Zooplankton community structure : factors affecting distribution of Bosmina longirostris
Dempsey, Diane M. Ormsby
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Two general trends in the structure of zooplankton communities have been observed. One is characterized by the presence of planktivorous fish and small-sized zooplankton and by the absence of large-sized zooplankters. The other is characterized by the absence of planktivorous fish and small zooplankton, and by the presence of large-sized plankters. Two explanations have been offered for the case in which fish are absent and large plankters dominate. The first proposed that when fish are absent, small-sized zooplankton are competitively eliminated by large forms. The second proposed that small species were absent from lakes occupied by large plankters because of size-selective predation by invertebrate predators and not because they are outcompeted by large zooplankters. This study examined The question of why small zooplankters tend not to coexist with large-sized species. Experiments were carried out in several Michigan lakes to determine why the small species, Bosmina longirostris, does not coexist with the larger Daphnia pulex. Studies of pH, temperature, and dissolved oxygen of two lakes, Beaver Bog, where B. longirostris is found, and Tender Bog, where D. pulex is found, indicate that the two lakes are similar in these parameters. Field experiments showed that B. longirostris was not excluded from Tender Bog because it could not tolerate the physical-chemical conditions of this lake. Dispersal studies have indicated it is unlikely that B. longirostris is absent from Tender Bog because B. longirostris ephippia could not be passively dispersed to Tender Bog. However, predation studies were conducted in which the invertebrate predator, Chaoborus americanus, selectively preyed on and eliminated B. longirostris when alternate prey were available. Competition experiments showed that when D. pulex and B. longirostris were enclosed in containers, B. longirostris was typically eliminated in all but one experimental container. These field studies indicate that both predation and competition may be important in structuring zooplankton communities.