TROPHIC COUPLING WITHIN THE MICROBIAL FOOD-WEB - A STUDY WITH FINE TEMPORAL RESOLUTION IN A EUTROPHIC FRESH-WATER ECOSYSTEM

1. The major components of the microbial food web (dissolved organic c arbon, bacteria, protozoa, rotifers and algae) of Priest Pot, a small freshwater Fond, were investigated over a period of 5 months. Water sa mples were collected from the epilimnion every 1-3 days. 2. Time serie s analysis helped identify the trophic relationships within the plankt onic community. There were strong predator-prey relationships between both ciliates and large rotifers and the total nanoplankton, between r otifers and small ciliates and between the total microzooplankton comm unity and phytoplankton. Small rotifers and small ciliates probably sh are the same food resources. The major bacterivores in the system coul d not be identified with our methods. However, our previous results po int to a dominating role of nanoplanktonic (2-20 mu m) heterotrophic p rotists as the main grazers of bacteria. 3. Rotifers are the major typ e of metazoan zooplankton in Priest Pot; crustacean zooplankton are ab sent from the community. Bacterial production probably reaches rotifer s via a variety of pathways: there may be a three-step link from bacte ria to bacterivorous nanoplankton, to ciliates and then to rotifers. F urthermore, a strong correlation between the nanoplankton and rotifers suggests a direct link between these components, implying a much shor ter pathway. Some of the rotifers in the pond can graze directly on ba cteria, and many of the larger planktonic organisms (large ciliates an d rotifers) are algivores. The latter two predator-prey relationships suggest an efficient transfer of bacterial and primary production to h igher trophic levels.