Top-down vs. bottom-up effects on the planktonic microbial food web of a maritime Antarctic lake

Abstract This study evaluated the influence of nutrient (bottom-up) and biological (top-down) controls on the planktonic microbial food web of Lake Limnopolar, an oligotrophic lake in maritime Antarctica. Using a factorial experiment, we manipulated nutrient levels (nitrogen and phosphorus) and macrozooplankton abundance (absence/presence, and abundance increased by 30×) to simulate environmental variations. Nutrient additions led to moderate phytoplankton growth in the absence of zooplankton, indicating nutrient limitation. The presence of zooplankton enhanced algal growth, favoured chlorophytes over diatoms, and significantly increased bacterial abundance while reducing particularly ciliates. These results indicate a cascading effect typical of biological control and they highlight the role of biotic interactions, even in extreme environments where abiotic factors are thought to dominate. Understanding biological controls in such extreme environments is essential, as it underscores the complexity of ecosystem functioning in the polar regions, which are increasingly seen as early indicators of global change.