Effects of Fish Predation and Excretion on the Configuration of Aquatic Food Webs

The relative importance for algae of fish excretion and fish predation on zooplankton was studied in experimental pools. Even- (zoo- and phytoplankton) and odd-link systems (planktivorous fish, zoo- and phytoplankton) were established, in which the standing stock of primary producers was expected to follow the predictions from food chain theory sensu Oksanen. Two intermediate treatments (fish predation and nutrient regeneration by fish alone) were used to determine the relative power of fish in affecting the transition from an even-link system to an odd-link system, i.e. their relative power in the trophic cascade. Algal density and production were found to only be enhanced when both fish predation and fish excretion acted together. A reduction in the grazer biomass alone did not increase the algal biomass. Fish excretion alone increased the algal biomass somewhat, but not significantly so compared with the control. The results suggest that both fish predation on zooplankton and the regeneration of nutrients by fish may be necessary to create the difference in algal biomass between an even- and an odd-link system predicted by food chain models, at least in the absence of an external input of nutrients. In addition, the effects of increased nutrient regeneration due to use of non-zooplankton resources by the fish, nutrient maintenance in living biota, and decreased grazing pressure seem to reinforce the cascading effect of planktivorous fish on algae. Consequently, use of a fixed carrying capacity in simple predatorprey models may lead to loss of important information on the dynamics of aquatic food webs, preventing an adequate understanding of cascading trophic interactions from being achieved.