Effects of organophosphorus insecticide and inorganic nutrients on the planktonic microinvertebrates and algae in a prairie wetland

Abstract

Manipulations of the planktonic microinvertebrate community in a prairie wetland were carried out in experimental enclosures in Delta Marsh, Canada. Lorsban 4E (active ingredient chlorpyrifos) was applied once to treatment enclosures at a nominal concentration of 10 μg/L. Additions (thrice weekly) of inorganic nitrogen and phosphorus were made to treatment enclosures for the duration of the 10-week expermental period. One-way ANOVA was used to discern treatment effects on the microinvertebrate community at the group level. Impacts of insecticide or inorganic nutrient addition on major groups of microinvertebrates (Cladocera, Cyclopoida and Calanoida Copepoda, Rotifera) were limited, with few significant density changes observed during the experiment. Canonical correspondence analysis (CCA) was used to analyse the structure of the microinvertebrate community at the species leve. Percent cover of enclosure bottom by submersed macrophytes and alkalinity were the only significant environmental variables: 10 environmental variables in the CCA accounted for 90 % of the variance in the microinvertebrate species data. Differential mortality of arthropod microinvertebrates resulted from chlorpyrifos addition; calanoid copepods were more tolerant than cladocerans and cyclopoid copepods. An increase in the proportional abundance of small planktonic rotifers was observed after insecticide treatment, probably due to decreased competition with cladocerans and reduced cyclopoid copepod predation. Inorganic nutrient addition did not substantially alter the microinvertebrate community structure.