Seasonal variation in zooplankton assemblages in Ryder Bay, Adelaide Island, Antarctic Peninsula

Seasonal and inter-annual variations in zooplankton play a vital role in marine ecosystems. In Antarctica, because of the extreme seasonality and ice extent, zooplankton studies are predominantly conducted in summer and are based offshore. Therefore the drivers of variability are poorly understood. Here, we present one of the few year-round studies of seasonality of zooplankton communities at a coastal site in Ryder Bay, Western Antarctic Peninsula (WAP), encompassing both holoplankton and meroplankton. While there were consistent species components to the assemblage, there was also substantial intra- and inter-annual variation throughout the study, suggesting short-term and single-season evaluations are insufficient to provide a comprehensive understanding of zooplanktonic assemblage dynamics and controlling factors. Small copepods and nemertean pilidium (32% and 28% of all individuals seen, respectively) were present throughout all seasons, and both organisms were present in all but one event, peaking at 89% and 90% of individuals sampled, respectively. However, some organisms, such as some species of pteropoda, cnidaria, and echinodermata, were only present once in the 3-year period, suggesting greater overall diversity than this study found and possibly indicating unusual spatial patchiness or multi-year phenologies. Planktonic larvae of benthic species were more abundant in deeper water samples, closer to the seabed, and assemblage composition and abundance were affected by chlorophyll levels. Larvae and zooplankton of pelagic species assemblages were significantly correlated with temperature, salinity, and pigment levels, strongly suggesting links between specific species and environmental conditions, relationships that are likely to be affected by climate change.