Dual role of fish in regulating primary producer biomass in lakes: Effects from the top and bottom of the food web

The impacts of fish on primary producers have been the subject of several hundred studies published since the 1960s. Based on these studies, three mechanisms have been proposed to explain the impact of fish on primary producer communities: (1) fish predation on herbivores reduces their abundance, consequently reducing grazing pressure on primary producers; (2) fish alter the structure of prey communities, therefore rates of nutrient cycling increase and primary producers are released from nutrient limitation; and (3) fish excrete nutrients that are available to primary producers.; The relative importance of these mechanisms is the focus of this work. In an enclosure study, I found that the mechanisms by which fish impact primary producers vary in the littoral and pelagic zones. In the pelagic, fish excretion was the primary mechanism influencing phytoplankton. Herbivore grazing appeared to have negligible impacts on phytoplankton. However, in the littoral zone, periphyton responded both to nutrients excreted by fish and to herbivore grazing.; In a separate pond study, I examined nutrient transfer by fish. I placed periphyton enhanced with 15N in the littoral zone, and then traced the movement of 15N through the littoral and pelagic food webs in the presence and absence of fish. Higher concentrations of 15N were found in phytoplankton in the presence of fish, suggesting that nutrient transport by fish may be an important source of nutrients for primary producers.; In the final chapter, I conducted a meta-analysis of the primary literature that focused on the role of fish in food webs to determine whether the density of fish used in experimental manipulations was a factor in the outcome of the study. More than half of the studies used fish biomass levels that were higher than would be expected in nature. I found significant negative relationships between the biomass of fish used in experiments and the magnitude of the fish effect on three prey variables. These findings suggest that investigators may overestimate the importance of fish in food webs based on the results of published studies.