BALANCE BETWEEN PHYTOPLANKTON PRODUCTION AND PLANKTON RESPIRATION IN LAKES

We analyzed published rates of algal photosynthesis and plankton community respiration to test the hypothesis that the ratio of planktonic primary production to community respiration (P/R) varies systematically with lake trophy. Regression analyses show that algal production and plankton respiration are closely related to chlorophyll concentrations for lakes spanning a wide trophic range. More surprisingly, plankton respiration exceeds algal photosynthesis in oligotrophic lakes, and P/R rises above unity only when chlorophyll concentrations are above 17 mg.m-3. A simple allometric model based on the predicted biomasses of the different planktonic components yields rates of community respiration that are in good agreement with measured values. Moreover, the model suggests that in oligotrophic lakes, microbial respiration may greatly exceed the current estimates based on bacterial production data and that heterotrophs contribute proportionately more to total plankton metabolism than they do in eutrophic lakes. Because such high respiration rates require external energy subsidies, these results challenge the view that pelagial communities of most lakes are even approximately self-supporting.