Seasonal microbial activity in Antarctic freshwater lake sediments
Seasonal fluctuations in population numbers and activity were monitored in bottom sediments of oligotrophic Moss Lake, mesotrophic Heywood Lake and eutrophic Amos Lake on Signy Island, South Orkney Islands, during 1976–78. Heywood and Amos Lakes became anoxic under winter ice cover (8–10 months) and significant populations of facultatively anaerobic heterotrophs and sulphate-reducing bacteria developed. In contrast, Moss Lake surface sediments never became anoxic and anaerobic bacteria were virtually absent. Direct microscopic counts and viable plate counts fluctuated relatively little in Moss Lake throughout the study period, whereas distinct seasonality was observed in the more enriched lake systems. Similarly, measurements of oxygen consumption and dark 14CO2 uptake by mud cores indicated no obvious seasonal fluctuations in Moss Lake data, in contrast to the marked seasonal pattern observed in data from the other lakes. In these latter systems, oxygen uptake rates were highest in summer (c. 400 mg O2 m-2 d-1) and virtually undetectable in winter. Comparison of oxygen uptake with oxygen concentration and temperature revealed differences, between lakes, in uptake response to oxygen concentration, whereas uptake response to temperature did not differ significantly between lakes. Chemosynthetic production in the Signy Island lake sediments was in the range 1.6–35.3 μg C m-2 (mud surface) d-1 with highest values recorded in Amos Lake under winter ice cover and anoxic conditions. The findings from this and earlier studies of the three lakes have been assembled to indicate the relative importance of green plants and bacteria to the carbon cycle in these permanently cold systems.