MICROPLANKTON COMMUNITY STRUCTURE OF THE SHALLOW COASTAL ZONE AT ADMIRALTY BAY, ANTARCTICA: COMPARISON BETWEEN TWO CONSECUTIVE AUSTRAL SUMMERS

The main objective of this work was to investigate interannual changes of phytoplankton structure as part of a long-term monitoring program in Admiralty Bay, Antarctic Peninsula. Based on microscopic analysis, phytoplankton taxonomic composition and biomass are investigated since 2009. This report presents results from the 2009/2010 and 2010/2011 summer surveys regarding the phytoplankton size-structure and biomass. Four scenarios were proposed in our study for environmental conditions and phytoplankton community: 1) 2009/2010 Early Summer; diatom growth was promoted by high Si:N, but inhibited by low N:P and low temperature, 2) 2009/2010 Late Summer; diatom growth was inhibited by low N:P and low Si:N, and high temperature and low N:P promoted dinoflagellates growth, due to their low optimum N:P ratios compared to diatoms; 3) 2010/2011 Early Summer; diatom growth was promoted by high N:P, high Si:N and high temperature and 4) 2010/2011 Late Summer; blooms of chain-forming diatoms (e.g. Thalassiosira spp.), favoured by high Si:N and high temperature, and it could explain the drastic reduction both in nitrate and silicate. The microplankton community of Admiralty Bay presented a high variability during the studied period that was clearly dictated by environmental factors. The time window covered by this study gives us only a glimpse of the direction of long term changes that the Antarctic environment might be experiencing. A more complete picture of such trends relies heavily on the continuity of the long-term monitoring program.