Datensatz

Zusammenfassung

Recent investigations have shown that the presence of Daphnia can substantially alter the structure and functioning of microbial food webs in freshwater ecosystems. Due to its ability to filter a large spectrum of particle sizes, Daphnia can exert a significant grazing pressure on all components of microbial food webs, including pico-, nano- and microplankton. Planktonic bacteria are at the lower boundary of ingestible particle sizes and the retention efficiency depends on the Daphnia species and the size structure of the bacterial population. Nanoplanktonic protozoans (flagellates and small ciliates) are in the optimum size range and are, thus, severely top-down controlled during population peaks of Daphnia. In such situations the cladocerans can become the main bacterivores and consume the major portion of bacterial production. In contrast to phytoplankton, there is little evidence of the development of grazing resistant forms within protozoans and bacteria in reponse to Daphnia. Besides their top-down impact, Daphnia grazing may have a positive feedback effect on bacterioplankton. The supply of bacterial substrates is advanced by a fast reccycling by a fast recycling of particulate organic carbon and release of dissolved organic carbon. The absence or presence of Daphnia depends on the whole food web structure (especially the fish stock) and is a key factor in the propagation of trophic cascades from fish to bacteria. The main differences in microbial communities between systems or seasons with and without large populations of daphnids will be examined