Predation, Enrichment, and Phytoplankton Community Structure

The significance of grazing and enrichment to the Pleasant Pond phytoplankton community was examined through a series of enclosure experiments. The addition of planktivorous fish led to the removal of large herbivores and to an order-of-magnitude increase in total phytoplankton biomass. This was a result of the appearance of several new algal species as well as the increase of most initial resident species. Aplzanizomenonjlos-aquae was an exception to this pattern. This filamentous blue-green attains its rnaximum density in the presence of large Daphnia by forming large, ungrazeable colonies; Daplznia may provide a service to Aphanizomenon by removing most potential algal competitors. The addition of phosphorus and nitrogen had no quantitative effect on total phytoplankton biomass either in the presence or absence of fish; changes in species composition did occur, several algae disappearing with enrichment. We summarize the varied responses of lakes to enrichment and suggest that the community-level effects of enrichment can only be understood in the context of a framework that considers initial nutrient status and the structure of planktivorous fish populations. Our knowledge of factors affecting zoobody of water may influence the structure plankton populations has developed to the point where generalized models for the structure of zooplankton communities have begun to emerge (Dodson 1974; Lynch 1979). Given a description of the predators found in a body of water, these models predict which zooplankton species will be excluded from the community. Since planktonic herbivores may have a significant impact on the composition of phytoplankton communities (Porter 1977), it logically follows that predators, through their effects on the herbivore community, may be important determinants of phytoplankton community structure (HrbaEek 1962; Losos and