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