Effects of Nutrients and Zooplankton Size on the Structure of a Phytoplankton Community

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Abstract
In situ enclosure experiments were conducted over two summers (1980—1981) to assess the effects of nutrient (N and P) enrichment and zooplankton size structure on the phytoplankton community of an oligo—mesotrophic lake containing planktivorous fish and small zooplankton species. Exclusion of fish from the community resulted in an increase in mean individual zooplankton size, primarily because the cladocerans Ceriodaphnia and Diaphanosoma attained larger sizes. In 1980, total cladoceran biomass was also greater in the absence of fish. In 1980, total phytoplankton density was significantly lower in the fishless enclosures (where zooplankton were larger and grazing rates presumably higher), at a given nutrient level. The proportion of phytoplankton density comprising species with gelatinous sheaths or other protective coverings was greater in the fishless enclosures, under both enriched and unenriched conditions, presumably because these species are relatively resistant to zooplankton grazing. In 1981, when lower fish densities were used (than in 1980), total phytoplankton density was lower in the fishless enclosures only toward the end of the experiment and only in unenriched enclosures. In enriched enclosures the presence of fish had no effect on total phytoplankton density. However, the proportion of phytoplankton density made up of small, edible (to zooplankton) phytoplankton species was lower in the absence of fish throughout the experiment in both unenriched and enriched treatments. In both years, nutrient enrichment caused phytoplankton density to increase greatly, in the presence and abscence of fish. Nutrient enrichment increased phytoplankton density a much greater amount than did reduction in zooplankton size. The results of these experiments demonstrate that even relatively small changes in zooplankton size (i.e., shifts in size within only small species) can result in significant alterations of the phytoplankton community. However, the increase in zooplankton body size resulting from fish exclusion could not buffer all effects of nutrient enrichment on total phytoplankton density.