Seasonal changes of the quantitative importance of protozoans in a large lake : an ecosystem approach using mass-balanced carbon flow diagrams

Based on comprehensive measurements of plankton abundance and production, quantitative carbon flow diagrams were established for the pelagic community of a large lake (L. Constance) for ten successive time intervals during the seasonal course of 1987. Using reasonable diet compositions and parameters (e.g. trophic transfer efficiencies in the range of l0-35%), mass-balance conditions could be fulfilled for individual compartments and the entire food web, provided that ciliate growth rates used during summer and autumn were far below the maximum rates observed in the laboratory. Ciliates, rotifers, and most crusta- ceans were predominantty herbivorous in our model. Ciliates played an important role in the overall carbon flow especially during the fmt half of the year. They constituted a substantial portion of the overall grazing pressure on phytoplankton (up to 89%, seasonal average 45%). They also formed an important part of the diet of cyclopoid copepods in spring. Thus, ciliates which are generally assigned to the microbial food web, were strongly involved in flow dynamics commonly attributed to the classical food chain. Osmotrophic bacteria derived most of their organic matter from zooplankton mediated EOC (excreted organic carbon) flows. The fraction originating directly from phytoplankton was less important. On a seasonal average, microbial production appeared to be of minor importance to the nutrition of larger plankton (i.e. ciliates, rotifers, crustaceans) although a tight coupling between heterotrophic unicellular and multicellular organisms was assumed. Low microbial food web efficiencies indicated that a large amount of organic carbon was respired within the microbial food web which, in turn, points to a considerable recycling of nutrients. Our model supported the hypothesis that nutrient recycling is the primary function of bacterivorous flagellates in the plankton food web of Lake Constance.