Physical and biological controls on interannual variability of zooplankton in Marguerite Bay, Western Antarctic Peninsula, austral fall 2001 and 2002

M. Marrari, K.L. Daly, A. Timonin, T. Semenova

The zooplankton community of Marguerite Bay was studied during austral fall of 2001 and 2002 using net and concurrent environmental data. Total

macrozooplankton abundances were similar during both years (mean = 123.8 and 105.1 ind m^-2 for 2001 and 2002, respectively); however, species composition was significantly different. Thysanoessa macrura was the most abundant euphausiid in 2001, while Euphausia crystallorophias dominated in 2002. Adult E. superba had intermediate abundances during both years, whereas juveniles were only present in 2002 indicating a successful recruitment from 2001 larvae. Amphipods and mysids did not show interannual differences. Copepods were more abundant in 2001 by a factor of 2.6 and their composition also varied between years. Calanoides acutus and Metridia gerlachei were dominant in 2001 while several species dominated in 2002, including M. gerlachei and Ctenocalanus spp. Copepods and T. macrura showed a rapid population response to unusually high chlorophyll concentrations in the Bellingshausen Sea and Marguerite Bay during spring-summer 2000/2001, whereas E. superba and E. crystallorophias had a longer term response and showed increased recruitment in fall 2002.

In general, macrozooplankton and copepods showed opposite distribution trends. Macrozooplankton was most abundant in northern areas (Crystal Sound and Laubeuf Fjord) while copepods were most important in inner Marguerite Bay and the vicinity of Alexander Island to the south during both years, and also in Laubeuf Fjord in 2001. Although the mean depths for E. superba (102 and 105 m for 2001 and 2002, respectively), E. crystallorophias (107 and 79 m) and T. macrura (123 and 102 m) were not significantly different within or between years, the depths of maximum abundance for each species rarely overlapped at a given location suggesting that these species vertically partition their habitat. Mysids occurred deepest in the water column (289 and 267 m in 2001 and 2002, respectively), followed by amphipods (193 and 146 m), and euphausiids at the shallowest depths. Overall, copepods were distributed deeper in the water column in 2001 (173 m and 121 m for 2001 and 2002 respectively; p = 0.004). There were no clear associations between the distribution of zooplankton and environmental conditions in fall; however there was a link between chlorophyll concentrations in the Bellingshausen Sea and Marguerite Bay during the preceding spring-summer and zooplankton patterns during fall.