The impact of metazooplankton on the structure of the microbial food web in a shallow, hypertrophic lake

Shallow, hypertrophic Lake Sobygard is characterized by strong fluctuations in the plankton community structure over short time scales, and cascading predation effects from higher to lower trophic levels. We examined the coup ling between the classical and microbial food web for al month period, duri ng which the typical zooplankton summer succession from rotifers (mainly Br achionus spp.) to cyclopoid copepods and daphnids occurred. In addition to the analysis of the plankton succession, we performed mesocosm experiments, comparing the microbial community structure in treatments with and without metazooplankton. We focused on the development of different functional gro ups within the microbial food web: total heterotrophic bacteria, filamentou s bacteria (as grazing-resistant forms), prokaryotic and eukaryotic, autotr ophic picoplankton (pAPP and eAPP), heterotrophic nanoflagellates (HNF) and ciliates. During the first experiment, the metazooplankton was dominated b y rotifers which exerted only moderate top-down control on small ciliates, HNF and APP. Cascading predation effects were visible after the collapse of the rotifer population; enhanced protozoan grazing resulted in a decrease in single-celled bacteria and an increase in filamentous bacteria. During t he second experiment, characterized by dominance of Cyclops vicinus, strong alterations in the microbial food web structure occurred. The most obvious effects were an efficient predation control of planktonic ciliates by cope pods and a shift of the picoplankton towards filamentous bacteria and a ver y high biomass of pAPP, which we interpret as a result of enhanced protozoa n grazing pressure on bacterioplankton. After the peak in cyclopoids, Daphn ia spp. became the dominant zooplankton taxa which resulted in the well kno wn strong predation effects on all microbial components. The two experiment s confirm that metazooplankton species composition is an important structur ing factor for the microbial food web. Two functional groups deserve specia l attention: filamentous bacteria and pAPP. Filamentous bacteria, which att ained nearly 50% of total heterotrophic bacterial biomass during the second period, seem to be a sensitive indicator of the overall planktonic food we b structure and showed significant responses in the enclosures. With a spec ial staining procedure, we analysed the abundance of filamentous bacteria i n Lugol-fixed samples collected since 1984 during periods when either daphn ids, cyclopoids or rotifers dominated the metazooplankton community. On ave rage, high abundance of filaments was always associated with Cyclops-domina ted situations, low numbers with Daphnia dominance and high, but rather var iable numbers with rotifer dominance. A general close correlation of pAPP w ith filamentous bacteria might indicate that pAPP also possess a reduced ed ibility for protozoans.