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 carbon, bacteria, protozoa, rotifers and algae) of Priest Pot, a small freshwater Fond, were investigated over a period of 5 months. Water samples were collected from the epilimnion every 1-3 days. 2. Time series analysis helped identify the trophic relationships within the planktonic community. There were strong predator-prey relationships between both ciliates and large rotifers and the total nanoplankton, between rotifers and small ciliates and between the total microzooplankton community and phytoplankton. Small rotifers and small ciliates probably share the same food resources. The major bacterivores in the system could not be identified with our methods. However, our previous results point to a dominating role of nanoplanktonic (2-20 mu m) heterotrophic protists as the main grazers of bacteria. 3. Rotifers are the major type of metazoan zooplankton in Priest Pot; crustacean zooplankton are absent from the community. Bacterial production probably reaches rotifers via a variety of pathways: there may be a three-step link from bacteria to bacterivorous nanoplankton, to ciliates and then to rotifers. Furthermore, a strong correlation between the nanoplankton and rotifers suggests a direct link between these components, implying a much shorter pathway. Some of the rotifers in the pond can graze directly on bacteria, and many of the larger planktonic organisms (large ciliates and rotifers) are algivores. The latter two predator-prey relationships suggest an efficient transfer of bacterial and primary production to higher trophic levels.