Seasonal dynamics of the planktonic microbial community in a maritime Antarctic lake undergoing eutropication

The abundance, biomass and community structure of phytoplankton, bacterioplankton and protozooplankton in a maritime Antarctic lake were determined at approximately monthly intervals from December 1994 to February 1996 and compared with data from earlier studies. Heywood Lake has become significantly eutrophic during the last three decades because of excreta from the expanding fur seal population in its catchment. Marked seasonal variations in the abundance, composition and productivity of biota were correlated with the seasonality of both physical factors and nutrient levels. Protozooplankton were abundant, diverse and usually dominated by heterotrophic nanoflagellates (HNF), with HNF abundance peaking at 2.35 X 10(7) 1(-1) in summer. High numbers of naked amoebae were sometimes present, reaching a maximum of 4.8 X 10(3) 1(-1) in March. An estimated 89 species of protozoa were observed during the study, indicating substantially more diversity than is found in continental Antarctic lakes. Diversity was highest in spring and lowest in winter, when the entire water column became anoxic and the plankton were dominated by bacteria and a few species of relatively large anaerobic flagellates. The current status of the lake is compared with data for continental Antarctic and lower latitude lakes. Earlier studies of biota and physical/chemical parameters in Heywood Lake are used to examine the effects of eutrophication over three decades. Observed changes include increased microbial abundance and changes in both community structure and seasonal patterns.