Direct and indirect effects of fish predation and excretion in aquatic food webs
Abstract: The aim of this thesis was to elucidate the mechanisms by which planktivorous fish can affect planktonic communities in lakes and how they may affect benthic communities through their effects on planktonic communities. In a laboratory experiment, nutrients released by fish increased algal biomass and altered phytoplankton community structure, suggesting that fish can affect phytoplankton by directly releasing nutrients. The effects of fish on planktonic communities through predation on zooplankton and nutrient excretion were experimentally separated and their relative importance quantified in a eutrophic humic lake. Results show that fish predation on zooplankton was the most important mechanism accounting for the increase in nutrient concentrations, phytoplankton and microzooplankton abundance in the presence of fish. However, nutrient excretion by fish had important indirect effects on zooplankton which, for some small cladocerans and copepods, were even stronger than the effects of fish predation. Fish affected phytoplankton through nutrient recycling by directly releasing nutrients and by changing the rates and ratios at which zooplankton recycled nutrients. These effects of fish on cyanobacteria and chlorophytes through nutrient recycling were stronger than those through zooplankton grazing, suggesting that fish-mediated nutrient recycling may explain most of the "trophic cascade" effects of planktivorous fish when the phytoplankton community is dominated by cyanobacteria or chlorophytes. Results also indicate that the effects of fish on planktonic communities were more predictable in direction as the taxonomic resolution of the data decreased and the experimental duration increased. The analysis of a general food web model suggest that in systems where autochthonous production is the dominant source of detritus, planktivorous fish may have an important effect on benthic communities by reducing zooplankton grazing on phytoplankton and thereby increasing the proportion of primary production that enters benthic habitats through sedimentation. Results also suggest that fish should have weaker effects (per unit of biomass) on pelagic food chains when subsidized by prey from benthic food chains and vice-versa.
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