Phytoplankton ecology in an Antarctic lake

SUMMARY. The ecology of the phytoplankton of Heywood Lake, Signy Island, South Orkney Islands, Antarctica was investigated during 1969–72. The lake, which is ice‐covered for 8–10 months per year, is moderately eutrophic due to enrichment by seal excreta. The annual cycle of the phytoplankton is described. During the winter (approximately May‐September), very few algal cells could be detected in the water column and 14C fixation was below measurable limits. In spring (October‐November), a rapidly‐growing population of algae caused a large increase in the chlorophyll‐a concentration (maximum value 170 mg m−2) but carbon fixation remained low, with values <500 mg C m−2 day−1. The algae contributing to this peak were mainly small chlorophytes and chrysophytes. The summer open‐water period (December‐March) was characterized by a different phytoplankton population dominated by cryptophytes. Chlorophyll levels were lower (c. 40 mg m−2) but 14C fixation rates >3 g C m−2 day−1 were measured on bright days. Values for Assimilation Number were very high (maximum value 10.5 mg C h−1 mg−1 (chlorophyll‐a) in January (1971) though temperatures never exceeded 8°C. In autumn, the phytoplankton regressed to winter levels. Both spring and summer algal populations probably overwinter as resting stages.