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The rates of ingestion of natural bacterial assemblages by natural populations of zooplankton (&gt;50 μm in size) were measured during a 19-day period in eutrophic Frederiksborg Slotssø, Denmark, as well as in experimental enclosures (containing 5.3 m3 of lake water). The fish and nutrients of the enclosures were manipulated. In enclosures without fish, large increases in ingestion by zooplankton &gt;140 μm in size were found (up to 3 μg of C liter−1 h−1 ), compared with values less than 0.3 μg of C liter−1 h−1 in the enclosures with fish and in the open lake.Daphnia cucullata andD. galeata dominated the community of zooplankton of &gt;140 μm. Ingestion rates for zooplankton between 50 and 140 μm decreased after a period of about 8 days, in all enclosures and in the lake, to values below 0.1 μg of C liter−1 h−1 . On the last 2 sampling days, somewhat higher values were observed in the enclosures with fish present. The &gt;50-μm 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 &gt;50 μm 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 &gt;50 μm and thus left a large proportion of the available bacteria to zooplankton sized &lt;50 μm. In fact, there were 4.6 × 103 to 5.0 × 103 flagellates (4 to 8 μm in size) ml−1 in the enclosures with fish added as well as in the lake, compared with 0.5 × 102 to 2.3 × 102 ml−1 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.

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