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Laboratory experiments were conducted with sediments and organisms collected from Lake Erie's western basin to examine the effect of Coelotanypus (Chironomidae: Diptera) predation on tubificid oligochaetes. Populations of predator and prey organisms were kept together in replicate laboratory microcosms at densities ranging from 2,500–12,500 predators/m2 and 12,500–62,500 prey/m2 for 15- to 30-day periods. Control populations of predators and prey were maintained separately for comparative purposes. A separate selection experiment showed that larvae preferred sediments containing tubificids or previously settled larvae over sediments that contained neither. Mortality in worm populations kept with predatory Coelotanypus larvae averaged 56.9% (S.D. = 3.9%) and was significantly higher (p < 0.001, t-test) than the 9.7% (S.D. = 3.1%)average mortality in control populations. Simulated effects of chironomid burrowing and burrow irrigation did not cause increased worm mortality. An average predation rate of 0.11 (S.D. = 0.04) prey·predator−1·day−1 was calculated from the data. Based on average Coelotanypus and tubificid densities in western Lake Erie and assuming that natural feeding rates are comparable to those in the laboratory, Coelotanypus may be capable of reducing tubificid populations by about 20% over a 2-month period during the late summer. Natural abundance data indicate that tubificid population declines are associated with periods of peak Coelotanypus density. |
