Spring abundance, biomass and sizes of appendicularians between open sea MPA Namuncurá/ Burdwood Bank and the adjacent coastal area, Southwest Atlantic Ocean: Are they a key link in the trophic web?

Spring abundance, biomass and sizes of appendicularians between open sea MPA Namuncurá/ Burdwood Bank and the adjacent coastal area, Southwest Atlantic Ocean: Are they a key link in the trophic web? | 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 Spring abundance, biomass and sizes of appendicularians between open sea MPA Namuncurá/ Burdwood Bank and the adjacent coastal area, Southwest Atlantic Ocean: Are they a key link in the trophic web? Nadia Alves, Mariela Spinelli, Jacobo Martin, Andrea Malits, Fabiana Capitanio This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3553555/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 15 Jan, 2025 Read the published version in Polar Biology → Version 1 posted 10 You are reading this latest preprint version Abstract Appendicularians are recognized as one of the most abundant mesozooplankton groups in numerous pelagic environments and they play a crucial role in marine ecosystems as they bridge the gap between primary producers and higher trophic levels in the food webs. Zooplankton samples were collected during three spring oceanographic surveys conducted in 2014, 2015, and 2016, in the coastal zone (Staten Island) and oceanic zone (Namuncurá-Banco Burdwood Marine Protected Area). Our study focuses on a comparative analysis of species composition, density, biomass, and maturity stages of appendicularia between these oceanic and coastal regions, which are marked by distinct physical and biological attributes. Two species of appendicularians were found in the study area, Oikopleura fusiformis and Fritillaria borealis , the former being the dominant. Their distribution was different because F. borealis was mainly concentrated in the coastal zone while O. fusiformis was consistently recorded in both zones. Chlorophyll-a concentrations were found to be higher in the coastal zone than in the oceanic zone. These higher concentrations were accompanied by higher densities of O. fusiformis in that area. The surface current velocity seems to reflect the total phytoplankton biomass being higher in the oceanic zone. On the other hand, the temperature for the marine protected area was lower which could be related to the larger sizes of the appendicularians in that zone. Baseline data of the species in protected areas and surrounding areas is essential to contribute to the stakeholders and advise on future changes that translate into regional and global processes. larvaceans tunicate population structure environmental conditions Staten Island Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction The small planktonic appendicularians are circumglobal tunicates considered one of the most abundant mesozooplankton groups in many pelagic environments. They inhabit mainly coastal rather than oceanic environments and occupy an important trophic position in marine food webs (Lindsay and Williams, 2010 ; Capitanio et al. 2018 ). Their role in the matter flow within the water column is crucial since most species can filter and concentrate a wide range of particles including nano and picoplankton (Gorsky et al. 1999 ). In this way, appendicularians promote an efficient energy transfer from basal to higher trophic levels, as they are consumed by many invertebrates such as copepods, chaetognaths, jellyfish, and ctenophores, among others (Gorsky and Fenaux 1998 ; Purcell et al. 2005 ) and larvae and adult fish (Capitanio et al. 2018 ). Hence, they encompass both the classical trophic and the microbial trophic networks (Gorsky and Fenaux 1998 ; Touratier et al. 2003 ). However, despite their ecological importance in marine environments, the appendicularians are still poorly studied, due to as well as difficulties in taxonomic identification (Panasiuk and Kalarus 2021 ). Water temperature is one of the environmental factors and even the most relevant one that affects the dynamics of appendicularian populations (Troedsson et al. 2002 , 2012 ; Kalarius and Panasiuk 2021 ). It is a key driver regulating the sizes at maturity of appendicularians, even though other factors such as quantity and quality of food may be considered as well (Lombard et al. 2009 ). Generally, as temperature decreases, generation time and maximum trunk length increase (Fenaux 1985 ; Capitanio et al. 2018 ). In the Southwest Atlantic Ocean (SWAO), the Argentinean Continental Shelf is known to be a highly productive environment exhibiting high concentrations of chlorophyll-a (Chl-a) in surface waters (Rivas et al. 2006 ; Romero et al. 2006 ; Carreto et al. 2007 ) providing suitable niches for ecologically relevant as well as commercially exploited species (Acha et al. 1999 ; Franco et al. 2020 ). At the southernmost limit of the South Patagonian Shelf, Isla de Staten Island (SI; 54°55’ S; 64°42’ W; 54°43’ S; 63°48’W) is separated from Isla Grande de Tierra del Fuego by the Le Maire Strait (Fig. 1 ). The Burdwood Bank (BB, 53°40'-55°S; 62°-58°40'W), is an underwater plateau 150 km east of SI. The BB is part of the northern Scotia Arc and also represents the eastward extension of the Andes Mountains. It is limited to the north by the 1800 m depth Malvinas Chasm and to the south to the Yaghanes basin, while a passage of lesser depth separates the BB from the continental shelf to the west. The BB plateau (delimited approximately by the 200 m isobath) has an extension of 370 km in a west-east direction, a mean depth of 100 m and a minimum of 50 m. Both the SI and BB are under the direct influence of the Antarctic Circumpolar Current (ACC). In particular, the Subantarctic Front (SAF) and the northern branch of the ACC associated with it, are located just south of SI and BB. Upon encountering the dramatic bathymetry of the BB, this jet splits into two main branches that contour the bank from the passages at its eastern and western limits to join further north and flow along the shelf-break as the Malvinas Current (Piola and Gordon 1989 ; Smith et al. 2010 ) (Fig. 1 A). Intense tidal currents and mixing over the bank and upwelling on its sides are predicted by numerical models (Glorioso and Flather 1995 ; Matano et al. 2019 ). The BB is known to host fragile and valuable benthic ecosystems (e.g. Schejter et al. 2017 ) and is also the spawning area of pelagic species of ecological and commercial significance (e.g. García Alonso et al. 2018). This rich ecosystem provides appropriate habitat for spawning and breeding different fish species, such as the Fuegian sprat and the Patagonian toothfish (García Alonso et al. 2018; Riccialdelli et al. 2020 ). On these grounds, the BB was declared a Marine Protected Area by the Argentinian government in 2013, under the name “Namuncurá”. The general zoogeography of appendicularians in the SWAO, mainly based on morphology and distributional patterns, has been reviewed by Esnal ( 1999 ). Despite the ecological importance of these small tunicates, most of the studies carried out so far in the Argentinean Continental Shelf are rather coastal (eg. Spinelli et al. 2009 , 2012 ; Presta et al. 2015 ; Capitanio et al. 2018 ). Spinelli et al. ( 2020 ) described the first registration of zooplankton in the BB during a spring study. Concerning appendicularians, they only describe the presence of the two species, Oikopleura fusiformis and Fritillaria boreali s. The spring-summer season in the SWAO has been described as the most productive in terms of microbial plankton, particularly in the coast-shelf area (Malits et al. 2023a ), where the highest peaks of Chl-a were related to the presence of diatoms (Guinder et al, 2020 ). This primary production would sustain the subsequent peak of zooplankton production since the density of appendicularians increases rapidly in response to an increase in food availability (Deibel and Lowen 2011). In this context, the main objective of the present study is to compare the species composition, density, biomass, and maturity stages of appendicularians between oceanic and coastal areas of the SWAO with contrasting physical and biological features using data collected from three spring cruises. Materials and Methods Sampling Zooplankton samples were obtained during three oceanographic cruises in SI (coastal area) and BB (oceanic area) during three consecutive austral springs: November 2014 (4 to 27), December 2015 (1 to17) and December 2016 (6 to 15) (Fig. 1 B). A total of 28 zooplankton samples were collected with a small Bongo net of 67 µm mesh size with a 0.22 m diameter. The net was operated from the bottom to the surface using oblique tows. A mechanical flowmeter (Hydrobios, Germany) was used to measure the volume of filtered water. All samples were preserved in a 5% seawater formalin solution. During 2014 and 2016 at each station, water samples for pico- and nanophytoplankton and bacterial abundance analysis were collected with Niskin bottles from 10 m depth. Sub-samples for heterotrophic bacteria (1 mL) and pico-and nanophytoplankton (5 mL) were fixed with 0.2 µm-pre-filtered glutaraldehyde (0.5% and 0.1% final concentration for bacteria and pico-nanophytoplankton, respectively), incubated for 15–30 min at 4 ºC, subsequently flash-frozen in liquid nitrogen and stored at -20 ºC following the protocol of Marie et al. ( 2001 ). Oceanographic data At each station, vertical profiles were recorded with a factory calibrated CTD Rinko ASTD-102 (JFE, Japan). The original data, acquired at 10 Hz, was processed using common procedures and the SBE Data Processing Pack to derive water salinity (PSS-78) and density. The geo-referenced dataset was integrated and visualized with the Ocean Data View software (Schlitzer 2020 ). Temperature and salinity profiles were analyzed at each station using Data – Interpolating Variational. Satellite-derived surface concentrations of Chl-a were obtained from MODIS/Aqua ( https://oceancolor.gsfc.nasa.gov ) with a resolution of 4 km. Surface currents during the three springs considered in this study were obtained from the Ocean Surface Current Analysis Real-time, third-degree resolution (OSCAR; Bonjean and Lagerdoef 2002). Appendicularian analysis The appendicularians were separated and identified in the laboratory using a Leica S6 D Greenough stereo microscope. Those samples with more than 200 specimens were fractionated in a subsample of 20 or 40 mL depending on the number of appendicularians found. All the specimens were identified to the species level and the densities at each station were calculated. The sizes (TL - trunk length) of both Oikopleura fusiformis and Fritillaria borealis species were measured (total number = 13164) using an ocular micrometer and the total biomass was estimated from TL–dry weight relationships obtained from Capitanio et al. ( 2008 ). The maturity stages of appendicularians were determined as immature or mature according to Bückmann classification (Fenaux 1976 ; Bückmann 1972 ; Capitanio and Esnal 1996 ; Martinucci et al. 2005 ). At coastal (SI) and oceanic (BB) areas, stages of immature and mature specimens were determined only for O. fusiformis . Microbial abundances Samples for microbial abundances were analyzed with a FACSCalibur flowcytometer (Becton Dickinson) within a few months. Autotrophic populations ( Synechococcus , picoeukaryotes and nanoeukaryotes) were discriminated from unstained samples according to their light scatter (SSC) and specific autofluorescence properties (Marie et al. 2001 ). Samples to determine the abundance of heterotrophic bacteria ( sensu stricto heterotrophic Bacteria and Archaea) were stained with SYBR Green I (10 X in Dimethyl sulfoxide, DMSO) in the dark and determined in plots of 90◦light scatter (SSC) versus green DNA fluorescence (FL1) following Gasol and Moran (2015). Bacteria with low nucleic acid content (LNA) from those with high nucleic acid content (HNA) were distinguished based on their signature in the SSC versus green fluorescence (FL1-H) cytometric plots using the Flow Jo software. The sample flow rate was accurately calibrated following a modified protocol of Marie et al. ( 2001 ) and used to calculate in situ abundances of heterotrophic bacteria and pico- and nanophytoplankton. For details see Malits et al. ( 2023b ). Statistical analysis To compare the abundance of O. fusiformis between zones (coastal and oceanic) and spring years, a generalized linear model (GLM) was performed, assuming a Poisson distribution of errors and the most parsimonious model was selected based on information theory criteria (Venables and Ripley 2002). In this model, the abundance of O. fusiformis was considered the response variable. This kind of numerical data results in variances much greater than the means, allowing us to assume a negative binomial error distribution and a log link (Crawley, 2005 ). The explanatory variables included in the GLMs that attempted to explain the different abundances were zone (SI, BB) and years (2014, 2015 and 2016). The model with the lower value (most plausible model) of the Akaike information criterion (AIC) was selected as the best one and was weighed against the others using Akaike’s weight (Aw). Aw, values vary between 0 (poor fit) and 1 (good fit) and provide an estimation of the likelihood of the model given the data (Johnson and Omland, 2004 ). The model assumptions were checked using the package DHARMa. Parametric statistics were tested but did not meet the assumptions yet when the variance was modeled. Hence, a non-parametric analysis was carried out to compare the abundance of F. boreali s (response variable) between springs (explanatory variables) with Kruskal Wallis tests, respectively (Mcknight and Najab 2010 ). ANOVA was applied to the biomass data of F. boreali s, to analyze differences between zones (SI and BB), and the same analysis was done for comparing springs. The Tukey–Kramer method was applied to compare the different analyses. A correlation analysis was carried out between the surface temperature and TL of O. fusiformis to understand the changes between these variables. In addition, taking into account the feeding habits of the appendicularians, correlations were carried out between their abundance during the three springs and the abundance of possible prey and satellite-derived Chl-a concentrations. Statistical analyses were performed in the R environment (R Core Team 2017) with the packages stats (R Core Team 2017), glmmTMB (Magnusson et al. 2017 ), multcomp (Hothorn et al. 2008 ), Psych (Revelle and Revelle 2015 ), lsmeans (Lenth 2018) and MASS (Venables and Ripley 2002). Results Environmental features The oceanographic setting, as revealed by near-surface temperature and salinity distributions and surface currents (Figs. 2 , 3 ), is coherent with previous works, and signals the BB as an area of relative isolation within a highly dynamic regional context. The mean eastward flow of the Antarctic Circumpolar Current (ACC), coupled to the subantarctic front is consistently observed south of the BB and SI. Meanders and eddies are apparent within the ACC, but two features found at all survey dates stand out: a prominent northward deflection of the flow at the eastern passage and a large anticlockwise gyre in the Yaganes basin, southwest of the BB (Fig. 2 ). The latter, in spite of being detectable in all 3 periods considered, was more defined in spring 2014 when current vectors suggest transport of water from the western passage into the plateau. In comparison to its surroundings, currents are consistently lower over the BB plateau (however note that velocity fields in Fig. 2 are 5-day integrations and tidal currents are not accounted for by OSCAR). Longitudinal west-to-east gradients of temperature and salinity (negative and positive respectively) are consistent through the years between the coastal area and the BB (Fig. 3 ). Salinity oscillated between 32.81 and 34.12 in the 3 springs considered for the entire studied domain, but the range was much narrower and more stable over the BB. Spring 2016 was the warmest of the three springs considered (7.81 ± 1.11°C at SI; 5.66 ± 0.38°C at BB). In 2014 the average temperature was EI: 6.54 ± 0.07°C; BB: 5.41 ± 0.09°C), while in 2015 presented a temperature of 6.3°C, while over the bank the average temperature was 5.22 ± 0.04 (Fig. 3 ). Chl-a concentrations were in general much higher outside the bank on its western and northern flanks than on the plateau itself. The highest Chl-a concentrations on the bank were registered in 2014 with maxima around 0.93 mg m − 3 to the east. Meanwhile, Chl-a concentrations in SI were higher in 2015 and 2016 (Fig. 3 ). In 2015, the maximum Chl-a over the BB was 0.30 mg m − 3 at the western BB while In 2016 the Chl-a concentrations for this area were minimal (0.11 mg m − 3 ). Appendicularians distribution and species abundance Two species of appendicularians were found in the area during the three springs studied, Oikopleura fusiformis and Fritillaria borealis. The distribution pattern for O. fusiformis and F. borealis species seems to be different, as the former occupied the entire coastal and oceanic areas whereas F. borealis occurred mainly in the coastal one around SI although it was sporadically observed in the BB. Abundances of F. borealis were low being highest in 2014 (up to 5 ind m 3 ). In the case of O. fusiformis , significant differences were found between areas (SI and BB) along the three springs (X 2 test, LR = 4.89; p = 0.03) being more abundant in SI. On the other hand, when evaluating the difference between years, differences between 2014 and the remaining years were detected (X 2 test, LR = 13.36 p = 0.001). During the spring of 2014, the distribution of O. fusiformis was throughout all the sampling stations, particularly in the BB and their abundances were much higher than for the other springs (100 times more) (Fig. 4 A). The highest density for this species was registered to the east of the BB, with a value of 313 ind m − 3 , while in SI the densities were similar in all the samples with an average value of 57 ind m − 3 (range: 50 to 63 ind m − 3 ). As for 2015, the highest densities of the O. fusifomis were recorded at the east of the BB with a maximum value of 36 ind m − 3 . Finally, in the spring of 2016, its densities increased east of the BB and west of the SI and the maximum density registered were 146 and 19 ind m − 3 for SI and BB respectively (Fig. 4 A). Comparing the biomass for O. fusiformis , no significant differences were observed between the three springs (X 2 test, LR = 4.26; p > 0.12). However, when comparing the areas, significant differences were found (X 2 test, LR = 18.69; p < 0.05), being higher in SI. In SI, the values were similar between springs, while in BB the highest average biomass was recorded in 2015 with a value of 8 ± 12 µg C m − 3 . The biomass contribution to F. borealis was very low (Fig. 4 B). The sizes of O. fusiformis were always smaller in SI. In the BB, the highest mean size was found during 2016 with a value of 783 ± 387 µm; followed by 2015 (776 ± 251 µm) and lastly spring 2014 (770 ± 350 µm). On the other hand, in SI the pattern was different and more marked: the highest mean size was found in 2015 with a value of 747 ± 210 µm followed by 2014 with a value of 703 ± 239 µm and 2016 with an average size of 630 ± 281 µm. The same comparison but for F. borealis in SI showed that the highest mean size was found in 2015 with a value of 822 ± 201µm, followed by 2014 and lastly 2016 (683 ± 233 µm and 670 ± 270 µm, respectively). In this context, 2015 was the year that exhibited the highest sizes in SI, for both species. During spring 2014, covered the entire length ranges proposed for the O. fusiformis species in both areas. However, F. borealis in SI presented organisms between 300 and > 1000 µm, with specimens between 600 and 900 µm being the most frequent. In spring 2015, it was observed that the O. fusiformis species covered almost the entire size spectrum in both areas (BB and SI). Regarding F. borealis , the size distribution was limited between 400 and 1000 µm. The most frequent size range among them was between 800–900 µm. Finally, spring 2016 was noted for its wide variety of sizes, both in SI and in the BB for the species O. fusiformis exhibiting in SI the smallest sizes in this study. Regarding F. borealis , no individuals with small sizes (100–300 µm) were found in SI. In general, the largest sizes were the most frequent for O. fusiformis in BB throughout all the years under study but not in SI (Fig. 5 ). The year 2015 presented the highest percentage of O. fusiformis immature individuals in BB and SI. Focusing on spatial differences for O. fusiformis , immature and mature individuals were observed to mean larger sizes in BB than in SI during all three springs (Table 1). A significant negative correlation was found between appendicularian sizes and temperature on SI (Correlation Spearman = -0.11, p value = 0.039). This coincides with the spring of 2016 when the highest temperature value was recorded and the average sizes were smaller. Also, the inverse pattern is seen in 2015, when the lowest temperature value was recorded and the average sizes were higher (see Table 1). Distribution of nanoplankton, picoplankton, Synechococcus, and bacteria as potential food sources of appendicularians Of the two years (2014 and 2016), in which the abundance of the autotrophic nano- and picoeukaryotes and heterotrophic bacteria were analyzed, in spring 2016 the highest numbers were observed with an average abundance of autotrophic nano-picoeukaryotes and heterotrophic bacteria of 6835 ± 4886 cell mL − 1 and 3 ± 1.4 x 10 5 cell mL − 1 , respectively. The abundance of nano- and picoeukaryotes was higher in BB than in SI. The easternmost stations of the BB exhibited the highest picoeukaryotes values for the area along with the lowest chlorophyll values while at the western edge of the submarine plateau nanophytoplankton, i.e. larger cells, dominated along with total high phytoplankton biomass (in terms of chl-a). However, in the center of BB, Synechococcus abundance was very low, as well as westwards of SI. In the spring of 2014, Synechococcus reached the maximum value of 3 x 10 3 cells mL − 1 in the eastern region of BB along with high nano- and picoeukaryotes abundance. In the central area much lower densities were recorded in terms of the picoplanktonic fraction, the latter trend was maintained in SI, however, in this region, autotrophic picoeukaryotes dominated the pico-nanophytoplankton. It is important to highlight that in all the analyzed stations the cyanobacterium Synechococcus was present. In 2014, heterotrophic bacteria were most abundant eastwards of SI and in the central area of BB (Fig. 6 ). In spring 2016, bacterial abundance decreased from high values west of the SI throughout the transition zone and then increased to the east of the bank, indicating a higher average abundance in the BB compared to SI. In the spring of 2014, appendicularians abundance correlated negatively with Chl-a (r=-0.70, p = 0.0344) but positively with nanoplankton (r = 0.79, p = 0.0069) and picoplankton (r = 0.71, p = 0.0227). However, the correlation with the rest of the potential picoplankton prey ( Synechococcus and bacteria) was not significant (r = 0.39 and r = 0.33, p > 0.05). On the other hand, in 2015 and 2016 with significantly lower total phytoplankton biomass, appendicularian densities correlated positively with Chl-a (r = 0.87, p = 0.055 and r = 0.83, p = 0.0015, respectively). Discussion Spatial distribution of appendicularians Two species of appendicularians were found in this study: Oikopleura fusiformis and Fritillaria borealis , the former being the dominant one. Their longitudinal distribution along the inshore-offshore gradient of the study area is consistent with their known habits. F. borealis was mainly concentrated in the inshore waters of SI, which is coherent with its typical coastal distribution in this subarctic region (Aguirre et al. 2012 ; Presta et al. 2015 ). Kalarus and Panasiuk ( 2021 ) also found the greatest abundances of F. borealis near the coasts of Tierra del Fuego and the Antarctic Peninsula but, interestingly, they also recorded this species offshore in the Drake Passage. Likewise, in our study, in spite of a higher presence in SI, it is noteworthy that F. borealis occurred occasionally at the BB. Most of these records are restricted to the western limit of the BB, which is to some extent connected to SI by the prevailing currents (see Fig. 2 ) and a punctual record in the central BB close to the slope. Although the BB presents a rather closed circulation (Matano et al. 2019 ), casual advection into the bank from the Drake Passage has been detected by means of surface drifters (Martin et al. 2023a,b). F. borealis was also detected on the bank in spring 2014, at its western limit and in coincidence with an intensification of the eddy southwest of the bank. From these observations and given the rarity of F. borealis occurrences over the bank, we postulate that the water intrusions from outside the bank, and eventually the presence of this species inside would be used in the future as a tracer of such processes. F. borealis was an important component of zooplankton particularly in spring near SI, coinciding with maximum values of chlorophyll-a values (See Fig. 3 ). Particularly in 2014, a positive correlation was observed between F. borealis density and chlorophyll. The western part of the BB is also the most productive one in terms of phytoplankton biomass, such as the coastal regions, could this be the reason for their distribution Regarding, O. fusiformis was recorded consistently in both SI and the BB (Fig. 4 ), being a species of the more oceanic character (Zoppi de Roa, 1971) and usually found in both coastal and oceanic waters (e.g. Capitanio et al., 2018 ; Panasiuk and Kalarus, 2021 ). Population dynamics of appendicularians The population dynamics of appendicularians are usually related to the amount of available food, predators, salinity and temperature (Shiga 1985 ; Taggart and Frank 1987 ; Tomita et al. 2003 ; Troedsson et al. 2002 ; 2012 ). Appendicularians are a key link in marine food webs, preying on nano- and picoplankton at the base of the food web and linking them to higher trophic levels. Therefore, they are mediators in the trophic cascade between the components of the marine microbial food web and the main predators in the study area. The abundance of nano-, picoeukaryotes and bacteria was in general higher in BB than in SI for both years, while 2016 presented higher values than 2014 in contrast to the chlorophyll data presented here. The distribution of satellite Chl- a (proxy of phytoplankton biomass and thus possible food indicator) was different between years in the BB and similarly in SI. It was observed that concentrations of Chl-a are higher in SI than at BB and similar to those at the mouth of the Beagle Channel. Particularly in the SI area, the chlorophyll concentration was higher in 2015 and 2016 along with a higher density of O. fusiformis . Instead, the BB showed spatial differences: while during 2014, more Chl– a was found west of the BB, in 2015 the eastern part increased in total phytoplankton biomass and the spring of 2016 presented a similar distribution of chl-a over the plateau. The surface current's speed seems to reflect the total phytoplankton biomass, i.e. the higher the current speed, the greater the Chl-a concentration and accordingly, the lowest satellite Chl-a along with the lowest current speed as for spring of 2016. In the spring of 2014 Bertola et al. (2018) found the highest analytical in situ Chl-a values ​​towards the west of the BB, matching the satellite images selected here, and the dominance of the diatom Rhizosolenia crassa , followed by aloricated and flagellated ciliates in BB. Acuña et al. ( 2002 ) demonstrated that the appendicularians abundance is lower in the presence of phytoplankton blooms dominated by large diatoms in Baffin Bay, North America. This could explain the lower appendicularian densities registered at the western part of the BB in contrast to the other areas. In any case, Guinder et al. ( 2020 ) studied the distribution and composition of microbial plankton during the spring of 2016. They observed a decreasing concentration gradient of Chl-a in surface layers from the Beagle Channel to the BB being the Beagle Channel - Shelf enriched in diatoms. At the same time, the transition zone located between SI and BB was more abundant in coccolithophorids and flagellates, and microheterotrophs abounded in the BB. All of these potential prey for appendicularians could support the abundance in a site with complex oceanographic dynamics for organisms as fragile as appendicularians. SI is largely bathed by the outermost waters of the mouth of the Beagle Channel, carrying nutrients that favor high levels of Chl-a and primary productivity. In addition to a tidal front that interacts there with the bottom producing an energy transfer, an abrupt change in the water depths from the order of 4000 m to 80 m promotes the fertilization of surface waters due to local upwellings (Matano et al. 2019 ; Guinder et al. 2020 ). This leads to large phytoplankton blooms (Paparazzo 2010) which allow zooplankton to develop (bottom-up control). Quantitative microscopy studies of faecal pellets indicate bacteria, cyano-bacteria, pennate and centric diatoms, dinoflagellates, choanoflagellates, ciliates, and coccolithophores as important dietary constituents of zooplankton (Deibel and Turner 1985 ; Urban et al. 1992 ; Acuña et al. 2002 ). More recently, in situ studies combined with flow cytometry and sequencing revealed that appendicularians are capable of grazing picocyanobacteria, i.e. Synechococcus and Prochlorococcus , at high rates (Scheinberget al. 2005 ; Dadon-Pilosof et al. 2017 ). Therefore, this explains the presence of high abundances of appendicularians (mainly in 2014) in an oligotrophic oceanic zone such as the BB. Just as phytoplankton is vital for the herbivorous link in the food web to thrive, the proper growth of zooplankton is vital for the higher links. From the perspective of higher trophic levels, appendicularians are one of the main components of the marine mesozooplankton, they are characterized by being primary consumers and food for pelagic fish larvae. The Fuegian sprat, Sprattus fuegensis is one of the most abundant zooplanktophagous pelagic fishes in the southern sector of the Patagonian shelf (Sánchez et al. 1995) and is known to spawn in the BB (Garcia Alonso et al. 2018 ). In the same campaign, Garcia Alonso et al ( 2018 ) found high densities of fuegian sprat larvae, mainly towards the eastern zone of the BB. These results match the high densities of appendicularians found in the present study, particularly in spring 2014. The mouth opening of the fuegian sprat larvae collected in spring 2014, ranged between 680 µm and 760 µm, with a maximum value of 1000 µm (Spinelli et al. 2020 ). The range of food particles ingested by sardine larvae oscillates in these values, therefore the recorded sizes of appendicularian indicate that they would be a potential prey. Although the total densities of appendicularians are lower in SI, it was seen that the biomass was higher in that zone, which would indicate a greater contribution of carbon available for higher trophic levels. Unfortunately, for the area of SI, there is no information on sardine larvae. In relation to environmental variables (salinity and temperature), given the low oscillation between the salinity values registered between areas and springs, this would not be affecting the distribution of the appendicularians species and nor could the recorded abundances be explained by said parameter. Temperature is a key driver in regulating the sizes at maturity of appendicularians since its increase causes the acceleration of gonadal maturation (Fenaux 1985 ; Capitanio et al. 2018 ). Numerous studies have shown that, when temperature decreases, generation time is maximum. The temperature of the BB was lower than in SI in spring and this could be related to the larger appendicularian sizes found in the BB. This is also clearly seen for both species between springs, with for example spring 2015 having the lowest temperatures and the largest mature and immature specimens. Panasiuk and Kalarus ( 2021 ) remarked that temperature was the strongest environmental factor influencing the larvacean community structure in the Drake Passage. Nevertheless, in the present study, there was no direct relationship between temperature and appendicularian density and distribution in both areas. Appendicularians can become reproductively mature and spawn over a wide range of trunk lengths, and larger animals generally produce more eggs. When temperature increases, trunk length at the maturity stage and fecundity generally decrease (Lombard et al. 2009 ). Given that in spring 2015 appendicularians showed larger sizes on maturity, higher egg production would be also expected. The mature: juvenile ratio in spring 2014 and 2016 suggests that a certain time span had elapsed since the last reproductive pulse. Instead, the lower mature: juvenile ratio registered during spring 2015 is indicative of a recent reproductive event. However, this was not reflected in a high abundance of appendicularians. On SI, temperatures were higher, and the proportion of mature organisms was similar to that registered in BB during the three springs analyzed. Importance of appendicularians in the oceanic marine protected area Namuncurá/ Burdwood Bank Appendicularians represent an important food item for larvae and adult fish (Capitanio et al. 2005 ; Gorsky et al. 2005 ) and play a key role in both the formation of marine snow and the flow of organic matter. Despite this relevance, there are no ecological studies on appendicularians in the marine protected area. In this context, the present study allowed discerning the following ecological aspects of appendicularians from Sub-antarctic environments. It is also important to highlight that the appendicularians are the second group that dominates after the copepods in the zooplankton samples recorded in the area (Spinelli et al. 2020 ). Thus, the presence of bacteria, pico and nanoplankton fraction, and high-concentration of Chl-a (spring 2014) in this area constitutes a suitable environment for appendicularians reproduction in spring, thus enhancing the survival and growth of several small pelagic fishes such as S. fuegensis . BB zones play a key role in ecological processes in the ocean, allowing an exceptionally large primary production, offering adequate feeding and reproductive habitats for planktivorous species and acting as retention areas for larvae. So, the biomass of appendicularians contributes to the transfer of carbon to higher trophic levels and is probably important for the survival and growth of various small pelagic fish such as the Fuegian sprat. On the other hand, the Southern Ocean and Antarctica are warming and their waters are particularly susceptible to ocean acidification (Swart et al. 2018 ; Trull et al. 2018 ). Experimental studies showed that the appendicularian abundance was positively correlated with increased pCO2 (Troedsson et al. 2012 ). However, the ecological importance of these organisms, along with progressive climate changes, would be in line with the general trend of the increasing importance of jellyfish organisms (e.g., tunicates and cnidarians) in all marine environments, including polar regions (Panasiuk et al. 2020 ; Kalarus and Panasiuk 2021 ). When facing global change, basic information on these small tunicates is necessary, particularly regarding their important trophic role. The conservation of marine resources is now a major scientific and social goal. Direct and indirect anthropogenic pressures are threatening the functioning and diversity of marine ecosystems and the services they provide (Gattuso et al. 2015 ). Baseline data on biodiversity, the abundance of species and their distribution in protected areas and surrounding areas is essential to contribute to the stakeholders in these areas and advise on future changes that translate into regional and global processes. This study is the first to document the population dynamics of appendicularians at the BB and SI, a region of particular interest in the Southwest Atlantic Ocean for its intense hydrographic activity, complex circulation, and high conservation value, and also provide new knowledge on the ecology of appendicularioans in sub-Antarctic waters. To know the behaviour of the appendicularians species that are represented and their role in the food web future studies should be focused on determining through which mechanisms and to what extent the changes in spatial and interannual appendicularians abundance are the result of either physical or/and biological processes. Also incorporate seasonal studies, to understand with better resolution the dynamics of appendicularia in this sub-antarctic zone Declarations Acknowledgements This study was supported by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina (PIP 11220150100109CO and PIP 11220150100368CO), Universidad de Buenos Aires, Argentina (UBACYT 20020190100133BA 2020-2024), ANPCyT (PICT- 2015- 0384 and PICT-2019-04049) and the Pampa Azul Interministerial Initiative implemented by the Argentinian Ministry for Science, Technology and Productive Innovation. The authors thank the leaders and coordinators of the cruises throughout the years, as well as Maria Laura Presta, Alejandro Martinez, and everyone who participated in the oceanographic surveys for their cooperation during sample collection and preparation. Clara Natalia Rodriguez Flores for her help onboard with sampling for the microbial abundances . Virginia García Alonso and Harold Fenco for their valuable contributions in terms of statistics and satellite image acquisition. This is Marine Protected Area Namuncurá-Burdwood Bank (Law 26,875) contribution No. XXX. References Acha EM, Pajaro M, Sanchez RP (1999) The reproductive response of clupeoid fishes to different physical scenarios. Three study cases in the Southwest Atlantic. ICES-CM-1999/K, 12 Acuña JL, Deibel D, Saunders PA et al (2002) Phytoplankton ingestion by appendicularians in the North Water.Deep Sea Res Part II Top Stud Oceanogr 49 (22-23):5101-5115. https://doi.org/10.1016/S0967-0645(02)00180-7 Aguirre GE, Capitanio FL, Lovrich GA, Esnal GB (2012). Seasonal variability of metazooplankton in coastal sub-Antarctic waters (Beagle Channel). Mar Biol Res 8(4):341-353 https://doi.org/10.1080/17451000.2011.627922 Bértola G, Olguín Salinas H, Iachetti C, Lovrich G, Alder V (2018) Estructura del plancton unicelular eucariota del AMP Namuncurá- Banco Burdwood en primavera. Conexiones con las adyacencias. In:Menoret A, Guller M (eds) Libro de resúmenes X JNCM, Buenos Aires, 74 pp Bonjean F, Lagerloef GSE (2002) Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean. J Phys Oceanogr 32:2938-2954 https://doi.org/10.1175/1520-0485(2002)0322.0.CO;2 Bückmann A (1972) Die Appendicalarien von den Fahrten der Meteor, derAnton Bruum und der Discovery in das Arabische Meer imRahmen der IIOE. “Meteor" Forsch Ergebn D 10:l-45 Capitanio FL, Esnal GB (1996) Caracterización morfométrica de los estadios de madurez de Oikopleura fusiformis (Tunicata, Appendicularia). Phys 52(122-123):13-17 Capitanio FL, Pájaro M, Esnal GB (2005). Appendicularians: an important food supply for the Argentine anchovy Engraulis anchoita in coastal waters. J Appl Ichthyol, 21(5): 414-419. https://doi.org/10.1111/j.1439-0426.2005.00657.x Capitanio FL, Curelovich J, Tresguerres M, Negri RM, Viñas MD, Esnal GB (2008) Seasonal Cycle of Appendicularians at a Coastal Station (38° 28′ s, 57° 4′ w) of the SW Atlantic Ocean. Bull Mar Sci 82(2):171-184 Capitanio FL, Spinelli ML, Presta ML et al (2018) “Ecological role of common appendicularian species from shelf waters of Argentina”.In: Hoffmeyer MS, Sabatini ME, Brandini FP, Calliari DL, Santinelli NH (eds) Plankton ecology of the Southwestern Atlantic - From the subtropical to the subantarctic realm. Springer, Cham, pp 201–218 https://doi.org/10.1007/978-3-319-77869-3_10 Carreto JI, Carignan MO, Montoya NG, Cucchi Colleoni AD (2007) Ecología del fitoplancton en los sistemas frontales del Mar Argentino.In: Sánchez RP, Bezzi SI (eds) El Mar Argentino y sus recursos pesqueros. El ecosistema marino. Publicaciones Especiales INIDEP, Mar del Plata, pp 11–31 Crawley MJ (2005) Statistics: an introduction using R, 2005. Wiley, ISBN 0, 470(02298), 1 Dadon-Pilosof A, Conley KR, Jacobi Y, et al (2017). Surface properties of SAR11 bacteria facilitate grazing avoidance. Nat. Microbiol, 2(12):1608-1615. https://doi.org/10.1038/s41564-017-0030-5 Deibel D, Turner JT (1985). Zooplankton feeding ecology: contents of fecal pellets of the appendicularian Oikopleura vanhoeffeni . Mar Ecol Prog Ser, 67-78 Esnal GB (1999) Appendicularia. In: Boltovskoy D (ed) South Atlantic zooplankton. Leiden,The Netherlands: Backhuys, págs. 1375–1399 Fenaux R (1976) Cycle vital, croissance et production chez Fritillaria pellucida (Appendicularia), dans la baie de Villefranche-sur-Mer, France. Mar Biol 34(3):229-238. https://doi.org/10.1007/BF00388800 Fenaux R (1985) Rhythm of secretion of oikopleurid's houses. Bull Mar Sci 37(2): 498-503 Franco BC, Combes V, Gonzalez Carman V (2020) Subsurface ocean warming hotspots and potential impacts on marine species: the southwest South Atlantic Ocean case study. Front Mar Sci 7, 824 https://doi.org/10.3389/fmars.2020.563394 Garcia Alonso VA, Brown D, Martín J, Pájaro M, Capitanio FL (2018) Seasonal patterns of Patagonian sprat Sprattus fuegensis early life stages in an open sea Sub-Antarctic Marine Protected Area. Polar Biol 41(11):2167-2179 https://doi.org/10.1007/s00300-018-2352-z Gasol JM, Del Giorgio PA (2000) Using flow cytometry for counting natural planktonic bacteria and understanding the structure of planktonic bacterial communities. Sci Mar 64(2):197-224. https://doi.org/10.3989/scimar.2000.64n2197 Gattuso JP, Magnan A, Billé R et al (2015). Contrasting futures for ocean and society from different anthropogenic CO2 emissions scenarios. Sci, 349(6243):aac4722. DOI: 10.1126/science.aac4722 Glorioso PD, Flather RA (1995) A barotropic model of the currents off SE South America. J Geophys Res Oceans 100:13427-13440. https://doi.org/10.1029/95JC00942 Gorsky, G, Fenaux, F (1998) The role of Appendicularians in marine food webs. In: Bone Q. (Ed.). The biology of Pelagic Tunicates. Oxford Univ. Press., Oxford, pp 161-169 Gorsky G, Chritennot-Dinet MJ, Blanchot J, Palazzoli (1999) Picoplankton and nanoplankton aggregation by appendicularians Fecal pellet contents of Megalocercus huxleyi in the equatorial Pacific. J Geophys Res 104:3381-3390 Gorsky G, Youngbluth MJ, Deibel D (2005) Response of Marine Ecosystems to Global Change: Ecological Impact of Appendicularians. GB Scientific Publisher, Paris. 440 pp Guinder VA, Malits A, Ferronato C, Krock B, Garzón-Cardona J, Martínez A (2020) Microbial plankton configuration in the epipelagic realm from the Beagle Channel to the Burdwood Bank, a Marine Protected Area in Sub-Antarctic waters. Plos one, 15(5): e0233156 Hothorn T, Bretz F, Westfall P (2008) Simultaneous inference in general parametric models. Biom J 50(3):346-363. https://doi.org/10.1002/bimj.200810425 Johnson JB, Omland KS. (2004) Model selection in ecology and evolution. Trends Ecol Evol 19(2): 101-108. https://doi.org/10.1016/j.tree.2003.10.013 Kalarus M, Panasiuk A (2021) Spatial Distribution Patterns of Appendicularians in the Drake Passage: Potential Indicators of Water Masses?. Diversity, 13(7):286. https://doi.org/10.3390/d13070286 Lenth R, Singmann H, Love J (2018). Emmeans: Estimated marginal means, aka least-squares means. R package version, 1(1) Lindsay, MCM, Williams, GD (2010) Distribution and abundance of Larvaceans in the Southern Ocean between 30 and 80°E. Deep-Sea Res 57:905–915.doi:10.1016/j.dsr2.2009.04.021. Lombard F, Renaud F, Sainsbury C, Sciandra A,Gorsky G (2009) Appendicularian ecophysiology I. Food concentration dependentclearance rate, assimilation efficiency, growth and reproduction of Oikopleura dioica . J Marine Syst 78: 606-616. https://doi.org/10.1016/j.jmarsys.2009.01.004 Magnusson A, Skaug H, Nielsen A et al (2017). Package ‘glmmTMB’. R Package Version 0.2. 0. Malits A, Ibarbalz FM, Martín J, Flombaum P (2023a). Higher biotic than abiotic natural variability of the plankton ecosystem revealed by a time series along a subantarctic transect. J Mar Syst 238: 103843.https://doi.org/10.1016/j.jmarsys.2022.103843 Malits A, Monforte C, Iachetti C, Gerea M, Latorre M (2023b). Source characterization of dissolved organic matter in the eastern Beagle Channel from a spring situation. J Mar Syst 240: 103863.https://doi.org/https://doi.org/10.1016/j.jmarsys.2023.103863 Marie D, Partensky F, Vaulot D, Brussaard C (2001). Enumeration of phytoplankton, bacteria, and viruses in marine samples. Curr Protoc 10(1):11-11.https://doi.org/10.1002/0471142956.cy1111s10 Martín J, Alonso G, Dragani W, Meyerjürgens J, Giesecke R, Cucco A, Fenco H (2023a). General circulation and tidal wave propagation along the Beagle Channel. J Mar Syst 240: 103889. https://doi.org/10.1016/j.jmarsys.2023.103889. Martín J, Meyerjürgens, J, Badewien TH, Alonso G, Dragani W, Cucco A, Giesecke R, Wollschläger J (2023b). Assessment of currents and ecosystem connectivity in subantarctic waters (Beagle Channel - Burdwood Bank). ASLO Aquatic Sciences Meeting. Martinucci G, Brena C, Cima F, Burighel P (2005) Synchronous spermatogenesis in appendicularians. In: Gorsky G, Youngbluth M, Deibel D (eds) Response of marine ecosystems to global changes: ecological impact of appendicularians. Paris: Contemporary Publishing International pp, 113-125. Matano RP, Palma ED, Combes V (2019) The Burdwood Bank Circulation. J Geophys Res Oceans 124:(10), 6904-6926.https://doi.org/10.1029/2019JC015001 McKnight PE, Najab J (2010) Mann‐Whitney U Test. The Corsini encyclopedia of psychology, 1-1.https://doi.org/10.1002/9780470479216.corpsy0524 Panasiuk A, Grzonka L, Prątnicka P, Wawrzynek-Borejko J, Szymelfenig M (2020). Zonal variability of pelagic Siphonophora (Cnidaria) in the atlantic sector of the southern ocean. J Sea Res, 165, 101951. https://doi.org/10.1016/j.seares.2020.101951 Panasiuk A, Kalarus M (2021). Appendicularia (Tunicata) in an Antarctic Glacial Fjord–Chaotic Fjordic Structure Community or Good Indicators of Oceanic Water Masses?. Diversity 13(12):675. https://doi.org/10.3390/d13120675 Piola AR, Gordon AL (1989) Intermediate waters in the southwest South Atlantic. Deep-Sea Res.I Oceanogr Res Pap 36:1–16.https://doi.org/10.1016/01980149(89)90015-0 Presta ML, Hoffmeyer, MS, Capitanio FL (2015). "Population structure and maturity stages of Fritillaria borealis (Appendicularia, Tunicata): seasonal cycle in Ushuaia Bay (Beagle Channel)." Braz J Oceanogr 63(3): 279-288.https://doi.org/10.1590/S1679-87592015087206303 Purcell JE, Studervant MV, Galt CP (2005) A review of appendicularians as prey of invertebrates and fish predators. In: Gorsky G, Youngbluth MJ, Deibel D (eds) Response of marine ecosystems to global change: ecological impact of appendicularians. Contemporary Publishing International, Paris, pp, 359–435 Revelle W, Revelle MW (2015). Package ‘psych’. The Comprehensive R Archive Network Riccialdelli L, Becker YA, Fioramonti NE, Torres M, Bruno DO, Rey AR, Fernández DA (2020). Trophic structure of southern marine ecosystems: a comparative isotopic analysis from the Beagle Channel to the oceanic Burdwood Bank area under a wasp-waist assumption. Mar Ecol Prog Ser, 655:1-27.https://doi.org/10.3354/meps13524 Rivas AL, Dogliotti AI, Gagliardini DA (2006). Seasonal variability in satellite-measured surface chlorophyll in the Patagonian Shelf. Cont Shelf Res 26(6):703-720. https://doi.org/10.1016/j.csr.2006.01.013 Romero SI, Piola AR, Charo M, Garcia CAE (2006). Chlorophyll a variability off Patagonia based on SeaWiFS data. J Geophys Res Oceans 111. https://doi.org/10.1029/2005JC003244 Sánchez RP, Ciechomski JD (1995) Spawning and nursery grounds of pelagic fish species in the sea-shelf off Argentina and adjacent areas Sci Mar 59:455–478 Scheinberg RD, Landry MR, Calbet A (2005). Grazing of two common appendicularians on the natural prey assemblage of a tropical coastal ecosystem. Mar Ecol Prog Ser, 294: 201-212. doi:10.3354/meps294201 Schejter LS, Martin J, Lovrich G (2017). Unveiling the submarine landscape of the Namuncurá marine protected area, Burdwood Bank, SW Atlantic Ocean. Pan-Am J Aquat Sci 12:248–253 Schlitzer R (2020) Ocean Data View, https://odv.awi.de. Shiga N (1985) Seasonal and vertical distributions of appendicularia in Volcano Bay, Hokkaido, Japan. Bull Mar Sci 37(2):425-439 Smith IJ, Stevens DP, Heywood KJ, Meredith MP (2010) The flow of the Antarctic circumpolar current over the north scotia ridge. Deep-Sea Res I Oceanogr Res Pap. 57:14–28.https://doi.org/10.1016/j.dsr.2009.10.010. Spinelli M, Martos P, Esnal, GB, Capitanio FL (2009) Appendicularian assemblages and their space-time variability of the La Plata River, SW Atlantic Ocean. Estuar Coast Shelf Sci 85: 97-106 Spinelli ML, Pájaro M, Martos P, Esnal GB, Sabatini M, Capitanio FL (2012) Potential zooplankton preys (Copepoda and Appendicularia) for Engraulis anchoita in relation to early larval and spawning distributions in the Patagonian frontal system (SW Atlantic Ocean). Sci Mar 76(1): 39-47 Spinelli ML, Malits A, Alonso VAG, Martín J, Capitanio FL (2020). Spatial gradients of spring zooplankton assemblages at the open ocean sub-Antarctic Namuncurá Marine Protected Area/Burdwood Bank, SW Atlantic Ocean. J Mar Syst, 210: 103398. https://doi.org/10.1016/j.jmarsys.2020.103398 Swart NC, Gille ST, Fyfe JC, Gillett NP (2018). Recent Southern Ocean warming and freshening driven by greenhouse gas emissions and ozone depletion. Nat Geosci 11:836–841. https://doi.org/10.1038/s41561-018-0226-1 Taggart CT, Frank KT (1987). Coastal upwelling and Oikopleura occurrence (" slub"): a model and potential application to inshore fisheries. Can J Fish Aquat Sci 44(10):1729-1736. https://doi.org/10.1139/f87-211 Tomita M, Shiga N, Ikeda T (2003). Seasonal occurrence and vertical distribution of appendicularians in Toyama Bay, southern Japan Sea. J Plankton Res 25(6):579-589. https://doi.org/10.1093/plankt/25.6.579 Touratier F, Carlotti F, Gorsky G (2003) Individual growth model for the appendicularian Oikopleura dioica . Mar EcoL Prog Series 248: 141-163 Troedsson C, Bouquet JM, Aksnes DL, Thompson EM (2002). Resource allocation between somatic growth and reproductive output in the pelagic chordate Oikopleura dioica allows opportunistic response to nutritional variation. Mar Ecol Prog Ser, 243:83-91. doi:10.3354/meps243083 Troedsson C, Bouquet JM, Lobon CM et al (2012) Effects of ocean acidification, temperature and nutrient regimes on the appendicularian Oikopleura dioica : A mesocosm study. Mar Biol 160: 2175–2187.35. https://doi.org/10.1007/s00227-012-2137-9 Trull TW, Passmore A, Davies DM, Smith T, Berry K, Tilbrook B (2018) Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: A baseline for ocean acidification impact assessment. Biogeosci 15:31. https://doi.org/10.1007/s00227-012-2137-9 Urban, J L, McKenzie CH, Deibel D (1992) Seasonal differences in the content of Oikopleura vanhoeffeni and Calanus finmarchicus faecal pellets: illustrations of zooplankton food web shifts in coastal Newfoundland waters. Mar Ecol Prog Series 84(3): 255–264. http://www.jstor.org/stable/24829563 Venables WN, Ripley BD (2013). Modern applied statistics with S-PLUS. Springer Science & Business Media. Zoppi de Roa E (1971). Apendicularias de la región oriental de Venezuela. Stud fauna Curaçao other Caribb isl 38(1):76-109. Tables Table 1 is available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files Table1..pdf Table 1. Results corresponding to both species recorded in this study ( Oikopleura fusiformis and Fritillaria borealis ) in terms of percentages and mean length by mature and immature stages (mean ± SD). According to springs (2014, 2015 and 2016) and their zones (Staten Island, and Burdwood Bank). Cite Share Download PDF Status: Published Journal Publication published 15 Jan, 2025 Read the published version in Polar Biology → Version 1 posted Editorial decision: Revision requested 02 Aug, 2024 Reviews received at journal 19 Feb, 2024 Reviewers agreed at journal 24 Jan, 2024 Reviews received at journal 11 Jan, 2024 Reviewers agreed at journal 07 Dec, 2023 Reviewers agreed at journal 28 Nov, 2023 Reviewers invited by journal 28 Nov, 2023 Editor assigned by journal 28 Nov, 2023 Submission checks completed at journal 04 Nov, 2023 First submitted to journal 03 Nov, 2023 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. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3553555","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":262135196,"identity":"86ef867b-cc42-462a-8cd9-b5f5799d8ee4","order_by":0,"name":"Nadia Alves","email":"","orcid":"","institution":"Instituto Nacional de Investigacion y Desarrollo Pesquero","correspondingAuthor":false,"prefix":"","firstName":"Nadia","middleName":"","lastName":"Alves","suffix":""},{"id":262135197,"identity":"23decabf-a23f-4070-b686-6ee641c4cd19","order_by":1,"name":"Mariela Spinelli","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABBElEQVRIiWNgGAWjYJCCAwwFQJIZiB8USMiBRR4Q1GIA1ZJgIGEMFkkgaI8BlE4wYEhsADPwKNZtP/vwwAcDO3tzdt6DDxIMLNLnhx1+CLTFTk63AbsWszPpBgdnGCQn7mzmSzYAOix34+00A6CWZGOzAzi0HEhjOMxjwJxgcJjHTAKsZXYCSMuBxG24tJx/xnD4j0G9PVCL+Q+glnTD2ekf8Gu5AbSFweAw4wagLaAQS5CXziFgy41nDAd7DI4D/cJjDHKY4QbpnIIDCQZ4/HI+jfnDj4pqe3P+M4YfPlTUycvPTt8MZNjJ4dICBwZwxgEULjFa5BuIUD0KRsEoGAUjCgAAoJlgRyyQ910AAAAASUVORK5CYII=","orcid":"","institution":"Instituto de Biodiversidad y Biología Experimental y Aplicada","correspondingAuthor":true,"prefix":"","firstName":"Mariela","middleName":"","lastName":"Spinelli","suffix":""},{"id":262135198,"identity":"f49e8d7c-ef5d-4265-8e87-4c867ce209ed","order_by":2,"name":"Jacobo Martin","email":"","orcid":"","institution":"Centro Austral de Investigaciones Científicas","correspondingAuthor":false,"prefix":"","firstName":"Jacobo","middleName":"","lastName":"Martin","suffix":""},{"id":262135199,"identity":"6b5dae1f-26e3-4be9-82ea-5d78c3e5575e","order_by":3,"name":"Andrea Malits","email":"","orcid":"","institution":"Centro Austral de Investigaciones Científicas","correspondingAuthor":false,"prefix":"","firstName":"Andrea","middleName":"","lastName":"Malits","suffix":""},{"id":262135200,"identity":"a23de6c4-4e7e-40fd-bca1-4d7658b7119e","order_by":4,"name":"Fabiana Capitanio","email":"","orcid":"","institution":"Instituto de Biodiversidad y Biología Experimental y Aplicada","correspondingAuthor":false,"prefix":"","firstName":"Fabiana","middleName":"","lastName":"Capitanio","suffix":""}],"badges":[],"createdAt":"2023-11-03 18:44:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3553555/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3553555/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00300-024-03331-z","type":"published","date":"2025-01-15T15:57:39+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":58809458,"identity":"4c781d8d-f7f6-466a-8070-45a887a9f036","added_by":"auto","created_at":"2024-06-21 11:35:42","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":284333,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA. \u003c/strong\u003eLocation of Staten Island (SI) and the Namuncurá Marine Protected Area/\u003cbr\u003e\nBurdwood Bank. The path of the more relevant regional currents is schematized with arrows, including an anticyclonic gyre contouring the bank (noted as AC). Modified from Garcia Alonso et al. (2018). \u003cstrong\u003eB. \u003c/strong\u003eLocation of the sampling sites during the three surveys considered in this study. Isla Grande de Tierra del Fuego (IGTF) and SLM (Le Maire Strait) were indicated.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-3553555/v1/d1b5a336f20dabb056fff942.png"},{"id":58809457,"identity":"ab6ce342-b7c7-401c-bfbf-3ab45120bdf5","added_by":"auto","created_at":"2024-06-21 11:35:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":728116,"visible":true,"origin":"","legend":"\u003cp\u003eSurface currents in the study area during the three sampled periods. Data from OSCAR third-degree resolution ocean surface currents (see the text for details)\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-3553555/v1/20e5950a5c91409ca0d333bd.png"},{"id":58809456,"identity":"dd812bb9-0f1c-46a4-a1cd-a5f2485e2f01","added_by":"auto","created_at":"2024-06-21 11:35:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":695494,"visible":true,"origin":"","legend":"\u003cp\u003eOceanographic and bio-optical settings during the three studied periods. Left and middle panels: water temperature and salinity respectively. The data has been linearly interpolated from bin-averaged (centered at 10 m depth) CTD measurements taken at oceanographic stations (marked with crosses); right panel: Chl-a surface concentration as provided by MODIS/Aqua.\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-3553555/v1/4b7a5298ffff431cbdba6a71.png"},{"id":58809463,"identity":"7e6a6c32-bccf-487b-9f59-fe94e8b20431","added_by":"auto","created_at":"2024-06-21 11:35:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":367527,"visible":true,"origin":"","legend":"\u003cp\u003eSpring distribution of appendicularians (\u003cem\u003eOikopleura fusiformis \u003c/em\u003eand \u003cem\u003eFritillaria borealis\u003c/em\u003e) abundances (ind m\u003csup\u003e-3\u003c/sup\u003e) (A) and biomasses (µg C m\u003csup\u003e-3\u003c/sup\u003e) (B) in the Namuncurá Marine Protected Area/Burdwood Bank and Staten Island during spring\u0026nbsp; 2014, 2015 and 2016.\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-3553555/v1/c3dfb59b9e406f3215a3583a.png"},{"id":58809462,"identity":"550d6d16-dd40-41d4-b0d5-7e93df45f721","added_by":"auto","created_at":"2024-06-21 11:35:43","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":113500,"visible":true,"origin":"","legend":"\u003cp\u003eSize structure (in µm) of \u003cem\u003eOikopleura fusiformis\u003c/em\u003e and\u003cem\u003e Fritillaria borealis\u003c/em\u003ein the Namuncurá Marine Protected Area/Burdwood Bank and Staten Island during the springs of 2014, 2015 and 2016.\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-3553555/v1/48f121b756c928a872e7fa14.png"},{"id":58809460,"identity":"43f5b0b7-a2bd-49a1-bd5c-e44831b77390","added_by":"auto","created_at":"2024-06-21 11:35:42","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":350008,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of bacteria, nanoeukaryotes and picoeukaryotes densities during 2014 and 2016 in the Namuncurá Marine Protected Area/Burdwood Bank and Staten Island.\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-3553555/v1/0e977f5ec85c7e2e55374754.png"},{"id":74284849,"identity":"2820c7d6-6f17-4464-85d9-690d71a70a48","added_by":"auto","created_at":"2025-01-20 16:13:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3165814,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3553555/v1/6928ffd3-aeee-44f0-9e4f-e2cea5525d47.pdf"},{"id":58810117,"identity":"40eb4f98-5108-4018-86df-81071424d6f2","added_by":"auto","created_at":"2024-06-21 11:43:42","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":68145,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTable 1.\u003c/strong\u003e Results corresponding to both species recorded in this study (\u003cem\u003eOikopleura fusiformis\u003c/em\u003e and \u003cem\u003eFritillaria borealis\u003c/em\u003e) in terms of percentages and mean length by mature and immature stages (mean ± SD). According to springs (2014, 2015 and 2016) and their zones (Staten Island, and Burdwood Bank).\u003c/p\u003e","description":"","filename":"Table1..pdf","url":"https://assets-eu.researchsquare.com/files/rs-3553555/v1/302819f051491cd2064afd2a.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Spring abundance, biomass and sizes of appendicularians between open sea MPA Namuncurá/ Burdwood Bank and the adjacent coastal area, Southwest Atlantic Ocean: Are they a key link in the trophic web?","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe small planktonic appendicularians are circumglobal tunicates considered one of the most abundant mesozooplankton groups in many pelagic environments. They inhabit mainly coastal rather than oceanic environments and occupy an important trophic position in marine food webs (Lindsay and Williams, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Capitanio et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Their role in the matter flow within the water column is crucial since most species can filter and concentrate a wide range of particles including nano and picoplankton (Gorsky et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). In this way, appendicularians promote an efficient energy transfer from basal to higher trophic levels, as they are consumed by many invertebrates such as copepods, chaetognaths, jellyfish, and ctenophores, among others (Gorsky and Fenaux \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Purcell et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) and larvae and adult fish (Capitanio et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Hence, they encompass both the classical trophic and the microbial trophic networks (Gorsky and Fenaux \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Touratier et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). However, despite their ecological importance in marine environments, the appendicularians are still poorly studied, due to as well as difficulties in taxonomic identification (Panasiuk and Kalarus \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWater temperature is one of the environmental factors and even the most relevant one that affects the dynamics of appendicularian populations (Troedsson et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2002\u003c/span\u003e, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Kalarius and Panasiuk \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). It is a key driver regulating the sizes at maturity of appendicularians, even though other factors such as quantity and quality of food may be considered as well (Lombard et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Generally, as temperature decreases, generation time and maximum trunk length increase (Fenaux \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Capitanio et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the Southwest Atlantic Ocean (SWAO), the Argentinean Continental Shelf is known to be a highly productive environment exhibiting high concentrations of chlorophyll-a (Chl-a) in surface waters (Rivas et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Romero et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Carreto et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) providing suitable niches for ecologically relevant as well as commercially exploited species (Acha et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Franco et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). At the southernmost limit of the South Patagonian Shelf, Isla de Staten Island (SI; 54\u0026deg;55\u0026rsquo; S; 64\u0026deg;42\u0026rsquo; W; 54\u0026deg;43\u0026rsquo; S; 63\u0026deg;48\u0026rsquo;W) is separated from Isla Grande de Tierra del Fuego by the Le Maire Strait (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The Burdwood Bank (BB, 53\u0026deg;40'-55\u0026deg;S; 62\u0026deg;-58\u0026deg;40'W), is an underwater plateau 150 km east of SI. The BB is part of the northern Scotia Arc and also represents the eastward extension of the Andes Mountains. It is limited to the north by the 1800 m depth Malvinas Chasm and to the south to the Yaghanes basin, while a passage of lesser depth separates the BB from the continental shelf to the west. The BB plateau (delimited approximately by the 200 m isobath) has an extension of 370 km in a west-east direction, a mean depth of 100 m and a minimum of 50 m. Both the SI and BB are under the direct influence of the Antarctic Circumpolar Current (ACC). In particular, the Subantarctic Front (SAF) and the northern branch of the ACC associated with it, are located just south of SI and BB. Upon encountering the dramatic bathymetry of the BB, this jet splits into two main branches that contour the bank from the passages at its eastern and western limits to join further north and flow along the shelf-break as the Malvinas Current (Piola and Gordon \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Smith et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). Intense tidal currents and mixing over the bank and upwelling on its sides are predicted by numerical models (Glorioso and Flather \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Matano et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe BB is known to host fragile and valuable benthic ecosystems (e.g. Schejter et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) and is also the spawning area of pelagic species of ecological and commercial significance (e.g. Garc\u0026iacute;a Alonso et al. 2018). This rich ecosystem provides appropriate habitat for spawning and breeding different fish species, such as the Fuegian sprat and the Patagonian toothfish (Garc\u0026iacute;a Alonso et al. 2018; Riccialdelli et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). On these grounds, the BB was declared a Marine Protected Area by the Argentinian government in 2013, under the name \u0026ldquo;Namuncur\u0026aacute;\u0026rdquo;.\u003c/p\u003e \u003cp\u003eThe general zoogeography of appendicularians in the SWAO, mainly based on morphology and distributional patterns, has been reviewed by Esnal (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Despite the ecological importance of these small tunicates, most of the studies carried out so far in the Argentinean Continental Shelf are rather coastal (eg. Spinelli et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Presta et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Capitanio et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSpinelli et al. (\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) described the first registration of zooplankton in the BB during a spring study. Concerning appendicularians, they only describe the presence of the two species, \u003cem\u003eOikopleura fusiformis\u003c/em\u003e and \u003cem\u003eFritillaria boreali\u003c/em\u003es. The spring-summer season in the SWAO has been described as the most productive in terms of microbial plankton, particularly in the coast-shelf area (Malits et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e), where the highest peaks of Chl-a were related to the presence of diatoms (Guinder et al, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). This primary production would sustain the subsequent peak of zooplankton production since the density of appendicularians increases rapidly in response to an increase in food availability (Deibel and Lowen 2011). In this context, the main objective of the present study is to compare the species composition, density, biomass, and maturity stages of appendicularians between oceanic and coastal areas of the SWAO with contrasting physical and biological features using data collected from three spring cruises.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSampling\u003c/h2\u003e \u003cp\u003eZooplankton samples were obtained during three oceanographic cruises in SI (coastal area) and BB (oceanic area) during three consecutive austral springs: November 2014 (4 to 27), December 2015 (1 to17) and December 2016 (6 to 15) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB). A total of 28 zooplankton samples were collected with a small Bongo net of 67 \u0026micro;m mesh size with a 0.22 m diameter. The net was operated from the bottom to the surface using oblique tows. A mechanical flowmeter (Hydrobios, Germany) was used to measure the volume of filtered water. All samples were preserved in a 5% seawater formalin solution. During 2014 and 2016 at each station, water samples for pico- and nanophytoplankton and bacterial abundance analysis were collected with Niskin bottles from 10 m depth. Sub-samples for heterotrophic bacteria (1 mL) and pico-and nanophytoplankton (5 mL) were fixed with 0.2 \u0026micro;m-pre-filtered glutaraldehyde (0.5% and 0.1% final concentration for bacteria and pico-nanophytoplankton, respectively), incubated for 15\u0026ndash;30 min at 4 \u0026ordm;C, subsequently flash-frozen in liquid nitrogen and stored at -20 \u0026ordm;C following the protocol of Marie et al. (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2001\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eOceanographic data\u003c/h2\u003e \u003cp\u003eAt each station, vertical profiles were recorded with a factory calibrated CTD Rinko ASTD-102 (JFE, Japan). The original data, acquired at 10 Hz, was processed using common procedures and the SBE Data Processing Pack to derive water salinity (PSS-78) and density. The geo-referenced dataset was integrated and visualized with the Ocean Data View software (Schlitzer \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTemperature and salinity profiles were analyzed at each station using Data \u0026ndash; Interpolating Variational. Satellite-derived surface concentrations of Chl-a were obtained from MODIS/Aqua (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://oceancolor.gsfc.nasa.gov\u003c/span\u003e\u003cspan address=\"https://oceancolor.gsfc.nasa.gov\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003cspan type=\"Underline\" class=\"Underline\" name=\"Emphasis\"\u003e)\u003c/span\u003e with a resolution of 4 km.\u003c/p\u003e \u003cp\u003eSurface currents during the three springs considered in this study were obtained from the Ocean Surface Current Analysis Real-time, third-degree resolution (OSCAR; Bonjean and Lagerdoef 2002).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eAppendicularian analysis\u003c/h2\u003e \u003cp\u003eThe appendicularians were separated and identified in the laboratory using a Leica S6 D Greenough stereo microscope. Those samples with more than 200 specimens were fractionated in a subsample of 20 or 40 mL depending on the number of appendicularians found. All the specimens were identified to the species level and the densities at each station were calculated.\u003c/p\u003e \u003cp\u003eThe sizes (TL - trunk length) of both \u003cem\u003eOikopleura fusiformis\u003c/em\u003e and \u003cem\u003eFritillaria borealis\u003c/em\u003e species were measured (total number\u0026thinsp;=\u0026thinsp;13164) using an ocular micrometer and the total biomass was estimated from TL\u0026ndash;dry weight relationships obtained from Capitanio et al. (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). The maturity stages of appendicularians were determined as immature or mature according to B\u0026uuml;ckmann classification (Fenaux \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; B\u0026uuml;ckmann \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1972\u003c/span\u003e; Capitanio and Esnal \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Martinucci et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). At coastal (SI) and oceanic (BB) areas, stages of immature and mature specimens were determined only for \u003cem\u003eO. fusiformis\u003c/em\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eMicrobial abundances\u003c/h2\u003e \u003cp\u003eSamples for microbial abundances were analyzed with a FACSCalibur flowcytometer (Becton Dickinson) within a few months. Autotrophic populations (\u003cem\u003eSynechococcus\u003c/em\u003e, picoeukaryotes and nanoeukaryotes) were discriminated from unstained samples according to their light scatter (SSC) and specific autofluorescence properties (Marie et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Samples to determine the abundance of heterotrophic bacteria (\u003cem\u003esensu stricto\u003c/em\u003e heterotrophic Bacteria and Archaea) were stained with SYBR Green I (10 X in Dimethyl sulfoxide, DMSO) in the dark and determined in plots of 90◦light scatter (SSC) versus green DNA fluorescence (FL1) following Gasol and Moran (2015). Bacteria with low nucleic acid content (LNA) from those with high nucleic acid content (HNA) were distinguished based on their signature in the SSC versus green fluorescence\u003c/p\u003e \u003cp\u003e(FL1-H) cytometric plots using the Flow Jo software. The sample flow rate was accurately calibrated following a modified protocol of Marie et al. (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) and used to calculate \u003cem\u003ein situ\u003c/em\u003e abundances of heterotrophic bacteria and pico- and nanophytoplankton. For details see Malits et al. (\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2023b\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eStatistical analysis\u003c/h2\u003e \u003cp\u003eTo compare the abundance of \u003cem\u003eO. fusiformis\u003c/em\u003e between zones (coastal and oceanic) and spring years, a generalized linear model (GLM) was performed, assuming a Poisson distribution of errors and the most parsimonious model was selected based on information theory criteria (Venables and Ripley 2002). In this model, the abundance of \u003cem\u003eO. fusiformis\u003c/em\u003e was considered the response variable. This kind of numerical data results in variances much greater than the means, allowing us to assume a negative binomial error distribution and a log link (Crawley, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The explanatory variables included in the GLMs that attempted to explain the different abundances were zone (SI, BB) and years (2014, 2015 and 2016). The model with the lower value (most plausible model) of the Akaike information criterion (AIC) was selected as the best one and was weighed against the others using Akaike\u0026rsquo;s weight (Aw). Aw, values vary between 0 (poor fit) and 1 (good fit) and provide an estimation of the likelihood of the model given the data (Johnson and Omland, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). The model assumptions were checked using the package DHARMa.\u003c/p\u003e \u003cp\u003eParametric statistics were tested but did not meet the assumptions yet when the variance was modeled. Hence, a non-parametric analysis was carried out to compare the abundance of \u003cem\u003eF. boreali\u003c/em\u003es (response variable) between springs (explanatory variables) with Kruskal Wallis tests, respectively (Mcknight and Najab \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). ANOVA was applied to the biomass data of \u003cem\u003eF. boreali\u003c/em\u003es, to analyze differences between zones (SI and BB), and the same analysis was done for comparing springs. The Tukey\u0026ndash;Kramer method was applied to compare the different analyses. A correlation analysis was carried out between the surface temperature and TL of \u003cem\u003eO. fusiformis\u003c/em\u003e to understand the changes between these variables. In addition, taking into account the feeding habits of the appendicularians, correlations were carried out between their abundance during the three springs and the abundance of possible prey and satellite-derived Chl-a concentrations.\u003c/p\u003e \u003cp\u003eStatistical analyses were performed in the R environment (R Core Team 2017) with the packages stats (R Core Team 2017), glmmTMB (Magnusson et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), multcomp (Hothorn et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), Psych (Revelle and Revelle \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), lsmeans (Lenth 2018) and MASS (Venables and Ripley 2002).\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eEnvironmental features\u003c/h2\u003e \u003cp\u003eThe oceanographic setting, as revealed by near-surface temperature and salinity distributions and surface currents (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), is coherent with previous works, and signals the BB as an area of relative isolation within a highly dynamic regional context. The mean eastward flow of the Antarctic Circumpolar Current (ACC), coupled to the subantarctic front is consistently observed south of the BB and SI. Meanders and eddies are apparent within the ACC, but two features found at all survey dates stand out: a prominent northward deflection of the flow at the eastern passage and a large anticlockwise gyre in the Yaganes basin, southwest of the BB (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The latter, in spite of being detectable in all 3 periods considered, was more defined in spring 2014 when current vectors suggest transport of water from the western passage into the plateau. In comparison to its surroundings, currents are consistently lower over the BB plateau (however note that velocity fields in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e are 5-day integrations and tidal currents are not accounted for by OSCAR). Longitudinal west-to-east gradients of temperature and salinity (negative and positive respectively) are consistent through the years between the coastal area and the BB (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Salinity oscillated between 32.81 and 34.12 in the 3 springs considered for the entire studied domain, but the range was much narrower and more stable over the BB. Spring 2016 was the warmest of the three springs considered (7.81\u0026thinsp;\u0026plusmn;\u0026thinsp;1.11\u0026deg;C at SI; 5.66\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38\u0026deg;C at BB). In 2014 the average temperature was EI: 6.54\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07\u0026deg;C; BB: 5.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u0026deg;C), while in 2015 presented a temperature of 6.3\u0026deg;C, while over the bank the average temperature was 5.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Chl-a concentrations were in general much higher outside the bank on its western and northern flanks than on the plateau itself. The highest Chl-a concentrations on the bank were registered in 2014 with maxima around 0.93 mg m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e to the east. Meanwhile, Chl-a concentrations in SI were higher in 2015 and 2016 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). In 2015, the maximum Chl-a over the BB was 0.30 mg m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e at the western BB while In 2016 the Chl-a concentrations for this area were minimal (0.11 mg m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eAppendicularians distribution and species abundance\u003c/h2\u003e \u003cp\u003eTwo species of appendicularians were found in the area during the three springs studied, \u003cem\u003eOikopleura fusiformis\u003c/em\u003e and \u003cem\u003eFritillaria borealis.\u003c/em\u003e The distribution pattern for \u003cem\u003eO. fusiformis\u003c/em\u003e and \u003cem\u003eF. borealis\u003c/em\u003e species seems to be different, as the former occupied the entire coastal and oceanic areas whereas \u003cem\u003eF. borealis\u003c/em\u003e occurred mainly in the coastal one around SI although it was sporadically observed in the BB. Abundances of \u003cem\u003eF. borealis\u003c/em\u003e were low being highest in 2014 (up to 5 ind m\u003csup\u003e3\u003c/sup\u003e). In the case of \u003cem\u003eO. fusiformis\u003c/em\u003e, significant differences were found between areas (SI and BB) along the three springs (X\u003csup\u003e2\u003c/sup\u003e test, LR\u0026thinsp;=\u0026thinsp;4.89; p\u0026thinsp;=\u0026thinsp;0.03) being more abundant in SI. On the other hand, when evaluating the difference between years, differences between 2014 and the remaining years were detected (X\u003csup\u003e2\u003c/sup\u003e test, LR\u0026thinsp;=\u0026thinsp;13.36 p\u0026thinsp;=\u0026thinsp;0.001). During the spring of 2014, the distribution of \u003cem\u003eO. fusiformis\u003c/em\u003e was throughout all the sampling stations, particularly in the BB and their abundances were much higher than for the other springs (100 times more) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA). The highest density for this species was registered to the east of the BB, with a value of 313 ind m \u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e, while in SI the densities were similar in all the samples with an average value of 57 ind m \u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e (range: 50 to 63 ind m \u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e). As for 2015, the highest densities of the \u003cem\u003eO. fusifomis\u003c/em\u003e were recorded at the east of the BB with a maximum value of 36 ind m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e. Finally, in the spring of 2016, its densities increased east of the BB and west of the SI and the maximum density registered were 146 and 19 ind m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e for SI and BB respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA). Comparing the biomass for \u003cem\u003eO. fusiformis\u003c/em\u003e, no significant differences were observed between the three springs (X\u003csup\u003e2\u003c/sup\u003e test, LR\u0026thinsp;=\u0026thinsp;4.26; p\u0026thinsp;\u0026gt;\u0026thinsp;0.12). However, when comparing the areas, significant differences were found (X\u003csup\u003e2\u003c/sup\u003e test, LR\u0026thinsp;=\u0026thinsp;18.69; p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), being higher in SI. In SI, the values were similar between springs, while in BB the highest average biomass was recorded in 2015 with a value of 8\u0026thinsp;\u0026plusmn;\u0026thinsp;12 \u0026micro;g C m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e. The biomass contribution to \u003cem\u003eF. borealis\u003c/em\u003e was very low (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe sizes of \u003cem\u003eO. fusiformis\u003c/em\u003e were always smaller in SI. In the BB, the highest mean size was found during 2016 with a value of 783\u0026thinsp;\u0026plusmn;\u0026thinsp;387 \u0026micro;m; followed by 2015 (776\u0026thinsp;\u0026plusmn;\u0026thinsp;251 \u0026micro;m) and lastly spring 2014 (770\u0026thinsp;\u0026plusmn;\u0026thinsp;350 \u0026micro;m). On the other hand, in SI the pattern was different and more marked: the highest mean size was found in 2015 with a value of 747\u0026thinsp;\u0026plusmn;\u0026thinsp;210 \u0026micro;m followed by 2014 with a value of 703\u0026thinsp;\u0026plusmn;\u0026thinsp;239 \u0026micro;m and 2016 with an average size of 630\u0026thinsp;\u0026plusmn;\u0026thinsp;281 \u0026micro;m. The same comparison but for \u003cem\u003eF. borealis\u003c/em\u003e in SI showed that the highest mean size was found in 2015 with a value of 822\u0026thinsp;\u0026plusmn;\u0026thinsp;201\u0026micro;m, followed by 2014 and lastly 2016 (683\u0026thinsp;\u0026plusmn;\u0026thinsp;233 \u0026micro;m and 670\u0026thinsp;\u0026plusmn;\u0026thinsp;270 \u0026micro;m, respectively). In this context, 2015 was the year that exhibited the highest sizes in SI, for both species. During spring 2014, covered the entire length ranges proposed for the \u003cem\u003eO. fusiformis\u003c/em\u003e species in both areas. However, \u003cem\u003eF. borealis\u003c/em\u003e in SI presented organisms between 300 and \u0026gt;\u0026thinsp;1000 \u0026micro;m, with specimens between 600 and 900 \u0026micro;m being the most frequent. In spring 2015, it was observed that the \u003cem\u003eO. fusiformis\u003c/em\u003e species covered almost the entire size spectrum in both areas (BB and SI). Regarding \u003cem\u003eF. borealis\u003c/em\u003e, the size distribution was limited between 400 and 1000 \u0026micro;m. The most frequent size range among them was between 800\u0026ndash;900 \u0026micro;m. Finally, spring 2016 was noted for its wide variety of sizes, both in SI and in the BB for the species \u003cem\u003eO. fusiformis\u003c/em\u003e exhibiting in SI the smallest sizes in this study. Regarding \u003cem\u003eF. borealis\u003c/em\u003e, no individuals with small sizes (100\u0026ndash;300 \u0026micro;m) were found in SI. In general, the largest sizes were the most frequent for \u003cem\u003eO. fusiformis\u003c/em\u003e in BB throughout all the years under study but not in SI (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The year 2015 presented the highest percentage of \u003cem\u003eO. fusiformis\u003c/em\u003e immature individuals in BB and SI. Focusing on spatial differences for \u003cem\u003eO. fusiformis\u003c/em\u003e, immature and mature individuals were observed to mean larger sizes in BB than in SI during all three springs (Table\u0026nbsp;1). A significant negative correlation was found between appendicularian sizes and temperature on SI (Correlation Spearman = -0.11, p value\u0026thinsp;=\u0026thinsp;0.039). This coincides with the spring of 2016 when the highest temperature value was recorded and the average sizes were smaller. Also, the inverse pattern is seen in 2015, when the lowest temperature value was recorded and the average sizes were higher (see Table\u0026nbsp;1).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eDistribution of nanoplankton, picoplankton, Synechococcus, and bacteria as potential food sources of appendicularians\u003c/h2\u003e \u003cp\u003eOf the two years (2014 and 2016), in which the abundance of the autotrophic nano- and picoeukaryotes and heterotrophic bacteria were analyzed, in spring 2016 the highest numbers were observed with an average abundance of autotrophic nano-picoeukaryotes and heterotrophic bacteria of 6835\u0026thinsp;\u0026plusmn;\u0026thinsp;4886 cell mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4 x 10\u003csup\u003e5\u003c/sup\u003e cell mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The abundance of nano- and picoeukaryotes was higher in BB than in SI. The easternmost stations of the BB exhibited the highest picoeukaryotes values for the area along with the lowest chlorophyll values while at the western edge of the submarine plateau nanophytoplankton, i.e. larger cells, dominated along with total high phytoplankton biomass (in terms of chl-a). However, in the center of BB, \u003cem\u003eSynechococcus\u003c/em\u003e abundance was very low, as well as westwards of SI. In the spring of 2014, \u003cem\u003eSynechococcus\u003c/em\u003e reached the maximum value of 3 x 10\u003csup\u003e3\u003c/sup\u003e cells mL\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the eastern region of BB along with high nano- and picoeukaryotes abundance. In the central area much lower densities were recorded in terms of the picoplanktonic fraction, the latter trend was maintained in SI, however, in this region, autotrophic picoeukaryotes dominated the pico-nanophytoplankton. It is important to highlight that in all the analyzed stations the cyanobacterium \u003cem\u003eSynechococcus\u003c/em\u003e was present. In 2014, heterotrophic bacteria were most abundant eastwards of SI and in the central area of BB (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn spring 2016, bacterial abundance decreased from high values west of the SI throughout the transition zone and then increased to the east of the bank, indicating a higher average abundance in the BB compared to SI. In the spring of 2014, appendicularians abundance correlated negatively with Chl-a (r=-0.70, p\u0026thinsp;=\u0026thinsp;0.0344) but positively with nanoplankton (r\u0026thinsp;=\u0026thinsp;0.79, p\u0026thinsp;=\u0026thinsp;0.0069) and picoplankton (r\u0026thinsp;=\u0026thinsp;0.71, p\u0026thinsp;=\u0026thinsp;0.0227). However, the correlation with the rest of the potential picoplankton prey (\u003cem\u003eSynechococcus\u003c/em\u003e and bacteria) was not significant (r\u0026thinsp;=\u0026thinsp;0.39 and r\u0026thinsp;=\u0026thinsp;0.33, p\u0026thinsp;\u0026gt;\u0026thinsp;0.05). On the other hand, in 2015 and 2016 with significantly lower total phytoplankton biomass, appendicularian densities correlated positively with Chl-a (r\u0026thinsp;=\u0026thinsp;0.87, p\u0026thinsp;=\u0026thinsp;0.055 and r\u0026thinsp;=\u0026thinsp;0.83, p\u0026thinsp;=\u0026thinsp;0.0015, respectively).\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eSpatial distribution of appendicularians\u003c/h2\u003e \u003cp\u003eTwo species of appendicularians were found in this study: \u003cem\u003eOikopleura fusiformis\u003c/em\u003e and \u003cem\u003eFritillaria borealis\u003c/em\u003e, the former being the dominant one. Their longitudinal distribution along the inshore-offshore gradient of the study area is consistent with their known habits.\u003c/p\u003e \u003cp\u003e \u003cem\u003eF. borealis\u003c/em\u003e was mainly concentrated in the inshore waters of SI, which is coherent with its typical coastal distribution in this subarctic region (Aguirre et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Presta et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Kalarus and Panasiuk (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) also found the greatest abundances of \u003cem\u003eF. borealis\u003c/em\u003e near the coasts of Tierra del Fuego and the Antarctic Peninsula but, interestingly, they also recorded this species offshore in the Drake Passage. Likewise, in our study, in spite of a higher presence in SI, it is noteworthy that \u003cem\u003eF. borealis\u003c/em\u003e occurred occasionally at the BB. Most of these records are restricted to the western limit of the BB, which is to some extent connected to SI by the prevailing currents (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and a punctual record in the central BB close to the slope. Although the BB presents a rather closed circulation (Matano et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), casual advection into the bank from the Drake Passage has been detected by means of surface drifters (Martin et al. 2023a,b). \u003cem\u003eF. borealis\u003c/em\u003e was also detected on the bank in spring 2014, at its western limit and in coincidence with an intensification of the eddy southwest of the bank. From these observations and given the rarity of \u003cem\u003eF. borealis\u003c/em\u003e occurrences over the bank, we postulate that the water intrusions from outside the bank, and eventually the presence of this species inside would be used in the future as a tracer of such processes. \u003cem\u003eF. borealis\u003c/em\u003e was an important component of zooplankton particularly in spring near SI, coinciding with maximum values of chlorophyll-a values (See Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Particularly in 2014, a positive correlation was observed between \u003cem\u003eF. borealis\u003c/em\u003e density and chlorophyll. The western part of the BB is also the most productive one in terms of phytoplankton biomass, such as the coastal regions, could this be the reason for their distribution \u003cem\u003eRegarding, O. fusiformis\u003c/em\u003e was recorded consistently in both SI and the BB (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), being a species of the more oceanic character (Zoppi de Roa, 1971) and usually found in both coastal and oceanic waters (e.g. Capitanio et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Panasiuk and Kalarus, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003ePopulation dynamics of appendicularians\u003c/h2\u003e \u003cp\u003eThe population dynamics of appendicularians are usually related to the amount of available food, predators, salinity and temperature (Shiga \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Taggart and Frank \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e1987\u003c/span\u003e; Tomita et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Troedsson et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Appendicularians are a key link in marine food webs, preying on nano- and picoplankton at the base of the food web and linking them to higher trophic levels. Therefore, they are mediators in the trophic cascade between the components of the marine microbial food web and the main predators in the study area. The abundance of nano-, picoeukaryotes and bacteria was in general higher in BB than in SI for both years, while 2016 presented higher values than 2014 in contrast to the chlorophyll data presented here.\u003c/p\u003e \u003cp\u003eThe distribution of satellite Chl-\u003cem\u003ea\u003c/em\u003e (proxy of phytoplankton biomass and thus possible food indicator) was different between years in the BB and similarly in SI. It was observed that concentrations of Chl-a are higher in SI than at BB and similar to those at the mouth of the Beagle Channel. Particularly in the SI area, the chlorophyll concentration was higher in 2015 and 2016 along with a higher density of \u003cem\u003eO. fusiformis\u003c/em\u003e. Instead, the BB showed spatial differences: while during 2014, more Chl\u0026ndash;\u003cem\u003ea\u003c/em\u003e was found west of the BB, in 2015 the eastern part increased in total phytoplankton biomass and the spring of 2016 presented a similar distribution of chl-a over the plateau. The surface current's speed seems to reflect the total phytoplankton biomass, i.e. the higher the current speed, the greater the Chl-a concentration and accordingly, the lowest satellite Chl-a along with the lowest current speed as for spring of 2016. In the spring of 2014 Bertola et al. (2018) found the highest analytical \u003cem\u003ein situ\u003c/em\u003e Chl-a values ​​towards the west of the BB, matching the satellite images selected here, and the dominance of the diatom \u003cem\u003eRhizosolenia crassa\u003c/em\u003e, followed by aloricated and flagellated ciliates in BB. Acu\u0026ntilde;a et al. (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) demonstrated that the appendicularians abundance is lower in the presence of phytoplankton blooms dominated by large diatoms in Baffin Bay, North America. This could explain the lower appendicularian densities registered at the western part of the BB in contrast to the other areas. In any case, Guinder et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) studied the distribution and composition of microbial plankton during the spring of 2016. They observed a decreasing concentration gradient of Chl-a in surface layers from the Beagle Channel to the BB being the Beagle Channel - Shelf enriched in diatoms. At the same time, the transition zone located between SI and BB was more abundant in coccolithophorids and flagellates, and microheterotrophs abounded in the BB. All of these potential prey for appendicularians could support the abundance in a site with complex oceanographic dynamics for organisms as fragile as appendicularians. SI is largely bathed by the outermost waters of the mouth of the Beagle Channel, carrying nutrients that favor high levels of Chl-a and primary productivity. In addition to a tidal front that interacts there with the bottom producing an energy transfer, an abrupt change in the water depths from the order of 4000 m to 80 m promotes the fertilization of surface waters due to local upwellings (Matano et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Guinder et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). This leads to large phytoplankton blooms (Paparazzo 2010) which allow zooplankton to develop (bottom-up control). Quantitative microscopy studies of faecal pellets indicate bacteria, cyano-bacteria, pennate and centric diatoms, dinoflagellates, choanoflagellates, ciliates, and coccolithophores as important dietary constituents of zooplankton (Deibel and Turner \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Urban et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Acu\u0026ntilde;a et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). More recently, in situ studies combined with flow cytometry and sequencing revealed that appendicularians are capable of grazing picocyanobacteria, i.e. \u003cem\u003eSynechococcus\u003c/em\u003e and \u003cem\u003eProchlorococcus\u003c/em\u003e, at high rates (Scheinberget al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Dadon-Pilosof et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Therefore, this explains the presence of high abundances of appendicularians (mainly in 2014) in an oligotrophic oceanic zone such as the BB.\u003c/p\u003e \u003cp\u003eJust as phytoplankton is vital for the herbivorous link in the food web to thrive, the proper growth of zooplankton is vital for the higher links. From the perspective of higher trophic levels, appendicularians are one of the main components of the marine mesozooplankton, they are characterized by being primary consumers and food for pelagic fish larvae. The Fuegian sprat, \u003cem\u003eSprattus fuegensis\u003c/em\u003e is one of the most abundant zooplanktophagous pelagic fishes in the southern sector of the Patagonian shelf (S\u0026aacute;nchez et al. 1995) and is known to spawn in the BB (Garcia Alonso et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). In the same campaign, Garcia Alonso et al (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) found high densities of fuegian sprat larvae, mainly towards the eastern zone of the BB. These results match the high densities of appendicularians found in the present study, particularly in spring 2014. The mouth opening of the fuegian sprat larvae collected in spring 2014, ranged between 680 \u0026micro;m and 760 \u0026micro;m, with a maximum value of 1000 \u0026micro;m (Spinelli et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The range of food particles ingested by sardine larvae oscillates in these values, therefore the recorded sizes of appendicularian indicate that they would be a potential prey. Although the total densities of appendicularians are lower in SI, it was seen that the biomass was higher in that zone, which would indicate a greater contribution of carbon available for higher trophic levels. Unfortunately, for the area of SI, there is no information on sardine larvae.\u003c/p\u003e \u003cp\u003eIn relation to environmental variables (salinity and temperature), given the low oscillation between the salinity values registered between areas and springs, this would not be affecting the distribution of the appendicularians species and nor could the recorded abundances be explained by said parameter. Temperature is a key driver in regulating the sizes at maturity of appendicularians since its increase causes the acceleration of gonadal maturation (Fenaux \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Capitanio et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Numerous studies have shown that, when temperature decreases, generation time is maximum. The temperature of the BB was lower than in SI in spring and this could be related to the larger appendicularian sizes found in the BB. This is also clearly seen for both species between springs, with for example spring 2015 having the lowest temperatures and the largest mature and immature specimens. Panasiuk and Kalarus (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) remarked that temperature was the strongest environmental factor influencing the larvacean community structure in the Drake Passage. Nevertheless, in the present study, there was no direct relationship between temperature and appendicularian density and distribution in both areas.\u003c/p\u003e \u003cp\u003eAppendicularians can become reproductively mature and spawn over a wide range of trunk lengths, and larger animals generally produce more eggs. When temperature increases, trunk length at the maturity stage and fecundity generally decrease (Lombard et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Given that in spring 2015 appendicularians showed larger sizes on maturity, higher egg production would be also expected. The mature: juvenile ratio in spring 2014 and 2016 suggests that a certain time span had elapsed since the last reproductive pulse. Instead, the lower mature: juvenile ratio registered during spring 2015 is indicative of a recent reproductive event. However, this was not reflected in a high abundance of appendicularians. On SI, temperatures were higher, and the proportion of mature organisms was similar to that registered in BB during the three springs analyzed.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eImportance of appendicularians in the oceanic marine protected area Namuncur\u0026aacute;/ Burdwood Bank\u003c/h2\u003e \u003cp\u003eAppendicularians represent an important food item for larvae and adult fish (Capitanio et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Gorsky et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) and play a key role in both the formation of marine snow and the flow of organic matter. Despite this relevance, there are no ecological studies on appendicularians in the marine protected area. In this context, the present study allowed discerning the following ecological aspects of appendicularians from Sub-antarctic environments. It is also important to highlight that the appendicularians are the second group that dominates after the copepods in the zooplankton samples recorded in the area (Spinelli et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Thus, the presence of bacteria, pico and nanoplankton fraction, and high-concentration of Chl-a (spring 2014) in this area constitutes a suitable environment for appendicularians reproduction in spring, thus enhancing the survival and growth of several small pelagic fishes such as \u003cem\u003eS. fuegensis\u003c/em\u003e. BB zones play a key role in ecological processes in the ocean, allowing an exceptionally large primary production, offering adequate feeding and reproductive habitats for planktivorous species and acting as retention areas for larvae. So, the biomass of appendicularians contributes to the transfer of carbon to higher trophic levels and is probably important for the survival and growth of various small pelagic fish such as the Fuegian sprat. On the other hand, the Southern Ocean and Antarctica are warming and their waters are particularly susceptible to ocean acidification (Swart et al. \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Trull et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Experimental studies showed that the appendicularian abundance was positively correlated with increased pCO2 (Troedsson et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). However, the ecological importance of these organisms, along with progressive climate changes, would be in line with the general trend of the increasing importance of jellyfish organisms (e.g., tunicates and cnidarians) in all marine environments, including polar regions (Panasiuk et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Kalarus and Panasiuk \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). When facing global change, basic information on these small tunicates is necessary, particularly regarding their important trophic role.\u003c/p\u003e \u003cp\u003eThe conservation of marine resources is now a major scientific and social goal. Direct and indirect anthropogenic pressures are threatening the functioning and diversity of marine ecosystems and the services they provide (Gattuso et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Baseline data on biodiversity, the abundance of species and their distribution in protected areas and surrounding areas is essential to contribute to the stakeholders in these areas and advise on future changes that translate into regional and global processes. This study is the first to document the population dynamics of appendicularians at the BB and SI, a region of particular interest in the Southwest Atlantic Ocean for its intense hydrographic activity, complex circulation, and high conservation value, and also provide new knowledge on the ecology of appendicularioans in sub-Antarctic waters. To know the behaviour of the appendicularians species that are represented and their role in the food web future studies should be focused on determining through which mechanisms and to what extent the changes in spatial and interannual appendicularians abundance are the result of either physical or/and biological processes. Also incorporate seasonal studies, to understand with better resolution the dynamics of appendicularia in this sub-antarctic zone\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by the Consejo Nacional de Investigaciones Cient\u0026iacute;ficas y T\u0026eacute;cnicas (CONICET), Argentina (PIP 11220150100109CO and PIP 11220150100368CO), Universidad de Buenos Aires, Argentina (UBACYT 20020190100133BA 2020-2024), ANPCyT (PICT- 2015- 0384 and PICT-2019-04049) and the Pampa Azul Interministerial Initiative implemented by the Argentinian Ministry for Science, Technology and Productive Innovation. The authors thank the leaders and coordinators of the cruises throughout the years, as well as Maria Laura Presta, Alejandro Martinez, and everyone who participated in the oceanographic surveys for their cooperation during sample collection and preparation. Clara Natalia Rodriguez Flores for her help onboard with sampling for the microbial abundances\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003eVirginia Garc\u0026iacute;a Alonso and Harold Fenco for their valuable contributions in terms of statistics and satellite image acquisition.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eThis is Marine Protected Area Namuncur\u0026aacute;-Burdwood Bank (Law 26,875) contribution No. XXX.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAcha EM, Pajaro M, Sanchez RP (1999) The reproductive response of clupeoid fishes to different physical scenarios. Three study cases in the Southwest Atlantic. ICES-CM-1999/K, 12\u003c/li\u003e\n\u003cli\u003eAcu\u0026ntilde;a JL, Deibel D, Saunders PA et al (2002) Phytoplankton ingestion by appendicularians in the North Water.Deep Sea Res Part II Top Stud Oceanogr 49 (22-23):5101-5115. https://doi.org/10.1016/S0967-0645(02)00180-7\u003c/li\u003e\n\u003cli\u003eAguirre GE, Capitanio FL, Lovrich GA, Esnal GB (2012). Seasonal variability of metazooplankton in coastal sub-Antarctic waters (Beagle Channel). Mar Biol Res 8(4):341-353 https://doi.org/10.1080/17451000.2011.627922\u003c/li\u003e\n\u003cli\u003eB\u0026eacute;rtola G, Olgu\u0026iacute;n Salinas H, Iachetti C, Lovrich G, Alder V (2018) Estructura del plancton unicelular eucariota del AMP Namuncur\u0026aacute;- Banco Burdwood en primavera. Conexiones con las adyacencias. In:Menoret A, Guller M (eds) Libro de res\u0026uacute;menes X JNCM, Buenos Aires, 74 pp\u003c/li\u003e\n\u003cli\u003eBonjean F, Lagerloef GSE (2002) Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean. J Phys Oceanogr 32:2938-2954 https://doi.org/10.1175/1520-0485(2002)032\u0026lt;2938:DMAAOT\u0026gt;2.0.CO;2\u003c/li\u003e\n\u003cli\u003eB\u0026uuml;ckmann A (1972) Die Appendicalarien von den Fahrten der Meteor, derAnton Bruum und der Discovery in das Arabische Meer imRahmen der IIOE. \u0026ldquo;Meteor\u0026quot; Forsch Ergebn D 10:l-45 \u003c/li\u003e\n\u003cli\u003eCapitanio FL, Esnal GB (1996) Caracterizaci\u0026oacute;n morfom\u0026eacute;trica de los estadios de madurez de \u003cem\u003eOikopleura fusiformis\u003c/em\u003e (Tunicata, Appendicularia). Phys 52(122-123):13-17\u003c/li\u003e\n\u003cli\u003eCapitanio FL, P\u0026aacute;jaro M, Esnal GB (2005). Appendicularians: an important food supply for the Argentine anchovy \u003cem\u003eEngraulis anchoita\u003c/em\u003e in coastal waters. J Appl Ichthyol, 21(5): 414-419. https://doi.org/10.1111/j.1439-0426.2005.00657.x\u003c/li\u003e\n\u003cli\u003eCapitanio FL, Curelovich J, Tresguerres M, Negri RM, Vi\u0026ntilde;as MD, Esnal GB (2008) Seasonal Cycle of Appendicularians at a Coastal Station (38\u0026deg; 28\u0026prime; s, 57\u0026deg; 4\u0026prime; w) of the SW Atlantic Ocean. Bull Mar Sci 82(2):171-184\u003c/li\u003e\n\u003cli\u003eCapitanio FL, Spinelli ML, Presta ML et al (2018) \u0026ldquo;Ecological role of common appendicularian species from shelf waters of Argentina\u0026rdquo;.In: Hoffmeyer MS, Sabatini ME, Brandini FP, Calliari DL, Santinelli NH (eds) Plankton ecology of the Southwestern Atlantic - From the subtropical to the subantarctic realm. Springer, Cham, pp 201\u0026ndash;218 https://doi.org/10.1007/978-3-319-77869-3_10\u003c/li\u003e\n\u003cli\u003eCarreto JI, Carignan MO, Montoya NG, Cucchi Colleoni AD (2007) Ecolog\u0026iacute;a del fitoplancton en los sistemas frontales del Mar Argentino.In: S\u0026aacute;nchez RP, Bezzi SI (eds) El Mar Argentino y sus recursos pesqueros. El ecosistema marino. Publicaciones Especiales INIDEP, Mar del Plata, pp 11\u0026ndash;31\u003c/li\u003e\n\u003cli\u003eCrawley MJ (2005) Statistics: an introduction using R, 2005. Wiley, ISBN 0, 470(02298), 1\u003c/li\u003e\n\u003cli\u003eDadon-Pilosof A, Conley KR, Jacobi Y, et al (2017). Surface properties of SAR11 bacteria facilitate grazing avoidance. Nat. Microbiol, 2(12):1608-1615. https://doi.org/10.1038/s41564-017-0030-5\u003c/li\u003e\n\u003cli\u003eDeibel D, Turner JT (1985). Zooplankton feeding ecology: contents of fecal pellets of the appendicularian \u003cem\u003eOikopleura vanhoeffeni\u003c/em\u003e. Mar Ecol Prog Ser, 67-78\u003c/li\u003e\n\u003cli\u003eEsnal GB (1999) Appendicularia. In: Boltovskoy D (ed) South Atlantic zooplankton. Leiden,The Netherlands: Backhuys, p\u0026aacute;gs. 1375\u0026ndash;1399\u003c/li\u003e\n\u003cli\u003eFenaux R (1976) Cycle vital, croissance et production chez\u003cem\u003e Fritillaria pellucida\u003c/em\u003e (Appendicularia), dans la baie de Villefranche-sur-Mer, France. Mar Biol 34(3):229-238. https://doi.org/10.1007/BF00388800\u003c/li\u003e\n\u003cli\u003eFenaux R (1985) Rhythm of secretion of oikopleurid\u0026apos;s houses. Bull Mar Sci 37(2): 498-503\u003c/li\u003e\n\u003cli\u003eFranco BC, Combes V, Gonzalez Carman V (2020) Subsurface ocean warming hotspots and potential impacts on marine species: the southwest South Atlantic Ocean case study. Front Mar Sci 7, 824 https://doi.org/10.3389/fmars.2020.563394\u003c/li\u003e\n\u003cli\u003eGarcia Alonso VA, Brown D, Mart\u0026iacute;n J, P\u0026aacute;jaro M, Capitanio FL (2018) Seasonal patterns of Patagonian sprat \u003cem\u003eSprattus fuegensis\u003c/em\u003e early life stages in an open sea Sub-Antarctic Marine Protected Area. Polar Biol 41(11):2167-2179 https://doi.org/10.1007/s00300-018-2352-z\u003c/li\u003e\n\u003cli\u003eGasol JM, Del Giorgio PA (2000) Using flow cytometry for counting natural planktonic bacteria and understanding the structure of planktonic bacterial communities. Sci Mar 64(2):197-224. https://doi.org/10.3989/scimar.2000.64n2197\u003c/li\u003e\n\u003cli\u003eGattuso JP, Magnan A, Bill\u0026eacute; R et al (2015). Contrasting futures for ocean and society from different anthropogenic CO2 emissions scenarios. Sci, 349(6243):aac4722. DOI: 10.1126/science.aac4722\u003c/li\u003e\n\u003cli\u003eGlorioso PD, Flather RA (1995) A barotropic model of the currents off SE South America. J Geophys Res Oceans 100:13427-13440. https://doi.org/10.1029/95JC00942\u003c/li\u003e\n\u003cli\u003eGorsky, G, Fenaux, F (1998) The role of Appendicularians in marine food webs. In: Bone Q. (Ed.). The biology of Pelagic Tunicates. Oxford Univ. Press., Oxford, pp 161-169 \u003c/li\u003e\n\u003cli\u003eGorsky G, Chritennot-Dinet MJ, Blanchot J, Palazzoli (1999) Picoplankton and nanoplankton aggregation by appendicularians Fecal pellet contents of \u003cem\u003eMegalocercus huxleyi\u003c/em\u003e in the equatorial Pacific. J Geophys Res 104:3381-3390 \u003c/li\u003e\n\u003cli\u003eGorsky G, Youngbluth MJ, Deibel D (2005) Response of Marine Ecosystems to Global Change: Ecological Impact of Appendicularians. GB Scientific Publisher, Paris. 440 pp \u003c/li\u003e\n\u003cli\u003eGuinder VA, Malits A, Ferronato C, Krock B, Garz\u0026oacute;n-Cardona J, Mart\u0026iacute;nez A (2020) Microbial plankton configuration in the epipelagic realm from the Beagle Channel to the Burdwood Bank, a Marine Protected Area in Sub-Antarctic waters. Plos one, 15(5): e0233156\u003c/li\u003e\n\u003cli\u003eHothorn T, Bretz F, Westfall P (2008) Simultaneous inference in general parametric models. Biom J 50(3):346-363. https://doi.org/10.1002/bimj.200810425\u003c/li\u003e\n\u003cli\u003eJohnson JB, Omland KS. (2004) Model selection in ecology and evolution. Trends Ecol Evol 19(2): 101-108. https://doi.org/10.1016/j.tree.2003.10.013\u003c/li\u003e\n\u003cli\u003eKalarus M, Panasiuk A (2021) Spatial Distribution Patterns of Appendicularians in the Drake Passage: Potential Indicators of Water Masses?. Diversity, 13(7):286. https://doi.org/10.3390/d13070286\u003c/li\u003e\n\u003cli\u003eLenth R, Singmann H, Love J (2018). Emmeans: Estimated marginal means, aka least-squares means. R package version, 1(1)\u003c/li\u003e\n\u003cli\u003eLindsay, MCM, Williams, GD (2010) Distribution and abundance of Larvaceans in the Southern Ocean between 30 and 80\u0026deg;E. Deep-Sea Res 57:905\u0026ndash;915.doi:10.1016/j.dsr2.2009.04.021.\u003c/li\u003e\n\u003cli\u003eLombard F, Renaud F, Sainsbury C, Sciandra A,Gorsky G (2009) Appendicularian ecophysiology I. Food concentration dependentclearance rate, assimilation efficiency, growth and reproduction of \u003cem\u003eOikopleura dioica\u003c/em\u003e. J Marine Syst 78: 606-616. https://doi.org/10.1016/j.jmarsys.2009.01.004\u003c/li\u003e\n\u003cli\u003eMagnusson A, Skaug H, Nielsen A et al (2017). Package \u0026lsquo;glmmTMB\u0026rsquo;. R Package Version 0.2. 0.\u003c/li\u003e\n\u003cli\u003eMalits A, Ibarbalz FM, Mart\u0026iacute;n J, Flombaum P (2023a). Higher biotic than abiotic natural variability of the plankton ecosystem revealed by a time series along a subantarctic transect. J Mar Syst 238: 103843.https://doi.org/10.1016/j.jmarsys.2022.103843\u003c/li\u003e\n\u003cli\u003eMalits A, Monforte C, Iachetti C, Gerea M, Latorre M (2023b). Source characterization of dissolved organic matter in the eastern Beagle Channel from a spring situation. J Mar Syst 240: 103863.https://doi.org/https://doi.org/10.1016/j.jmarsys.2023.103863\u003c/li\u003e\n\u003cli\u003eMarie D, Partensky F, Vaulot D, Brussaard C (2001). Enumeration of phytoplankton, bacteria, and viruses in marine samples. Curr Protoc 10(1):11-11.https://doi.org/10.1002/0471142956.cy1111s10\u003c/li\u003e\n\u003cli\u003eMart\u0026iacute;n J, Alonso G, Dragani W, Meyerj\u0026uuml;rgens J, Giesecke R, Cucco A, Fenco H (2023a). General circulation and tidal wave propagation along the Beagle Channel. J Mar Syst 240: 103889. https://doi.org/10.1016/j.jmarsys.2023.103889.\u003c/li\u003e\n\u003cli\u003eMart\u0026iacute;n J, Meyerj\u0026uuml;rgens, J, Badewien TH, Alonso G, Dragani W, Cucco A, Giesecke R, Wollschl\u0026auml;ger J (2023b). Assessment of currents and ecosystem connectivity in subantarctic waters (Beagle Channel - Burdwood Bank). ASLO Aquatic Sciences Meeting. \u003c/li\u003e\n\u003cli\u003eMartinucci G, Brena C, Cima F, Burighel P (2005) \u003cem\u003eSynchronous spermatogenesis\u003c/em\u003e in appendicularians. In: Gorsky G, Youngbluth M, Deibel D (eds) Response of marine ecosystems to global changes: ecological impact of appendicularians. Paris: Contemporary Publishing International pp, 113-125.\u003c/li\u003e\n\u003cli\u003eMatano RP, Palma ED, Combes V (2019) The Burdwood Bank Circulation. J Geophys Res Oceans 124:(10), 6904-6926.https://doi.org/10.1029/2019JC015001\u003c/li\u003e\n\u003cli\u003eMcKnight PE, Najab J (2010) Mann‐Whitney U Test. The Corsini encyclopedia of psychology, 1-1.https://doi.org/10.1002/9780470479216.corpsy0524\u003c/li\u003e\n\u003cli\u003ePanasiuk A, Grzonka L, Prątnicka P, Wawrzynek-Borejko J, Szymelfenig M (2020). Zonal variability of pelagic Siphonophora (Cnidaria) in the atlantic sector of the southern ocean. J Sea Res, 165, 101951. https://doi.org/10.1016/j.seares.2020.101951\u003c/li\u003e\n\u003cli\u003ePanasiuk A, Kalarus M (2021). Appendicularia (Tunicata) in an Antarctic Glacial Fjord\u0026ndash;Chaotic Fjordic Structure Community or Good Indicators of Oceanic Water Masses?. Diversity 13(12):675. https://doi.org/10.3390/d13120675\u003c/li\u003e\n\u003cli\u003ePiola AR, Gordon AL (1989) Intermediate waters in the southwest South Atlantic. Deep-Sea Res.I Oceanogr Res Pap 36:1\u0026ndash;16.https://doi.org/10.1016/01980149(89)90015-0\u003c/li\u003e\n\u003cli\u003ePresta ML, Hoffmeyer, MS, Capitanio FL (2015). \u0026quot;Population structure and maturity stages of \u003cem\u003eFritillaria borealis \u003c/em\u003e(Appendicularia, Tunicata): seasonal cycle in Ushuaia Bay (Beagle Channel).\u0026quot; Braz J Oceanogr 63(3): 279-288.https://doi.org/10.1590/S1679-87592015087206303 \u003c/li\u003e\n\u003cli\u003ePurcell JE, Studervant MV, Galt CP (2005) A review of appendicularians as prey of invertebrates and fish predators. In: Gorsky G, Youngbluth MJ, Deibel D (eds) Response of marine ecosystems to global change: ecological impact of appendicularians. Contemporary Publishing International, Paris, pp, 359\u0026ndash;435\u003c/li\u003e\n\u003cli\u003eRevelle W, Revelle MW (2015). Package \u0026lsquo;psych\u0026rsquo;. The Comprehensive R Archive Network\u003c/li\u003e\n\u003cli\u003eRiccialdelli L, Becker YA, Fioramonti NE, Torres M, Bruno DO, Rey AR, Fern\u0026aacute;ndez DA (2020). Trophic structure of southern marine ecosystems: a comparative isotopic analysis from the Beagle Channel to the oceanic Burdwood Bank area under a wasp-waist assumption. Mar Ecol Prog Ser, 655:1-27.https://doi.org/10.3354/meps13524\u003c/li\u003e\n\u003cli\u003eRivas AL, Dogliotti AI, Gagliardini DA (2006). Seasonal variability in satellite-measured surface chlorophyll in the Patagonian Shelf. Cont Shelf Res 26(6):703-720. https://doi.org/10.1016/j.csr.2006.01.013\u003c/li\u003e\n\u003cli\u003eRomero SI, Piola AR, Charo M, Garcia CAE (2006). Chlorophyll a variability off Patagonia based on SeaWiFS data. J Geophys Res Oceans 111. https://doi.org/10.1029/2005JC003244\u003c/li\u003e\n\u003cli\u003eS\u0026aacute;nchez RP, Ciechomski JD (1995) Spawning and nursery grounds of pelagic fish species in the sea-shelf off Argentina and adjacent areas Sci Mar 59:455\u0026ndash;478\u003c/li\u003e\n\u003cli\u003eScheinberg RD, Landry MR, Calbet A (2005). Grazing of two common appendicularians on the natural prey assemblage of a tropical coastal ecosystem. Mar Ecol Prog Ser, 294: 201-212. doi:10.3354/meps294201\u003c/li\u003e\n\u003cli\u003eSchejter LS, Martin J, Lovrich G (2017). Unveiling the submarine landscape of the Namuncur\u0026aacute; marine protected area, Burdwood Bank, SW Atlantic Ocean. Pan-Am J Aquat Sci 12:248\u0026ndash;253\u003c/li\u003e\n\u003cli\u003eSchlitzer R (2020) Ocean Data View, https://odv.awi.de.\u003c/li\u003e\n\u003cli\u003eShiga N (1985) Seasonal and vertical distributions of appendicularia in Volcano Bay, Hokkaido, Japan. Bull Mar Sci 37(2):425-439\u003c/li\u003e\n\u003cli\u003eSmith IJ, Stevens DP, Heywood KJ, Meredith MP (2010) The flow of the Antarctic circumpolar current over the north scotia ridge. Deep-Sea Res I Oceanogr Res Pap. 57:14\u0026ndash;28.https://doi.org/10.1016/j.dsr.2009.10.010.\u003c/li\u003e\n\u003cli\u003eSpinelli M, Martos P, Esnal, GB, Capitanio FL (2009) Appendicularian assemblages and their space-time variability of the La Plata River, SW Atlantic Ocean. Estuar Coast Shelf Sci 85: 97-106\u003c/li\u003e\n\u003cli\u003eSpinelli ML, P\u0026aacute;jaro M, Martos P, Esnal GB, Sabatini M, Capitanio FL (2012) Potential zooplankton preys (Copepoda and Appendicularia) for \u003cem\u003eEngraulis anchoita\u003c/em\u003e in relation to early larval and spawning distributions in the Patagonian frontal system (SW Atlantic Ocean). Sci Mar 76(1): 39-47\u003c/li\u003e\n\u003cli\u003eSpinelli ML, Malits A, Alonso VAG, Mart\u0026iacute;n J, Capitanio FL (2020). Spatial gradients of spring zooplankton assemblages at the open ocean sub-Antarctic Namuncur\u0026aacute; Marine Protected Area/Burdwood Bank, SW Atlantic Ocean. J Mar Syst, 210: 103398. https://doi.org/10.1016/j.jmarsys.2020.103398\u003c/li\u003e\n\u003cli\u003eSwart NC, Gille ST, Fyfe JC, Gillett NP (2018). Recent Southern Ocean warming and freshening driven by greenhouse gas emissions and ozone depletion. Nat Geosci 11:836\u0026ndash;841. https://doi.org/10.1038/s41561-018-0226-1\u003c/li\u003e\n\u003cli\u003eTaggart CT, Frank KT (1987). Coastal upwelling and Oikopleura occurrence (\u0026quot; slub\u0026quot;): a model and potential application to inshore fisheries. Can J Fish Aquat Sci 44(10):1729-1736. https://doi.org/10.1139/f87-211\u003c/li\u003e\n\u003cli\u003eTomita M, Shiga N, Ikeda T (2003). Seasonal occurrence and vertical distribution of appendicularians in Toyama Bay, southern Japan Sea. J Plankton Res 25(6):579-589. https://doi.org/10.1093/plankt/25.6.579\u003c/li\u003e\n\u003cli\u003eTouratier F, Carlotti F, Gorsky G (2003) Individual growth model for the appendicularian\u003cem\u003e Oikopleura dioica\u003c/em\u003e. Mar EcoL Prog Series 248: 141-163 \u003c/li\u003e\n\u003cli\u003eTroedsson C, Bouquet JM, Aksnes DL, Thompson EM (2002). Resource allocation between somatic growth and reproductive output in the pelagic chordate \u003cem\u003eOikopleura dioica \u003c/em\u003eallows opportunistic response to nutritional variation. Mar Ecol Prog Ser, 243:83-91. doi:10.3354/meps243083\u003c/li\u003e\n\u003cli\u003eTroedsson C, Bouquet JM, Lobon CM et al (2012) Effects of ocean acidification, temperature and nutrient regimes on the appendicularian \u003cem\u003eOikopleura dioica\u003c/em\u003e: A mesocosm study. Mar Biol 160: 2175\u0026ndash;2187.35. https://doi.org/10.1007/s00227-012-2137-9\u003c/li\u003e\n\u003cli\u003eTrull TW, Passmore A, Davies DM, Smith T, Berry K, Tilbrook B (2018) Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: A baseline for ocean acidification impact assessment. Biogeosci 15:31. https://doi.org/10.1007/s00227-012-2137-9\u003c/li\u003e\n\u003cli\u003eUrban, J L, McKenzie CH, Deibel D (1992) Seasonal differences in the content of \u003cem\u003eOikopleura vanhoeffeni\u003c/em\u003e and \u003cem\u003eCalanus finmarchicus\u003c/em\u003e faecal pellets: illustrations of zooplankton food web shifts in coastal Newfoundland waters. Mar Ecol Prog Series 84(3): 255\u0026ndash;264. http://www.jstor.org/stable/24829563\u003c/li\u003e\n\u003cli\u003eVenables WN, Ripley BD (2013). Modern applied statistics with S-PLUS. Springer Science \u0026amp; Business Media.\u003c/li\u003e\n\u003cli\u003eZoppi de Roa E (1971). Apendicularias de la regi\u0026oacute;n oriental de Venezuela. Stud fauna Cura\u0026ccedil;ao other Caribb isl 38(1):76-109.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTable 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"polar-biology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"pobi","sideBox":"Learn more about [Polar Biology](http://link.springer.com/journal/300)","snPcode":"300","submissionUrl":"https://submission.nature.com/new-submission/300/3","title":"Polar Biology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"larvaceans, tunicate, population structure, environmental conditions, Staten Island","lastPublishedDoi":"10.21203/rs.3.rs-3553555/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3553555/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAppendicularians are recognized as one of the most abundant mesozooplankton groups in numerous pelagic environments and they play a crucial role in marine ecosystems as they bridge the gap between primary producers and higher trophic levels in the food webs. Zooplankton samples were collected during three spring oceanographic surveys conducted in 2014, 2015, and 2016, in the coastal zone (Staten Island) and oceanic zone (Namuncur\u0026aacute;-Banco Burdwood Marine Protected Area). Our study focuses on a comparative analysis of species composition, density, biomass, and maturity stages of appendicularia between these oceanic and coastal regions, which are marked by distinct physical and biological attributes. Two species of appendicularians were found in the study area, \u003cem\u003eOikopleura fusiformis\u003c/em\u003e and \u003cem\u003eFritillaria borealis\u003c/em\u003e, the former being the dominant. Their distribution was different because \u003cem\u003eF. borealis\u003c/em\u003e was mainly concentrated in the coastal zone while \u003cem\u003eO. fusiformis\u003c/em\u003e was consistently recorded in both zones. Chlorophyll-a concentrations were found to be higher in the coastal zone than in the oceanic zone. These higher concentrations were accompanied by higher densities of \u003cem\u003eO. fusiformis\u003c/em\u003e in that area. The surface current velocity seems to reflect the total phytoplankton biomass being higher in the oceanic zone. On the other hand, the temperature for the marine protected area was lower which could be related to the larger sizes of the appendicularians in that zone. Baseline data of the species in protected areas and surrounding areas is essential to contribute to the stakeholders and advise on future changes that translate into regional and global processes.\u003c/p\u003e","manuscriptTitle":"Spring abundance, biomass and sizes of appendicularians between open sea MPA Namuncurá/ Burdwood Bank and the adjacent coastal area, Southwest Atlantic Ocean: Are they a key link in the trophic web?","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-21 11:35:38","doi":"10.21203/rs.3.rs-3553555/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-08-02T16:25:34+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-02-19T06:35:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"77e31ec1-fd66-4bda-894c-8e18b0c55b1e_SNPRID","date":"2024-01-24T12:02:15+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-01-11T22:24:54+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"a68234e9-e931-4430-8a04-d4bf8dd62989","date":"2023-12-07T17:08:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"0c1dcf6a-6a72-45d9-b9f7-aad90e3294f4_SNPRID","date":"2023-11-28T20:13:52+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2023-11-28T12:39:43+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2023-11-28T12:31:19+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2023-11-04T10:27:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Polar Biology","date":"2023-11-03T18:32:02+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"polar-biology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"pobi","sideBox":"Learn more about [Polar Biology](http://link.springer.com/journal/300)","snPcode":"300","submissionUrl":"https://submission.nature.com/new-submission/300/3","title":"Polar Biology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"08b3ffae-ad00-4c77-a467-64b140c3d87c","owner":[],"postedDate":"June 21st, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-01-20T16:06:36+00:00","versionOfRecord":{"articleIdentity":"rs-3553555","link":"https://doi.org/10.1007/s00300-024-03331-z","journal":{"identity":"polar-biology","isVorOnly":false,"title":"Polar Biology"},"publishedOn":"2025-01-15 15:57:39","publishedOnDateReadable":"January 15th, 2025"},"versionCreatedAt":"2024-06-21 11:35:38","video":"","vorDoi":"10.1007/s00300-024-03331-z","vorDoiUrl":"https://doi.org/10.1007/s00300-024-03331-z","workflowStages":[]},"version":"v1","identity":"rs-3553555","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3553555","identity":"rs-3553555","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}