NUTRIENT RECYCLING AND HERBIVORY AS MECHANISMS IN THE “TOP–DOWN” EFFECT OF FISH ON ALGAE IN LAKES

Abstract

In lakes, top predators (fish) often influence the abundance of primary producers (phytoplankton) through food web interactions: phytoplankton biomass is often greater when planktivorous fish are abundant than when they are rare, and phytoplankton community structure is often affected by fish. Three mechanisms can account for these “top–down” effects of fish: decreased herbivory by zooplankton when fish biomass is high; modification of nutrient recycling rates by the herbivorous zooplankton assemblage as fish biomass varies; and nutrient recycling by fish. These processes were experimentally separated and their relative importance quantified in Tuesday Lake, Michigan. This was accomplished by manipulating the abundance of planktivorous fish or zooplankton in enclosures containing natural phytoplankton communities, and by incubating phytoplankton in nutrient-permeable chambers (which excluded herbivores) placed inside these enclosures. In large enclosures with all trophic levels, several phytoplankton taxa and total phytoplankton biomass showed increased abundance in the presence of fish compared to enclosures without fish. Several taxa also showed significantly greater abundance in nutrient-permeable chambers incubated in enclosures with fish than in chambers incubated in enclosures without fish. The latter result indicates that some phytoplankton taxa respond to fish even when separated from direct herbivory but exposed to nutrients recycled by consumers. Thus, consumer-mediated nutrient recycling had strong effects on phytoplankton community dynamics and could partly explain the “top–down” effects of fish. Most phytoplankton taxa responded to consumer-mediated nutrient recycling, especially dinoflagellates and chrysophytes. In separate enclosures, phytoplankton were exposed to contrasting zooplankton assemblages shaped by fish predation but without fish being present. The response of phytoplankton was not as strong as in the case of fish manipulations. Furthermore, the community-level response of phytoplankton was weaker in nutrient-permeable chambers placed in these enclosures than in chambers incubated in enclosures with fish. Nutrient limitation assays showed that manipulation of fish decreased phosphorus limitation of phytoplankton, while direct manipulation of zooplankton had no effect on phosphorus limitation. These results provide experimental evidence that food web effects on nutrient recycling are important in controlling phytoplankton community dynamics. Results also suggest that some of these effects are expressed via direct recycling of nutrients by fish, as well as by fish-induced effects on nutrient recycling by herbivores. Nutrient-mediated effects of top predators on primary producers should be incorporated into future models of “top–down” control of food web dynamics.