Rapid establishment of test conditions and trophic‐level interactions in 0.04‐hectare earthen pond mesocosms

A mesocosm study of pesticide effects was conducted between November 1987 and November 1988 using 0 04‐ha earthen ponds The bottoms of the ponds were covered with sediments from an old farm pond, filled with aged reservoir water, and stocked with fifty 11‐ to 17‐cm blue‐gill sunfish Importing mature communities rapidly produced taxonomically rich ecosystems Coefficients of variance among ponds for taxonomic richness and population densities were modest (typically 10–30%) and usually inversely related to abundance Unchecked bluegill reproduction resulted in expected effects on lower trophic levels Crustacean zooplankton were nearly eliminated, which led to poor growth of the juvenile bluegill Phytoplankton densities appeared to increase as grazing pressure from zooplankton was reduced Insect emergence was not obviously impacted by fish predation Potential indirect effects on fish, such as growth impairment caused by pesticide induced reduction of food supply, could be obscured if zooplankton were nearly eliminated by over‐predation This suggests a need to control bluegill reproduction to achieve a more balanced ecosystem