Competition in zooplankton communities: Suppression of small species by Daphvlia pulex1

The hypothesis that a large zooplankton herbivore, Daphnia pulex, can competitively reduce the abundance of resident zooplankton when colonizing a community of small species was tested in two lakes. When introduced to enclosures in eutrophic Larimore Pond, D. pulex decreased phytoplankton abundance by an order of magnitude, resulting in drastically reduced densities of Tropocyclops prasinus and Mesocyclops edax, the two dominant crustaceans in the pond. Daphnia pulex was also added to enclosures in oligo-mesotrophic Dynamite Lake, along with nutrients, in a factorial design. Three Dynamite Lake zooplankters were reduced in density by D. pulex under both enriched and unenriched conditions: Bosmina Zongirostris, Trichocercu multicrinis, and copepod nauplii. Nutrient additions allowed these taxa to overcome some effects of competition with D. pulex. Daphnia pulex reduced the densities of copepods and rotifers common to both lakes more in eutrophic Larimore Pond than in unenriched Dynamite Lake enclosures. These results show that a large herbivore can reduce the density of some small zooplankton species and therefore contribute to the scarcity of small species in lakes dominated by large herbivores. Furthermore, the results suggest that the competitive effects of a large herbivore on rotifers and copepods may bc more pronounced in eutrophic systems because the large species can attain a higher population density, and subsequently alter the resource base to a greater extent, in eutrophic systems. Freshwater zooplankton communities typically are dominated by either largeor small-bodied species. Lakes containing abundant planktivorous fish contain mostly small species, the result of the elimination of large species by size-selective fish predation (e.g. Hrbacek 1962; Brooks and Dodson 1965; Lynch 1979). Fishless lakes are typically inhabited by large species, sometimes to the exclusion of small species (e.g. Brooks and Dodson 1965; Zaret 1980). Two hypotheses have been proposed to account for the scarcity (or absence) of small species in fishless lakes. One is that large herbivorous zooplankton, by monopolizing the food resources (mainly phytoplankton), competitively reduce the abundance of, or exclude, small species (the size-efficiency hypothesis by Brooks and Dodson 1965). An alternative hypothesis is that predation pressure, e.g. by large Chaoborus species and I Research supported by Illinois Water Resources Center grant S-093-ILL to M. Lynch. Data analysis and manuscript preparation facilitated by support from the Freshwater Institute and the Natural Sciences and Engineering Research Council of Canada. 2 Present address: Freshwater Institute, 50 1 University Crescent, Winnipeg, Manitoba R3T 2N6. large copepods, is greater in fishless lakes and that this invertebrate predation excludes, or reduces the abundance of, small species (Dodson 1974). It has been argued that predation by invertebrates is sufficient to account for the reduced abundance of small species in fishless lakes, while resource depression by large species is not sufficiently strong to drive smaller species to low densities (Dodson 1974; Zaret 1980). Many experimental field studies of competition between small and large zooplankters have been relatively short term, typically involving competition trials between two or three species in small containers. These investigations failed to distinguish a consistently dominant competitor; the competitively dominant species often varied from one experiment to the next, even within a single field season in one lake (Sprules 1972; Lynch 1978; Smith and Cooper 1982; DeMott 1983). While useful in assessing interactions among particular species, small-scale experiments do not address the potential effects of a large species invading an entire community of small species, as would be the case if, for example, planktivorous fish were reduced in abun-