Shifts in microphytoplankton species and cell size at Admiralty Bay, Antarctica

Phytoplankton (> 15 mu m) was investigated in three shallow coastal areas at Admiralty Bay (AB) between the summers of 2002-03 and 2008-09. Phytoplankton abundance was low (10(3) cells l(-1)) and, over time, the prevailing cell size decreased due to a shift in phytoplankton dominant species from diatoms to dinoflagellates. In situ and remote sensing data showed that oscillations in sea surface temperature, precipitation, ice formation/melting, irradiance (cloud cover) and bottom circulation (indexed by the Antarctic Oscillation Index; AAO) were shown to govern the structure of the phytoplankton. Under negative AAO, diatoms prevailed, with the dominance of large (> 80 mu m) benthic diatoms (e.g. Corethron pennatum and Navicula directa) in periods of low production (10(2) cells l(-1) in 2002-03), and medium-sized (31-80 mu m) centrics (e.g. Thalassiosira spp. and Stellarima microtrias) when the abundance was higher (10(4) cells l(-1) in 2003-04). Conversely, positive AAO led to the co-dominance of dinoflagellates and planktonic diatoms (e.g. Pseudo-nitzschia spp.) in the summers of 2007-08 and 2008-09. These results suggest that the AAO can be a good predictor of phytoplankton in coastal areas around the western Antarctic Peninsula, and may help our understanding of changes in other trophic levels of the food web.