Comparison of pelagic food webs in lakes along a trophic gradient and with seasonal aspects: influence of resource and predation

Composition and seasonal dynamics of phytoplankton, bacteria, and zooplankton (including heterotrophic flagellates, ciliates, rotifers and crustaceans) were studied in 55 lakes in Northern Germany with different trophic status, ranging from mesotrophic to hypertrophic. Mean abundance and biomass of all groups increased significantly with trophic level of the lake, but bacteria and metazooplankton showed only a weak correlation and a slight increase with chlorophyll concentration. Composition of phytoplankton showed a dominance of cyanobacteria in hypertrophic lakes, whereas the importance of chrysophytes and dinophytes decreased with an increase in trophic status. Protozoans (heterotrophic flagellates and ciliates) made up 24% (mesotrophic lakes) to 42% (hypertrophic lakes) of total zooplankton biomass on average, and were dominated by ciliates (62-80% of protozoan biomass). Seasonally, protozoans can build up to 60% of zooplankton biomass in spring, when heterotrophic flagellates can contribute similar to50% to protozoan biomass. Correlation analyses revealed significant relationships between the planktonic groups and indicated that pelagic food webs are influenced by both bottom-up as well as top-down mechanisms. Comparing lakes along a trophic gradient, resource parameters seem to be of major importance, whereas seasonal changes within a lake were perhaps regulated by the presence of predators.