u003eAverage values (points and bars) and standard deviations (vertical lines) ​​of environmental variables obtained hourly during the low tide period. a) temperature, b) dissolved oxygen, c) salinity, d) pH, e) chlorophyll-a, f) chromophoric dissolved organic matter, g) suspended solids (inorganics in grey and organic in white), and h) nutrients (ammonium in yellow, nitrate in blue, nitrite in grey and phosphate in red)\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/90a68b8a4f53c165f0ff258f.png"},{"id":69339113,"identity":"90f35659-c38c-4569-846c-2fcfceb77adc","added_by":"auto","created_at":"2024-11-19 10:39:41","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":12111,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between MPU density and temperature hourly during low tide period. Dots represent the measured values, the solid line represents the predicted values from the GLMM model, and the grey area indicates the confidence interval\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/66646ae48e6b23279ca1e44a.png"},{"id":69339116,"identity":"ca2df7a4-fcff-49fb-90ba-6cc3eb4ec228","added_by":"auto","created_at":"2024-11-19 10:39:41","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":8691,"visible":true,"origin":"","legend":"\u003cp\u003eAverage values (bars) and standard deviations (horizontal lines) of MPU density obtained throughout the water column at daytime high tide (white) and at night (grey). The letters above the bars show the result of the post-hoc test among depth at daytime high tide (in red) and at night (in grey)\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/9d48eab7b70c6d4f23e0b5f0.png"},{"id":69339108,"identity":"b9820b59-452b-4a60-b5d2-ae11c4a1a377","added_by":"auto","created_at":"2024-11-19 10:39:41","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":11292,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between MPU density and temperature registered at daytime and nighttime high tides. Dots represent the measured values, the solid line represents the predicted values from the GLMM model, and the grey area indicates the confidence interval\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/d0af312a34e1db249d340740.png"},{"id":78688910,"identity":"b74f9c4e-272c-4757-afc3-781a768f492f","added_by":"auto","created_at":"2025-03-17 16:06:51","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1790426,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/bd0b13fc-f6c8-4775-a14a-17df88965d75.pdf"},{"id":69339427,"identity":"d52b63dc-9fc8-4cad-a64e-40cafdb42657","added_by":"auto","created_at":"2024-11-19 10:47:42","extension":"docx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":113476,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-5320006/v1/cac0893807f872a906ecc44b.docx"}],"financialInterests":"","formattedTitle":"Seawater temperature and tidal action as modulators of Ulva spp. micropropagules density in a eutrophicated macrotidal coastal system","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003e \u003cem\u003eUlva\u003c/em\u003e Linnaeus 1753, is a cosmopolitan genus of green algae with species inhabiting all oceans and estuaries of the world. Species of \u003cem\u003eUlva\u003c/em\u003e are characterised by their opportunistic nature, including a simple thallus structure (sheet-like or tubular) and rapid nutrient uptake and growth rates, which favour its success in a wide variety of environments (Fong et al. 1996). These green macroalgae dominate coastal areas bearing wide variations in salinity, seawater temperature, and available light or nutrients (Chen et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Bews et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Regarding the latter, a high contribution of anthropogenic nitrogen often causes uncontrolled growth, forming blooms (Teichberg et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), and in extreme conditions, they can break away and float from the mats, developing green tides (Fletcher \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1996\u003c/span\u003e, Ding et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Ye et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). An increase in the eutrophication of coastal areas is expected in a near future scenario, mainly enhanced by anthropogenic pressure. This, combined with the effect of climate change (i.e. elevated temperature and carbon dioxide concentration), would increase the frequency and magnitude of macroalgal blooms (including green tides) globally, which in turn may result in habitat degradations and a more physiologically stressful environment at the community level (Worm et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Le Luherne et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; W\u0026aring;hlstr\u0026ouml;m et al. \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Ji \u0026amp; Gao \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cem\u003eUlva\u003c/em\u003e has both sexual and asexual reproduction through planktonic micropropagules, including biflagellate gametes and quadriflagellated zoospores, respectively, which are released into the environment and serve as dispersal agents (Smith \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1947\u003c/span\u003e; Hoxmark \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1975\u003c/span\u003e; Phillips \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). Studies on macroalgal blooms have been focused primarily on the macroscopic phases of this macroalgal life cycle, with less emphasis on micropropagules, probably due to the difficulty of quantifying them using traditional methods. In fact, in previous studies, the abundance of MPU was recorded only indirectly by culturing cumulative plankton samples and counting emerging seedlings (Hoffman \u0026amp; Camus \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Schories \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Granhag et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Heydt et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Huo et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Li et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Han et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Xiaoxiang et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe reproduction of \u003cem\u003eUlva\u003c/em\u003e is strongly controlled by physical conditions, such as photoperiod, temperature, light intensity, and water movement, among others (Balar \u0026amp; Mantri \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). For instance, the release of gametes increases with increasing daylight hours (L\u0026uuml;ning et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Light intensity affects the distribution of MPU in the water column with gametes (positively phototactic) in more superficial waters and zoospores in deeper waters (negatively phototactic; Jones \u0026amp; Babb \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1968\u003c/span\u003e). Warm seawater temperature favours the release of \u003cem\u003eUlva\u003c/em\u003e gametes and zoospores and their germination (Nordby \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1977\u003c/span\u003e; Song et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Given the limited mobility of the MPU, it is postulated that water movement would dilute the concentration of gametes, decreasing the probability of fertilization. On the contrary, calm waters increase the probability of encounters between gametes through positive phototactic and communication by pheromones (Brawley \u0026amp; Johnson \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1992\u003c/span\u003e). A single study has shown that water motion stimulates the gametes and zoospores release in \u003cem\u003eU. lactuca\u003c/em\u003e (Gordon \u0026amp; Brawley \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2004\u003c/span\u003e), while gamete release could be inhibited under calmer conditions (Stratmann et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1996\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSan Antonio Bay is a coastal marine area in northern Argentine Patagonia. Tidal channels characterize the intertidal zone of the bay. The tidal channel near San Antonio Oeste is a eutrophic system due to the extra input of organic and inorganic nutrients from residential wastewater (Martinetto et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). As a consequence, there is an increase in the biomass of macroalgae, mainly of \u003cem\u003eUlva\u003c/em\u003e species, which form frequent green macroalgal blooms from late winter to early summer (Teichberg et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Gastaldi et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Becherucci et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Coinciding with the time of most significant coverage of \u003cem\u003eUlva\u003c/em\u003e in the subtidal and intertidal zones of San Antonio Bay, Saad et al. (\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) found a high concentration of MPU during low tide in the spring months, constituting up to 95% of all planktonic cells. This finding is the first record of such high densities of macroalgal MPU.\u003c/p\u003e \u003cp\u003eThe main objective of this study is to evaluate the MPU density dynamics in the SAB. For this, we 1) describe the variation of MPU density over a year and during the tide cycle, 2) describe the variation of MPU density during the whole low tide period, and 3) contrast the distribution of MPU between a nocturnal and a diurnal high tide. Additionally, we measured physical and chemical variables in each sampling to explore its relationships with MPU variation. We hypothesize that (1) MPU production increases during the warmer months of the year due to the increased macroalgal biomass and other ambient variables such as seawater temperature and nutrients, (2) MPU density is modulated by the tides, increasing their concentration during low tide, while diluting them during high tide, and (3) MPU density is modulated by light, which increases their concentration at daytime.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Study site\u003c/h2\u003e \u003cp\u003eSan Antonio Bay (SAB) is a semi-desert coastal system with a semi-diurnal macrotidal regime featuring average amplitudes of 6.26 m and maximum amplitudes of 9.24 m (Naval Hydrography Service, SHN). SAB exchanges a significant volume of water with the San Mat\u0026iacute;as Gulf in each tidal cycle, and its semicircular shape protects against the high energy of tides and waves, allowing the formation of extensive tidal flats and channels (Carbone et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). It is a hypersaline system due to the low average precipitation (250 mm per year). The average annual atmospheric temperature is 15.1\u0026deg;C, with extreme temperatures in July (austral winter, minimum of -7.7\u0026deg;C) and February (austral summer, maximum of 41.4\u0026deg;C, respectively). The mean annual air humidity is 57%, and the average annual wind speed is 18 km h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, reaching an average of 25 km h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in spring-summer, with the predominant wind direction from the west (Genchi et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Gonz\u0026aacute;lez et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe study was carried out in the channel bordering San Antonio Oeste (40\u0026deg;43'37.1\"S 64\u0026deg;56'52.6\" W; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The samples were collected in the subtidal area covering 80 m\u003csup\u003e2,\u003c/sup\u003e characterized by a low tide of 0.5 m and a high tide of up to 5.3 m deep. Since the sampling site is located inland of the bay, it remains uncovered for approximately 5 hours (Gastaldi et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Supplementary Fig.\u0026nbsp;1), providing an ideal opportunity for experimentation. In the bay, both \u003cem\u003eUlva\u003c/em\u003e spp. tubular-shaped thalli in the intertidal and lamellar-shaped thalli in the subtidal were observed. We only worked with the lamellar form \u003cem\u003eUlva\u003c/em\u003e thalli (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAlthough in previous studies on the SAB, \u003cem\u003eUlva lactuca\u003c/em\u003e has been mentioned as the species responsible for the green blooms, its taxonomic identity has not yet been determined, so it may have been a complex of several \u003cem\u003eUlva\u003c/em\u003e species. Given that the taxonomic determination is beyond the objective of the present study, we use the term \u003cem\u003eUlva\u003c/em\u003e spp. to refer to the complex of species responsible for producing green blooms and micropropagules in the SAB.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Sample collection\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1. Annual cycle sampling\u003c/h2\u003e \u003cp\u003eA monthly sampling was performed from October 2021 to September 2022 to evaluate the temporal variation of the density of MPU. Seawater samples were collected in triplicate in 250 ml bottles during the low tide and neap tides (Service of Naval Hydrography, SHN). The samples were fixed with formaldehyde to a final concentration of 0.4% and stored in a cool, dark place until analysis. For counting (expressed in cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), the Uterm\u0026ouml;hl (\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e1958\u003c/span\u003e) method was applied, using sedimentation chambers in an inverted microscope (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). According to Venrick (\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e1978\u003c/span\u003e), the counting error was estimated in a random number of fields, with a maximum error of 20%.\u003c/p\u003e \u003cp\u003eMacroalgae biomass data were collected at the same time as the MPU sampling. Twenty quadrats (25x25 cm) were randomly thrown into the field in 200 m\u003csup\u003e2\u003c/sup\u003e, within which all \u003cem\u003eUlva\u003c/em\u003e spp. individuals were collected. The samples were transported cold, and in the laboratory, they were dried in an oven at 60 \u0026ordm;C for \u0026gt;\u0026thinsp;72 hours. The dry weight (g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) was determined by the gravimetric method. Additionally, 30 thalli were randomly collected in the same area and stored under cold and humid conditions until processing to determine the weight of mature and immature thalli. To determine the state of maturity, the thalli were thawed and subsequently inspected with a stereoscope to identify the reproductive structures (sporangia/gametangia), which were visualized as a dark spot of a cellular layer capable of detaching or depigmentation at the margins (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb,d). After inspection, the thalli were dried in an oven at 60\u0026deg;C for \u0026gt;\u0026thinsp;72 hours, and the biomass was determined by the gravimetric method (in terms of dry weight, in g).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe \u003cem\u003ein situ\u003c/em\u003e seawater temperature was recorded with a digital thermometer, and water samples were collected for subsequent laboratory analysis. Salinity, pH, and dissolved oxygen (DO, mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) were measured immediately (\u0026lt;\u0026thinsp;1 h of taken) with an Atlas Scientific multiparameter probe (kits EC-10, 103P, and 103DX). To determine chlorophyll-a (Chla-a) and suspended solids, water samples were filtered through 45 mm glass fiber filters (0.7 \u0026micro;m pore size). The extraction of Chla-a was carried out by incubating the filters with 96% ethanol for 12 hours, and the measurements were taken at absorbances of 665 and 750 nm with a Persee T7S spectrophotometer, applying a correction for phaeopigments by adding HCL 1N. The concentration was expressed in \u0026micro;g l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e following Marker et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1980\u003c/span\u003e). The filters were dried in an oven at 60\u0026deg;C to obtain the suspended solids, and the total concentration was obtained by applying the gravimetric method (APHA 1998). The inorganic fraction was calculated after the combustion of the filters at 500\u0026deg;C in a muffle for 3 hours (the concentrations were expressed in mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Chromophoric dissolved organic matter (CDOM, mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was measured from the filtered water by UV-visible spectral analysis (200\u0026ndash;800 nm) with spectrophotometer at an absorbance of 350 nm according to Helms et al. (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and nutrients (nitrate, nitrite, ammonium y phosphate, mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) using standard colorimetric methods with a spectrophotometer according to standardized protocols (Strickland \u0026amp; Parsons 1972).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2. Tide cycle sampling\u003c/h2\u003e \u003cp\u003eTwo different samplings were carried out to evaluate the variation of MPU density during a tidal cycle at the peak of MPU (December; Saad et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The first was carried out during the five-hour period of diurnal low tide. To do so, 250 ml of water was collected in bottles every hour from 7:40 a.m. to 12:40 p.m. The second sampling was conducted during two consecutive high tides, one at day and the other at night. Water samples were collected from the water column at four depths (0 m, 1.5 m, 3 m, and 4.5 m) using a 5 L Van Dorn bottle. The water characteristics (temperature, salinity, pH, and other reported parameters) were also measured and determined during low tide and at each depth during high tide samplings (daytime and nighttime). These measurements followed the protocols described in the \"Annual cycle sampling\" section.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Statistical analysis\u003c/h2\u003e \u003cp\u003eAll statistical analyses were performed with R program version 3.6.1 (R Core Team \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Differences in MPU density between months, hours, and high tides were determined using the Bonferroni adjusted 'emmeans' function from the emmeans package (Lenth et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) with a negative binomial error distribution and log link function determined from a Generalized Linear Model with a 95% confidence interval (GLM: function 'glm.nb', package MASS; Ripley et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Supplementary Tables\u0026nbsp;1). Differences in macroalgal biomass and mature thalli's dry weight were determined similarly but with a gamma error distribution and log link function (Supplementary Tables\u0026nbsp;2). To evaluate the effect of environment variables on MPU density, we used a generalized linear mixed model with the distribution mentioned above (GLMM, function 'glmmTMB', glmmTMB package; Brooks et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) and with the \u0026ldquo;months\u0026rdquo;, \u0026ldquo;hours\u0026rdquo; and \u0026ldquo;depth\u0026rdquo; as random factors according to models accounted the annual variations, low tide and day tide variations respectively. Possible proxy variables of the MPU, such as total and organic suspended solids, DO, and Chl-a, were discarded, leaving the following predictor variables available: inorganic suspended solids, chromophoric dissolved organic matter, temperature, salinity, nitrate, nitrite, ammonium and phosphate for the construction of the models. Due to possible correlations between variables, Pearson rank correlation coefficients were calculated for all environmental variables (Supplementary Table\u0026nbsp;3). Of the correlated ones, only one of the variables was selected to the extent possible. Model assumptions were verified by plotting the residuals against the fitted values ​​of the global model and by a QQ plot using the DHARMa (Hartig \u0026amp; Harting 2022) and performance (L\u0026uuml;decke et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) packages.\u003c/p\u003e \u003cp\u003eThe models (GLMM) were evaluated with information theory procedures (Burnham \u0026amp; Anderson \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). The Akaike information criterion corrected for small sample size (AICc) was calculated for each model. Model comparison was performed by evaluating the DAICc, defined as the difference between the lowest AICc value (i.e., the best of the suitable models) and the AICc of all other models, as well as the AICc weight. The AICc weight of a model (AICc Weight) indicates the relative probability that the specific model is the best of all the models. In cases where the AICc weight of the most appropriate model did not exceed a value of 0.7, we evaluated the support of the predictor variables. Parameter estimates were calculated using model-averaged parameter estimates based on the AICc weights of all candidate models, and we also calculated the 95% confidence interval limits of the parameter estimates.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Annual cycle\u003c/h2\u003e \u003cp\u003eMPU density varied between months (GLM: LRS\u0026thinsp;=\u0026thinsp;1924.9, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;36). MPU density was maximum in February (33983\u0026thinsp;\u0026plusmn;\u0026thinsp;9553 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and higher in November, December, and January (8048\u0026thinsp;\u0026plusmn;\u0026thinsp;654 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 5699\u0026thinsp;\u0026plusmn;\u0026thinsp;528 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 2922\u0026thinsp;\u0026plusmn;\u0026thinsp;448 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e respectively) than the cold months (from March to October, 239\u0026thinsp;\u0026plusmn;\u0026thinsp;285 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Macroalgae biomass varied between months (GLM: LRS\u0026thinsp;=\u0026thinsp;139.7, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;240), being higher in warm months (November to April, 53.4\u0026thinsp;\u0026plusmn;\u0026thinsp;38.4 g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) than cold months (May to October, 21.5\u0026thinsp;\u0026plusmn;\u0026thinsp;14.4 g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The dry weight of mature thalli varied between months (GLM: LRS\u0026thinsp;=\u0026thinsp;31.2, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;98). Only December (7.59\u0026thinsp;\u0026plusmn;\u0026thinsp;3.23 g) was higher than May and June (2.32\u0026thinsp;\u0026plusmn;\u0026thinsp;3.24 g; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), while in the remaining months, it was constant (4.40\u0026thinsp;\u0026plusmn;\u0026thinsp;4.13 g). The biomass of mature thalli was greater than that of immature thalli in all months (GLM: LRS\u0026thinsp;=\u0026thinsp;62.8, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;347).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe temperature, salinity, dissolved oxygen, and pH exhibited marked seasonal fluctuations, with higher values during the warm months (October to March; 25.36\u0026thinsp;\u0026plusmn;\u0026thinsp;2.32\u0026deg;C, 34.76\u0026thinsp;\u0026plusmn;\u0026thinsp;0.67; 17.33\u0026thinsp;\u0026plusmn;\u0026thinsp;2.96 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 8.67, respectively) and lower values during the cold months (April to September; 14.03\u0026thinsp;\u0026plusmn;\u0026thinsp;2.85\u0026deg;C, 31.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.29, 13.05\u0026thinsp;\u0026plusmn;\u0026thinsp;2.58 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 7.79, respectively; Supplementary Table\u0026nbsp;4.I; Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea,d). Chl-a described two peaks that coincided with those of MPU. In November, the Chl-a concentration reached 18.9\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4 \u0026micro;g l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and in February, it peaked at 44.3\u0026thinsp;\u0026plusmn;\u0026thinsp;12.5 \u0026micro;g l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Throughout the remaining months, the Chl-a values remained below 12.9 \u0026micro;g l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee). CDOM revealed higher values during January and February (0.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.19 and 0.58\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16 \u0026micro;g l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively), coinciding with the maximum MPU density (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). The organic suspended solids showed two maxima, one in December of 9.33\u0026thinsp;\u0026plusmn;\u0026thinsp;0.58 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which coincides with the maximum macroalgal biomass, and one in February of 8.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which coincided with the maximum of MPU; the remaining months presented values lower than 6.34 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg). For nutrients, nitrate concentration was maximum and nitrite minimum, both with constant values ​​throughout the year (average concentration 2.910\u0026thinsp;\u0026plusmn;\u0026thinsp;0.797 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 0.065\u0026thinsp;\u0026plusmn;\u0026thinsp;0.046 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively). Ammonium and phosphate had intermediate concentrations with peaks in December (1.540\u0026thinsp;\u0026plusmn;\u0026thinsp;0.409 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 2.010\u0026thinsp;\u0026plusmn;\u0026thinsp;0.095 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively) that coincided with the first peak of MPU density and the maximum of macroalgal biomass (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegarding the role of environmental variables in MPU annual variation, the global model (MPU\u0026thinsp;~\u0026thinsp;temperature\u0026thinsp;+\u0026thinsp;nitrate\u0026thinsp;+\u0026thinsp;ammonium\u0026thinsp;+\u0026thinsp;phosphate\u0026thinsp;+\u0026thinsp;ISS + (1|month)) accounted for a dispersion value of 0.83. Model validation for all dependent variables tested indicated normality and homogeneity of residuals. The best model describing the MPU variation included temperature as an explanatory variable (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e,\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The best model accounted for 97% of the variation. The MPU increased exponentially with temperature starting at 25\u0026deg;C, reaching the maximum value at 28.9\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\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\u003eSummary of model-selection results for models explaining variations in MPU during low tide with temperature (T), inorganic suspended solid (ISS), and the nutrients nitrate (N), ammonium (A), and phosphate (P). Models are listed in decreasing order of importance.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\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=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAssociated hypothesis\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCandidate models\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003edf\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAICc\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eΔAICc\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eWeight\u003csub\u003eAICc\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eMPU varied only by T\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eT\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e4\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e538.9\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e0.00\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003e0.53\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMPU varied by T and ISS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT\u0026thinsp;+\u0026thinsp;ISS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e539.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.41\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMPU varied by T and nutrients\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT\u0026thinsp;+\u0026thinsp;N\u0026thinsp;+\u0026thinsp;A\u0026thinsp;+\u0026thinsp;P\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e545.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.025\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNeither of the variables is associated with the MPU\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNull model\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e545.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.019\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMPU varied by ISS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eISS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e546.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e7.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.011\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGlobal model\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT\u0026thinsp;+\u0026thinsp;ISS\u0026thinsp;+\u0026thinsp;N\u0026thinsp;+\u0026thinsp;A\u0026thinsp;+\u0026thinsp;P\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e547.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e8.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.008\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMPU varied by nutrients\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN\u0026thinsp;+\u0026thinsp;A\u0026thinsp;+\u0026thinsp;P\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e550.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e11.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.001\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMPU varied by ISS and nutrients\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eISS\u0026thinsp;+\u0026thinsp;N\u0026thinsp;+\u0026thinsp;A\u0026thinsp;+\u0026thinsp;P\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e553.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e14.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.00\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\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\u003eParameter estimates, standard errors (ES) and 95% confidence interval limits for explanatory variables describing variation in MPU during low tide. Explanatory variables excluding zero are in bold.\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=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" 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\"\u003e \u003cp\u003eExplanatory variable\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEstimate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSE\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003eConfidence interval\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLower\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eUpper\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIntercept\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-1.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eTemperature\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003e0.25\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003e0.06\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.12\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003e0.39\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eISS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e-0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.02\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\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Tidal cycle\u003c/h2\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1. Low tide\u003c/h2\u003e \u003cp\u003eThe density of MPU increased during the low tide period (GLM: LRS\u0026thinsp;=\u0026thinsp;94.88; p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;18). Start with a minimum of 559\u0026thinsp;\u0026plusmn;\u0026thinsp;125 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 7:40 h and reach maximum values ​​at 10:40 and 11:40 h of 8081\u0026thinsp;\u0026plusmn;\u0026thinsp;4410 and 10511\u0026thinsp;\u0026plusmn;\u0026thinsp;7345 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. There was a significant decrease in MPU density in the last hour (12:40 h) to 2839\u0026thinsp;\u0026plusmn;\u0026thinsp;540 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe environmental variables also showed a pronounced increasing trend over time (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e; Supplementary Table\u0026nbsp;4.II). At low tide, temperature increased by 9.3 \u0026ordm;C, dissolved oxygen by 14.92 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, salinity by 2.9, and pH by 0.52 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea,d). Chl-a concentrations started at 3.94\u0026thinsp;\u0026plusmn;\u0026thinsp;0.73 and peaked at 5.63\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38 \u0026micro;g l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ee). Similarly, CDOM started at 0.197\u0026thinsp;\u0026plusmn;\u0026thinsp;0.035 and peaked at 0.577\u0026thinsp;\u0026plusmn;\u0026thinsp;0.030 at 11:40. Organic suspended solids began at 4 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the initial hour and peaked at 9 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 11:40 h, coinciding with the minimum and maximum MPU density (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ef,g). Within nutrients, nitrate experienced the most pronounced increase (3.63 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), while nitrite, ammonium and phosphate also rose but to a lesser extent (0.07, 0.62, and 0.23 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively; Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eh).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegarding the role of environmental variables in low tide MPU variation, the global model (MPU\u0026thinsp;~\u0026thinsp;temperature\u0026thinsp;+\u0026thinsp;nitrate\u0026thinsp;+\u0026thinsp;ammonium\u0026thinsp;+\u0026thinsp;phosphate\u0026thinsp;+\u0026thinsp;ISS + (1|hours)) accounted for a dispersion value of 1.8. Model validation for all dependent variables tested indicated normality and homogeneity of residuals. When the last time is removed, the best model is temperature. When not removed, the null model (Weight AICc\u0026thinsp;=\u0026thinsp;0.66) followed by temperature (Weight AICc\u0026thinsp;=\u0026thinsp;0.21) were the models that best explained the MPU variation during low tide. However, according to multiple inferences, neither measured variable explains the MPU density (CI temperature = [ -0.29; 0.43]). The null model accounted for 80% of the variation. Although MPU density increases with temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e), in the last hour of low tide, where a temperature peak is observed, MPU density decreases dramatically (observed at 12:40 h in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2. High tide\u003c/h2\u003e \u003cp\u003eConsidering daytime versus nighttime during high tide, the MPU density was lower than at low tide (78\u0026thinsp;\u0026plusmn;\u0026thinsp;96 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 4665\u0026thinsp;\u0026plusmn;\u0026thinsp;4678 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively; GLM: LRS\u0026thinsp;=\u0026thinsp;139.42, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;42). The MPU density at daytime was higher than at nighttime (128\u0026thinsp;\u0026plusmn;\u0026thinsp;115 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 25\u0026thinsp;\u0026plusmn;\u0026thinsp;18 cel ml\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively; GLM: LRS\u0026thinsp;=\u0026thinsp;27.31, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;24; Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). The MPU density varied among depths in both high tides (daytime: GLM: LRS\u0026thinsp;=\u0026thinsp;4.48, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;12 and nighttime: GLM: LRS\u0026thinsp;=\u0026thinsp;11.34, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, n\u0026thinsp;=\u0026thinsp;12). In the daytime, the MPU density at 3 m depth was higher than at the bottom and the surface. In the nighttime the superficial MPU density was higher than at 1.5 m. Most physical and chemical variables experienced a decrease concerning low tide. Temperature decreased by 0.51 \u0026ordm;C, salinity by 1.33, OSS by 1.67 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, ISS by 2.75 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, Chl-a by 2.33 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, CDOM by 0.27 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, nitrate by 2.37 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, nitrite by 0.06 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, ammonium by 0.64 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, phosphate by 0.21 mg l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Supplementary Table\u0026nbsp;4.III).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe global model (MPU\u0026thinsp;~\u0026thinsp;temperature\u0026thinsp;+\u0026thinsp;pH\u0026thinsp;+\u0026thinsp;ISS\u0026thinsp;+\u0026thinsp;ammonium\u0026thinsp;+\u0026thinsp;phosphate + (1|depth)) accounted for a dispersion value of 1.4. Model validation for all dependent variables tested indicated normality and homogeneity of residuals. Temperature (Weight AICc\u0026thinsp;=\u0026thinsp;0.53) followed by temperature and ISS (Weight AICc\u0026thinsp;=\u0026thinsp;0.28) were the models that best explained the MPU variation during high tide. However, according to multiple inferences, only temperature explained the MPU density (CI temperature = [ 0.89; 2.12], CI ISS = [-0.28; 0.05]; Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). This model accounted for 54% of the variation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eWe found a temporal variation of MPU densities guided mainly by seawater temperature and tidal action. To our knowledge, this study represents the first comprehensive monthly monitoring of MPU density during a year and through direct counting. Although indirect methods (for example, cultivation) could help to determine the proportion of MPU that is converted into biomass, our direct counting results show a more realistic amount of MPU that circulates through the system and participates in biogeochemical cycles, mainly in summer when the density was notably high (Maggs \u0026amp; Callow \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Annual cycle\u003c/h2\u003e \u003cp\u003eField investigations have primarily focused on recording MPU density and environmental variables during the green tide events. For example, in the Yellow Sea, a high density of MPU was recorded during the world's largest green tide event (Huo et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). However, our study observed a MPU density peak after the macroalgal biomass peak. Therefore, at least in our study site, a high MPU density is not necessarily related to high macroalgal biomass. It could be thought that the observed pattern results from an important bias not to include information on macroalgae biomass in the intertidal zone. However, it has been shown that the cover of \u003cem\u003eUlva\u003c/em\u003e spp. it does not vary between the intertidal and subtidal zones (Gastaldi et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe strong association observed between MPU density and temperature and the coincidence between their maxima in February evidences the stimulating effect of high temperatures on the release of MPU. Nordby (\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1977\u003c/span\u003e) and Song et al. (\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) also found a strong dependence between these variables. On the other hand, low temperature and luminosity during the cold months have been shown to limit the release and germination of propagules, as well as the growth of seedlings (Steffensen et al. 1976, L\u0026uuml;ning et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Lyalina \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). This would explain the low density of propagules and macroalgal biomass observed during the autumn and winter (with temperatures of 10.9 to 24\u0026deg;C). Despite this, the propagules deposited in the sediment in all seasons can survive for up to 10 months under simulated winter conditions, germinating again when conditions are favourable (Schories \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Santelices et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). In this sense, it is highly plausible that a substantial percentage of \u003cem\u003eUlva\u003c/em\u003e spp. comes from an \u0026ldquo;MPU bank\u0026rdquo; accumulated in the previous months, with February possibly contributing the most to these deposits. In this context, our findings highlight the importance of future research evaluating the effect of increasing temperature in a climate change scenario on the increase in biomass and frequency of \u003cem\u003eUlva\u003c/em\u003e species (Ji \u0026amp; Gao \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe sensitivity of MPU to variations in nutrient concentrations is greater than that of adult thalli, mainly due to phosphate limitation (Sousa et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). While we did not find a significant relationship between MPU and nutrients, the high concentration of nitrates in the channel is well known (Martinetto et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2010\u003c/span\u003e, Teichberg et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2010\u003c/span\u003e, Becherucci et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e, and this study), and therefore, these nutrients could regulate the production of MPU throughout the year in San Antonio Bay. It is essential to mention that the range of nutrient concentrations and temperatures estimated for optimal growth of \u003cem\u003eUlva\u003c/em\u003e thalli in previous studies are coincident with those recorded here (Dan et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Therefore, although these variables could limit the production of MPU at certain times of the year (Liu et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), they would not limit the germination and growth of adult thalli in San Antonio Bay.\u003c/p\u003e \u003cp\u003eThe strong correlation between Chla-a and MPU density indicates that a significant portion of Chla-a is due to the MPU. This phenomenon was previously observed by Saad et al. (\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) for San Antonio Bay. Currently, no precedent shows that macroalgal micropropagules dominate (at least during low tide) practically the entire phytoplankton assemblage in coastal systems. Furthermore, given that the maximum MPU densities recorded from November to February are comparable with those found in situations of microalgae blooms (Delgado \u0026amp; Alcarraz 1999; Place et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), we can define that the MPU in San Antonio Bay are capable to produce \u0026ldquo;merophytoplankton blooms\u0026rdquo;. Additionally, the strong correlation between dissolved oxygen and MPU shows the vital contribution of MPU to dissolved oxygen concentrations in San Antonio Bay through photosynthesis. This contribution is particularly noteworthy during the time of year when biological oxygen demand increases due to the decomposition process of adult \u003cem\u003eUlva\u003c/em\u003e thalli (Wallace et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Tidal cycle\u003c/h2\u003e \u003cp\u003eWe found strong changes in densities of MPU within the tidal cycle that are consistent with what we proposed in hypothesis 2. The increase in the density of MPU during low tide could be attributed to two phenomena. First, a biological phenomenon, characterized by a constant rate of MPU production by macroalgae. Second, a physical phenomenon suggests an accumulation of MPU from the innermost sectors of the bay and intertidal zone during the low tides. The biological phenomenon is supported by studies of cultures with \u003cem\u003eUlva\u003c/em\u003e species that have shown that elevated levels of temperature, radiation, and nutrients in seawater greatly facilitate the production of MPU (Nordby \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1977\u003c/span\u003e; L\u0026uuml;ning et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Song et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Additionally, laboratory experiments revealed that 1 cm\u003csup\u003e2\u003c/sup\u003e thallus fragments (single cell layer) of \u003cem\u003eUlva prolifera\u003c/em\u003e, can release up to 6.62 x 10\u003csup\u003e6\u003c/sup\u003e propagules after 48 h under optimal conditions (Zhang et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). The observed increase in temperature, the slight increase in nutrients, and, probably, the intense irradiation during low tide make it highly plausible that the progressive increase in MPU density is fundamentally due to a constant rate of in situ production by macroalgae.\u003c/p\u003e \u003cp\u003eThe physical phenomenon refers to the process of continuous drainage of water from higher parts of the channel during low tide, which may be enriched with MPU and contribute significantly to the MPU stock. During the tide cycle sampling, \u003cem\u003eUlva\u003c/em\u003e spp. covered a large portion of the seafloor of the studied channel (both subtidal and intertidal areas), exhibiting a conspicuous green macroalgal bloom previously documented (Gastaldi et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). This elevated macroalgal biomass is expected to provide an additional source of MPU to our study site, supplementing the production by the local \u003cem\u003eUlva\u003c/em\u003e spp.\u003c/p\u003e \u003cp\u003eThe tidal action notably influenced MPU density in the SAO channel, where a strong contrast in MPU density (and environmental variables) was observed between low and high tide. During high tide, the density of MPU and the magnitude of environmental variables decreased compared to low tide due to increased water volume. This, combined with the high energy associated with the flow of water from the mouth of the San Mat\u0026iacute;as Gulf, causes the MPU to experience strong dilution and advection throughout the water column, resulting in a homogeneous distribution rather than stratification as expected (Aliotta et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Vara \u0026amp; Mazio \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e1983\u003c/span\u003e). Studies also support this phenomenon by showing that tidal ebb and flow currents in tidal flat channels can transport up to 100% of nanoplanktonic biomass over long distances by advection (or horizontal transport, Rusch \u0026amp; Huettel \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). The minimum densities of MPU recorded during the first and last hour of low tide show the strong influence exerted by the speed of the ebb in the initial hour and the flow in the final hour on the horizontal displacement of the MPU (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDespite the strong dilution and washing of MPU during the tidal ebb and flow, a significant portion of MPU may settle to the seafloor during the low tide period. In a study on the rate of sinking and viability of algal propagules, it was found that 25% of the propagules of the species \u003cem\u003eUlva rigida\u003c/em\u003e with sizes less than 15 \u0026micro;, can be deposited on the bottom in a time interval of 135 minutes in turbulent conditions (Hoffmann \u0026amp; Camus 1989). In this study, the duration of low tide exceeds this time, and the ranges of MPU sizes recorded were between 4 and 16 \u0026micro; (Supplementary Fig.\u0026nbsp;2).\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThe present study allowed us to identify in the interior channel of San Antonio Bay the presence of MPU and the macroalgal phase in several months of the year, with a maximum density of MPU in February, which did not coincide with the occurrence of the macroalgal bloom event of December.\u003c/p\u003e \u003cp\u003eWater temperature is the main variable associated with MPU density. This finding has ecological relevance since the increase in temperature predicted by climate change could increase the density of MPU and, consequently, the mass and frequency of green tides.\u003c/p\u003e \u003cp\u003eThe macrotidal system of San Antonio Bay has a notable influence on the spatio-temporal dynamics of MPU, with a continuous cycle of MPU production and export throughout the tidal cycles. During tidal ebb, the water exports a large part of the MPU out of the bay, while another part is retained in subtidal pools. At low tide, the current flow decreases, increasing the temperature and the release and settling of MPU to the water. During tidal flow, the increase in the velocity and volume of water dilutes the MPU, decreasing their density at high tide relative to low tide.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that there are no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors' contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors have contributed substantially to its development given their experience in the study of plankton (JFS) and the \u003cem\u003eUlva\u003c/em\u003e macroalga (MAN, MEB and MG) in the area. Each has contributed to the conception or design of the work or the acquisition, analysis, or interpretation of data and has critically reviewed each version. MAC actively participated in the study design and each of the sampling and processing, analyzed and interpreted the data, wrote the manuscript and evaluated comments and revisions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to thank Maite A. Barrena, Giuliana M. Burgueño, Denis N. Landette, Patricio J. Pereyra, Mariano Rosset and Guillermo M. Svendsen for their collaboration in sampling and sample processing. The support provided by the staff of the Escuela Superior de Ciencias Marinas and the Centro de Investigación Aplicada y Transferencia Tecnológica en Recursos Marinos Almirante Storni is gratefully acknowledged.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was funded by Estímulo a las Vocaciones Científicas-Consejo Interuniversitario Nacional (EVC-CIN, Buenos Aires, Argentina) awarded to M.A.C. and Explorers Club Youth Activity Fund Rising Explorers Grant (New York, USA) to M.A.C. Also was supported by Universidad Nacional del Comahue (PIN1 o4/oo7) to J.F.S and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) (PIBAA-CONICET 28720210100721CO) to J.F.S.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAliotta, S., E.N. Schnack, F.I. Isla, and G.O. Lizasoain. 2000. 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Liu, G. Zhu, and B. Qin. 2009. The contribution of phytoplankton degradation to chromophoric dissolved organic matter (CDOM) in eutrophic shallow lakes: Field and experimental evidence. \u003cem\u003eWater Research\u003c/em\u003e 43 (18): 4685-4697. https://doi.org/10.1016/j.watres.2009.07.024\u003c/li\u003e\n\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":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"estuaries-and-coasts","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"esco","sideBox":"Learn more about [Estuaries and Coasts](https://www.springer.com/journal/12237)","snPcode":"12237","submissionUrl":"https://www.editorialmanager.com/esco/","title":"Estuaries and Coasts","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"green tides, macroalgal biomass, eutrophication, propagules, coastal area, Patagonia Argentina","lastPublishedDoi":"10.21203/rs.3.rs-5320006/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5320006/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMicropropagules are the reproductive and dispersal means of macroalgae, often linked to green macroalgal blooms in eutrophicated coastal areas. In San Antonio Bay (North Patagonia, Argentina), increased nutrients have led to \u003cem\u003eUlva\u003c/em\u003e spp. blooms in spring and early summer, coinciding with high \u003cem\u003eUlva\u003c/em\u003e spp. micropropagules (MPU) density at low tide. This study aimed to describe the variation in MPU densities throughout a year and in a tidal cycle\u0026nbsp;and their relationship with environmental variables. For this, MPU density, macroalgal biomass, weight of mature and immature thalli, and seawater physical and chemical variables were determined: 1) monthly for a year at low tide, 2) during a tidal cycle at one-hour intervals covering the low tide period (approx. five hours), and 3) at different depths in the water column during daytime and nighttime high tides. Maximum MPU density (33983±9553 cel ml\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e \u003c/sup\u003eoccurred in February, while macroalgal biomass peaked in December. MPU density, seawater temperature, salinity, chlorophyll-a, and nutrients increased during low tide but decreased at high tide, with no evidence of vertical stratification. MPU density was positively associated with seawater temperature during low tide and throughout the year. We conclude that MPU variation is associated with seawater temperature annually and with tidal action daily. High MPU densities during summer raise chlorophyll and dissolved oxygen levels, while tidal flow dilutes and exports MPU. These results provide insights into the dynamics of the dispersal phase of an opportunistic and globally distributed green algal genus for the first time.\u003c/p\u003e","manuscriptTitle":"Seawater temperature and tidal action as modulators of Ulva spp. micropropagules density in a eutrophicated macrotidal coastal system","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-11-19 10:39:36","doi":"10.21203/rs.3.rs-5320006/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-11-20T08:12:04+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-11-05T13:58:43+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Estuaries and Coasts","date":"2024-11-05T00:13:28+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-10-23T15:10:20+00:00","index":"","fulltext":""},{"type":"submitted","content":"Estuaries and Coasts","date":"2024-10-23T11:04:49+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"estuaries-and-coasts","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"esco","sideBox":"Learn more about [Estuaries and Coasts](https://www.springer.com/journal/12237)","snPcode":"12237","submissionUrl":"https://www.editorialmanager.com/esco/","title":"Estuaries and Coasts","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"93f93c82-1555-4ee9-922d-40ae2e80de47","owner":[],"postedDate":"November 19th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-03-17T15:59:57+00:00","versionOfRecord":{"articleIdentity":"rs-5320006","link":"https://doi.org/10.1007/s12237-025-01517-0","journal":{"identity":"estuaries-and-coasts","isVorOnly":false,"title":"Estuaries and Coasts"},"publishedOn":"2025-03-10 15:57:10","publishedOnDateReadable":"March 10th, 2025"},"versionCreatedAt":"2024-11-19 10:39:36","video":"","vorDoi":"10.1007/s12237-025-01517-0","vorDoiUrl":"https://doi.org/10.1007/s12237-025-01517-0","workflowStages":[]},"version":"v1","identity":"rs-5320006","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5320006","identity":"rs-5320006","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}