Phytoplankton dynamics at the ice-edge zone of the Lazarev Sea (Southern Ocean) during the austral summer 1994/1995 drogue study

Phytoplankton dynamics in the open waters of the ice-edge zone of the Lazar ev Sea were investigated over 12 consecutive days during a drogue study con ducted in austral summer (December/January) 1994/1995, Throughout the study , the upper water column (< 30 m) was stratified with a well-defined pycnoc line evident. Although a subsurface intrusion of colder, more saline water into the region was recorded on days 5-9 of the experiment, its effect on t he water column structure was negligible. Total surface chlorophyll a bioma ss doubled between days 1 and 5 (from 0.82 to 1.62 mg m(-3)), and then show ed a tendency to stabilise, while the depth-integrated chlorophyll a standi ng stock displayed an increasing trend during the entire experiment. All ch anges in biomass were associated with an increase in micro-phytoplankton. F lagellates and picoplankton dominated cell counts, while diatoms composed m ost of the phytoplankton biovolume. Results of the study indicate that, dur ing the period of investigation, average cell abundance decreased. Coupled with this decrease was an increase in the biovolume and average size of the phytoplankton. Phytoplankton succession was observed in the ice edge durin g the drogue study. Typical ice-associated species of genera Haslea, Fragil ariopsis and Chaetoceros, which dominated at the beginning of study were re placed by open-water species of genera Corethron, Dactyliosolen and Rhizoso lenia. The shift in phytoplankton species composition and size can likely b e related to high light intensities and grazing by microzooplankton. The in trusion of colder, more saline water on day 5 appeared to modify the diatom succession, indicating extreme variability in phytoplankton dynamics in th e near ice-edge zone of the Lazarev Sea.