Predation, enrichment, and phytoplankton community structure1

The significance of grazing and enrichment to the Pleasant Pond phytoplankton community was examined through a series of enclosure cxpcriments. The addition of planktivororls 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 rcsidcnt species. Aphanizomenon flos-aquae was an exception to this pattern. This filamentous blue-green attains its maximum density in the presence of large Daphnia by forming large, ungrazeable colonies; Daphnia may provide a service to Aphanixomenon by removing most potential algal competitors. The addition of phosphorus and nitrogen had no quantitative effect on total phytoplankton biomass cithcr in the presence or absence of fish; changes in species composition did occur, several algae disappearing with enrichment. WC summarize the varied rcsponscs 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 strrrcturc of planktivorous fish populations. Our knowledge of factors affecting zooplankton popul ation s has developed to the point where generalized models for the structure of zooplankton communities have begun to emerge (Dodson 1974; Lynch 1979). G iven 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 (Hrbacek 1962; Losos and Hetesa 1973). The significance of nutrients in aquatic environments has also long been appreciated, and several hypotheses have been proposed to explain the dominance of certain phylogenetic lines of algae under particular nutrient regimes (Schelske and Stoermer 1971; Shapiro 1973; Schindler 1977). The chemical characteristics of a ’ Contribution 173 from the Limnological Research Center. Rcscarch supported by NSF grant EMS 74-19490 and by a grant from the U.S. EPA to J. Shapiro. z Present address: Department of Ecology, EtholWY, and Evolution, Vivarirlm Bldg., University of I Hi noi s, Champaign 6 1820. body of water may influence the structure of phytoplankton communities by direct mediation of competitive interactions or by several indirect routes determined by the connections among the members of the community (Lane and Lcvins 1977). Consequently, while the response of a phytoplankton community to enrichment is often dramatic and predictable, the mechanisms promoting the response are rarely known. We examine here the phytoplankton community structure of Pleasant Pond, Minnesota. Since the factors responsible for the distribution of zooplankton species in this pond are known (Lynch 1978, 1979), any changes in the phytoplankton community associated with herbivore activity can ultimately be related to mechanisms regulating the zooplankton community. Here we report on the structure of the Pleasant Pond phytoplankton community in relation to predator-mediated changes in zooplankton community composition, the immediate effects of herbivory as indicated by the short term removal of zooplankton, and the results of longer term enclosure experiments to determine the relative effects of enrichment and predation on the phytoplankton community. A complete description of the pond, its predators, and zooplankton community