The Effects of Filter-Feeding Planktivorous Fish on Production of Protozoa and Carbon Flow from Protozoa to Zooplankton in a Eutrophic Lake

Abstract

ABSTRACT Enclosure experiments were performed in a eutrophic lake in order to evaluate the effects of filter-feeding fish (silver carp) on production of planktonic protozoa and the carbon flux from protozoa to zooplankton. The densities of heterotrophic nanoflagellates (HNF) and ciliates (<60 μm) increased when fish were introduced to enclosures, and then decreased after the fish were removed. The increased densities of the protozoan resulted from relaxed feeding pressure by crustaceans and rotifers, both of which were suppressed by the fish. The growth rates of HNF were not significantly different in fishless enclosures and in enclosures with fish. The growth rates were positively correlated with water temperature (p<0.05). Similarly, ciliate growth rates were not significantly different between the two enclosures, but they were not significantly correlated with any environmental variables. Grazing rates on HNF by zooplankton were significantly higher in fishless enclosures than those in enclosures with fish. The HNF were grazed more heavily by the 10–94 um predators than by those >94 μm. Grazing rates on ciliates were not different between treatments or between the two size groups of predators. Estimates of the ratio of zooplankton grazing rate on protozoan to zooplankton ingestion rate, indicated that both HNF and ciliates were more important food sources for zooplankton in enclosures with fish than in fishless enclosures. In the presence of fish, zooplankton depended almost exclusively on HNF for food, but in the absence of fish, zooplankton feeding shifted to phytoplankton. The introduction of silver carp to this eutrophic ecosystem changed the pathways of carbon flow from protozoan to zooplankton in the microbial food web.