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Abstract

The importance of grazer versus resource control has been extensively studied in the pelagic zone of lakes. In contrast, comparatively little is known about trophic interactions within the littoral zone. We conducted an experiment in the littoral zone of a eutrophic shallow lake using six 20 m(2) polyethylene enclosures, which 2.5 month prior to the experiments were mounted above the water surface on a stainless steel ring to allow development of natural pelagic, benthic and epiphytic communities. Half of the enclosures were kept free of plants by harvesting. In late July, the sheets were released from a distance of 5 in and trapped the present biotic community. Fish were removed and the experiment ran for 6 days. Major differences in trophic structure and dynamics were found amongst the treatments, which we attribute to differences in fish density and predation risk prior to the experiment. In the plant-free controls (M-), the zooplankton were dominated by rotifers and cyclopoid copepods, while large-bodied cladocerans dominated in the enclosures with plants (M+). In both sets of enclosures, diel variations in abundance of large-bodied zooplankton were found, with higher densities at night, whereas no significant differences were found amongst small-bodied species (rotifers, nauplii), ciliates, heterotrophic nanoflagellates and bacterioplankton. Phytoplankton dominated the living suspended particulate carbon pool in the plant-free enclosures (69 %), while zooplankton dominated in the enclosures with plants (85 %). Zooplankton (>140 mum) community clearance rates of phytoplankton and bacterioplankton were low (<54 ml/l/d) in M- and high (2200-3000 ml/l/d) in M+. Grazing exceeded phytoplankton production (3.7-fold) in M+, but only amounted to 6 % of the phytoplankton production in M- For bacterioplankton, zooplankton grazing was equivalent to production in M+, but amounted to <3 % of production in M Our results combined with earlier studies indicate that the zooplankton in plant-rich M-toral zones do not alone feed on particles produced in the water, but also exploit alternative sources such as periphyton or benthic food sources or particles entering from outside the plant beds. Thus, zooplankton in such environments may continuously maintain high densities even with low abundance of phytoplankton and bacterioplankton, and are thereby able to maintain high grazing pressure on these pelagic food sources. We hypothesize that the strong cascading effects of zooplankton on chlorophyll-a and microorganisms in the littoral zone at natural fish densities are restricted to eutrophic lakes with high plant densities.