Planktonic protozoa and the microbial food web in Lake Constance

We present a synopsis of the ecology of pelagic protozoans in Lake Constance that has been investigated for over one decade (1987-1996). The trophic significance of protozoa within the microbial food web has been studied mainly at the community level. Ciliates were found to be the single most important group contributing to, on average, two thirds of the total protozoan biomass and production. Heterotrophic nanoflagellates (HNF), due to their short generation times, were more significant in terms of production than in terms of biomass. The opposite holds true for large heterotrophic flagellates, in particular Gymnodinium helveticum. Heliozoa contributed very little (1%) to both biomass and production. Naked amoebae were found in relatively high numbers and biomasses during phytoplankton peaks. HNF were the most important bacterivores. Mixotrophic chrysomonads which have increased during the re-oligotrophication of Lake Constance were responsible for only a minor fraction of total bacterivory. The combined production and carbon consumption of protozoa reached similar levels or even exceeded the respective rates of metazooplankton, especially at the beginning of the growing season in spring. Seasonal successions of heterotrophic nanoflagellates, ciliates, and heliozoa were investigated at the genus or species level. Specific in situ measurements as well as laboratory investigations on dominant taxa isolated from the lake revealed that the response to environmental factors may vary greatly even among closely related species.