The zooplankton of Marguerite Bay, Western Antarctic Peninsula—Part I: Abundance, distribution, and population response to variability in environmental conditions

The zooplankton community of Marguerite Bay was studied during austral fall of 2001 and 2002 using net and concurrent environmental data. Interannual differences were observed in zooplankton species composition, developmental stages, and abundances, which were linked to unusually high chlorophyll concentrations in the Bellingshausen Sea and Marguerite Bay during spring–summer 2000/2001. Copepod abundance was significantly higher in 2001 than in 2002 (46.3 and 28.3indm⁻³ in 2001 and 2002, respectively). During 2001, the copepod community was dominated by two species. Calanoides acutus, a herbivore, and Metridia gerlachei, an omnivore, accounted for 46% and 45% of the community, respectively. During 2002, however, several species were relatively abundant, including M. gerlachei, Ctenocalanus spp., C. acutus, Oithona spp., and Paraeuchaeta spp. Euphausiids also showed a rapid population response to high chlorophyll levels in 2001. Even though average total euphausiid (juvenile/adult) abundances were similar during both years (0.20 and 0.15indm⁻³ for 2001 and 2002, respectively), species composition showed marked interannual differences due to varying life history strategies among species. Thysanoessa macrura, which has a relatively rapid development from larval to juvenile stages between spring and fall of the same year, was the most abundant euphausiid in 2001. In contrast, Euphausia crystallorophias and Euphausia superba juvenile/adult populations increased in 2002, owing to a slower development in which larval stages overwinter and recruit to juveniles during the following spring/summer. Other zooplankton groups those were abundant in Marguerite Bay, but showed little variability between years, included ostracods, pteropods, chaetognaths, medusae, amphipods, and mysids. Summer phytoplankton concentrations strongly influenced copepods and euphausiids; however, there were no clear associations between zooplankton distributions and fall environmental conditions (i.e., pigment concentrations and surface salinity) or bottom depth. It is notable that ostracods and pteropods had the highest abundances of non-copepod zooplankton.