BIOMASS DISTRIBUTION AMONG TROPHIC LEVELS IN LAKES LACKING VERTEBRATE PREDATORS

The relative importance of nutrient supply and food chain composition for the structure and function of ''two-level'' South Georgian (Sub-Antarctica) freshwater ecosystems was assessed. Along a productivity gradient of 19 lakes, we determined the abundance of bacteria, bacterivorous flagellates, and herbivorous macrozooplankters, as well as the concentrations of nutrients (phosphorus and nitrogen) and chlorophyll. Our aims were to test predictions from ''classical'' food chain theory on these simple ecosystems. and to include microbial components in the analysis. The abundance of all organisms increased with increasing productivity, although chlorophyll concentration showed a damped increase compared with lower latitude lakes with similar productivity. Moreover, the amount of chlorophyll per zooplankton grazer showed no increase with increasing productivity, indicating that grazing is a major structuring factor for algal biomass development. Similarly, there was no increase in the ratio between the abundances of flagellates (an alternative prey for zooplankters) and zooplankton with increasing productivity. Conversely, the number of bacteria per flagellate increased with increasing productivity, suggesting that bacteria are not regulated by flagellate grazing. We conclude that food chain theory may explain a considerable part of the variation in abundance of organisms of all size classes, including microorganisms.