Feeding of dominant zooplankton in Prydz Bay, Antarctica, during austral spring/summer: food availability and species responses

Information on the food and predation of zooplankton species is essential for an improved understanding of zooplankton community dynamics of the Southern Ocean. Feeding of dominant zooplankton species at locations across the neritic, slope and oceanic regions of Prydz Bay, Antarctica, was investigated with incubation experiments during austral spring/summer of 2009/2010 to identify the response of dominant copepods and krill to different feeding environments. Results showed distinct spatial and temporal variations in ingestion and filtration of predominant copepods and krill. In late spring, Calanoides acutus was inactive and the ingestion rate was only 1.30 μgC/ind/day. During early summer, a diatom bloom was in progress at station IS21, showing a peak biomass of Thalassiosira spp. and Fragilaria spp. Daily ingestion rates of Euphausia crystallorophias, C. acutus, Metridia gerlachei and Ctenocalanus citer were relatively high. By contrast, copepod ate predominantly ciliates in slope and oceanic regions where microplankton biomass were lower (<20 μgC/L). During late summer, microplankton of neritic regions, mainly composed of nano-sized Pseudo-nitzschia spp. and ciliates, was less than 10 μgC/L. C. acutus incubated in neritic regions mainly ate ciliates. The total microplankton biomass was lower (<5 μgC/L) and predominated by Pseudo-nitzschia spp. in slope regions north of Fram Bank, and daily ingestion rates of incubated copepods were less than 2.5 μgC/ind. Our results clearly demonstrated that copepods and krill had flexible feeding strategies to cope with temporally and spatially changing food availability in Prydz Bay. Meanwhile, ciliates appeared to represent an important carbon source for zooplankton, especially in regions with lower food concentrations.