Biomass allocation and carbon flow in the pelagic food web of Lake Constance

The seasonal succession of all major planktonic groups, i.e., phytoplankton, bacteria, heterotrophic flagellates, ciliates, rotifers, and crustaceans, was investigated during five years (1987-1991) of weekly sampling with elaborate sampling methods and microscopic countings. This allows to study the seasonal changes of the biomass and production allocation patterns within the epilimnetic food web of Lake Constance during five consecutive years. On an annual average. the contribution ofphytoplankton, microbial plankton (including bacteria and protozoans), and metazoan zooplankton to the total planktonic biomass was approximately 30%, 30%, and 40%, respectively. Hence, heterotrophic biomass was on average twice as large as phototrophic biomass. In contrast, phytoplankton contributes 69% to the total production, whereas the share of microbial plankton and metazooplankton amounts to 19 and 11 %, i.e., phototrophic production is twice the heterotrophic production. The seasonal biomass allocation pattern is characterized by a dominance of small organisms in early spring and of herbivorous crustaceans (mainly daphnids) during the clear-water phase and in early summer, when they contribute up to 66% of the total plankton biomass. Mass-balanced carbon flux charts could be established based on production measurements and estimates, which point to a high consistency of all measurements. However, initial production estimates of herbivorous crustaceans (calculated by the growth increment summation) during the clear-water phase and of ciliates (derived from an allometric relationship) during summer did not fit into the global network environment set up by the other measurements because of their failure to consider food limitation. The summer community exhibits a higher biomass allocation and carbon flux diversity and an increased importance of higher trophic levels as compared to the spring community. The heterotrophic biomass sustained by one unit of primary productivity in summer is twice the one in spring.