TROPHIC INTERACTIONS AMONG HETEROTROPHIC MICROPLANKTON, NANOPLANKTON, AND BACTERIA IN LAKE CONSTANCE

A considerable portion of the pelagic energy flow in Lake Constance (FRG) is channelled through a highly dynamic microbial food web. In-situ experiments using the lake water dilution technique according to Landry & Hasset (1982) revealed that grazing by heterotrophic nanoflagellates (HNF) smaller than 10 μm is the major loss factor of bacterial production. An average flagellate ingests 10 to 100 bacteria per hour. Nano- and micro-ciliates have been identified as the main predators of HNF. If no other food is used between 3 and 40 HNF are consumed per ciliate and hour. Other protozoans and small metazoans such as rotifers are of minor importance in controlling HNF population dynamics. Clearance rates varied between 0.2 and 122.8 nl HNF-1 h-1 and between 0.2 and 53.6 μl ciliate-1 h-1, respectively. Ingestion and clearance rates measured for HNF and ciliates are in good agreement with results obtained by other investigators from different aquatic environments and from laboratory cultures. Both the abundance of all three major microheterotrophic categories - bacteria, HNF, and ciliates - and the grazing pressure within the microbial loop show pronounced seasonal variations. © 1990 Kluwer Academic Publishers.