| Summary: | Grazer and microbial constituents of a prairie wetland food web were manipulated using mesocosms in Blind Channel, Delta Marsh, Canada. Lorsban TM 4E (active ingredient chlorpyrifos) was applied once to treatment enclosures at a concentration of 10 ug/L. Additions of inorganic nitrogen and phosphorus were made to treatment enclosures for the duration of the 10-week experimental period. Impacts of insecticide or nutrients on abundance of invertebrates (Cladocera, Cyclopoida and Calanoida Copepoda, Ostracoda, Rotifera, Insecta, Gastropoda, Amphipoda) and planktonic bacteria were limited, with relatively few significant density changes observed. In contrast, structure of invertebrate communities did change substantially in response to treatment. Differential mortality of arthropods resulted from chlorpyrifos addition; within the water column, calanoids were more tolerant than cladocerans and cyclopoids; associated with submersed macrophytes, calanoids and harpacticoid copepods were more tolerant than cladocerans, cyclopoids, and ostracods. An increase in the proportional abundance of planktonic rotifers, and macrophyte-associated rotifers and oligochaetes was observed after insecticide treatment. Nutrient enrichment did not substantially alter invertebrate community structure. Canonical correspondence analysis (CCA) was used to analyze the structure of the invertebrate communities at the species or group level. Percent cover of enclosure bottom by submersed macrophytes and alkalinity were the only significant variables in the CCA of the planktonic microinvertebrate community; 10 environmental variables in the CCA accounted for 90 % of the variance in the species data. Soluble reactive phosphorus was the only significant variable in the ccA for the macrophyte-associated microinvertebrate community; eight environmental variables in the CCA accounted for 89 % of the variance in the species data. Percent cover of enclosure bottom by submersed macrophytes and soluble reactive phosphorus were the only significant variables in the CCA of the macrophyte-associated macroinvertebrate community; eight environmental variables in the CCA accounted for 91 % of the variance in the taxa data.
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