ECOSYSTEM ANALYSIS BASED ON BIOMASS SIZE DISTRIBUTIONS - A CASE-STUDY OF A PLANKTON COMMUNITY IN A LARGE LAKE

The plankton biomass size distribution and its seasonal changes were used to estimate seasonal fluctuations of the metabolic activity of the community and of the efficiency to transfer biomass to larger sized organisms in a large and deep prealpine lake (Bodensee: Lake Constance). The efficiency of transfer of biomass from small (e.g. autotrophic) to larger (e.g. herbivorous) organisms increased considerably during the first half of the year and reached maximum values when daphnid densities were high. This trend was attributed to the different generation times of the organisms involved in the food web. The trophic transfer efficiency may depend on the predator-prey weight ratios, and the efficiency of growth and exploitation of the constituent organisms. The contribution of these factors to the overall change of the transfer efficiency was estimated from additional information on the food-web structure. The results derived from the size spectra agreed reasonably well with previous studies of single populations, suggesting that biomass size distributions may substantially contribute to ecosystem analysis. The seasonal trend in slopes of the size spectrum was related to predictions derived from conceptual models. Empirical results from Lake Constance could be reconciled with those models accounting for seasonal changes of exploitation efficiency. Inference from the size spectra supported the hypothesis that natural assemblages of pelagic bacteria are unlikely to attain the potential productivity implied by allometric relationships.