Potential Importance of Fish Predation and Zooplankton Grazing on Natural Populations of Freshwater Bacteria

The rates of ingestion of natural bacterial assemblages by natural populations of zooplankton (>50 mum in size) were measured during a 19-day period in eutrophic Frederiksborg Slotssø, Denmark, as well as in experimental enclosures (containing 5.3 m of lake water). The fish and nutrients of the enclosures were manipulated. In enclosures without fish, large increases in ingestion by zooplankton >140 mum in size were found (up to 3 mug of C liter h), compared with values less than 0.3 mug of C liter h in the enclosures with fish and in the open lake. Daphnia cucullata and D. galeata dominated the community of zooplankton of >140 mum. Ingestion rates for zooplankton between 50 and 140 mum decreased after a period of about 8 days, in all enclosures and in the lake, to values below 0.1 mug of C liter h. On the last 2 sampling days, somewhat higher values were observed in the enclosures with fish present. The >50-mum zooplankton ingested 48 to 51% of the bacterial net secondary production in enclosures without fish, compared to 4% in the enclosures with added fish. Considering the sum of bacterial secondary production plus biomass change, 35 to 41% of the available bacteria were ingested by zooplankton of >50 mum in the enclosures without fish, compared with 4 to 6% in the enclosures with added fish and 21% in the open lake. Fish predation reduced the occurrence of zookplankton sized >50 mum and thus left a large proportion of the available bacteria to zooplankton sized <50 mum. In fact, there were 4.6 x 10 to 5.0 x 10 flagellates (4 to 8 mum in size) ml in the enclosures with fish added as well as in the lake, compared with 0.5 x 10 to 2.3 x 10 ml in the enclosures without fish. This link in the food chain was reduced when fish predation on zooplankton was eliminated and a direct route of dissolved organic matter, via the bacteria to the zooplankton, was established.