Despite high densities in an upstream lake, the calanoid copepod Diaptomus leptopus is extremely scarce in oligotrophic montane Eunice Lake, British Columbia. Field experiments tested the hypothesis that competition from zooplankton resident in Eunice Lake prevents immigrant D. leptopus from colonizing.Experiments in 1979 exposed a standard density of D. leptopus to all Eunice Lake zooplankton species at lake densities (control) and at reduced densities (low density), and to all Eunice Lake species except one of Daphnia rosea (Daphnia removal), Diaptomus kenai (kenai removal), or Diaptomus tyrrelli (tyrrelli removal). Improved D. leptopus performance in noncontrol treatments was evidence for competition in controls. Performance measures included density, survival, and reproduction. Similar and poor performances of D. leptopus in control, Daphnia-removal, and kenai-removal treatments plus high concentrations of D. leptopus nauplii in the tyrrelli-removal treatment suggested that only D. tyrrelli competed with D. leptopus. However, similarity between tyrrelli-removal and low-density treatments in D. leptopus naupliar concentrations confounded competition from D. tyrrelli with diffuse competition from several Eunice Lake species combined. Experiments in 1980, designed to separate these effects, provided no evidence of interspecific interactions. Cool 1980 temperatures presumably reduced competition for algal foods by lowering zooplankton respiration rates.
Feeding of Chaoborus flavicans larvae (Diptera: Chaoboridae) on Ceratium hirundinella and Daphnia rosea in a eutrophic pond.