In two consecutive years rotifer and crustacean plankton displayed alternating dynamics in the meso-eutrophic Schoehsee (Ploen, Germany). During the clear-water phase daphnids increased and excluded rotifers from the plankton. Only after the DAPHNIA population had crashed rotifers attained higher densities. In summer, when phytoplankton biomass was highest, rotifers reached their peak densities. Copepods could be abundant during this time, daphnids, however, remained unimportant throughout the summer.

Based on these observations I performed a series of enclosure experiments in the lake. The objective was to assess the importance of bottom-up and top-down processes for the rotifer plankton. Depending on the experiment phytoplankton resources and/or crustacean plankton were manipulated.

Quantitative as well as qualitative alterations of phytoplankton resources had a strong impact on the birth rates of almost all rotifer species. When condensed natural lake seston was added to the enclosures birth rates increased dramatically for some rotifer species suggesting resource limitation of these species in the lake.

In enclosures containing crustacean plankton rotifers reached lower densities than in enclosures without crustaceans. In most cases rotifer birth rates decreased in the presence of crustaceans. Mortality increased only in a few instances. Thus, in this study crustaceans were most likely to influence rotifers as exploitative competitors (bottom-up). Top-down processes like predation were found to be less important. The susceptibility to bottom-up and top-down mechanisms, however, varied very much with the type of rotifer species present.

Daphnids had the highest potential to control rotifers among the crustacean taxa present in the Schoehsee. Exploitative competition for phytoplankton turned out to be much more important than mechanical interference which occurs when rotifers are killed by the movement of a cladoceranıs filtration apparatus. Cyclopoid copepods were found to be predators of rotifers whereas the predacious cladoceran Leptodora had no effect on the rotifers.

In conclusion, it was possible to show in a series of in situ experiments that biotic factors like competition and predation are able to influence natural populations in terms of abundance, diversity or reproductive success.