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

Manipulations of the planktonic microinvertebrate community in a prairie we tland were carried out in experimental enclosures in Delta Marsh, Canada. L orsban(TM) 4E (active ingredient chlorpyrifos) was applied once to treatmen t enclosures at a nominal concentration of 10 mu g/L. Additions (thrice wee kly) of inorganic nitrogen and phosphorus were made to treatment enclosures for the duration of the 10-week experimental period. One-way ANOVA was use d to discern treatment effects on the microinvertebrate community at the gr oup level. Impacts of insecticide or inorganic nutrient addition on major g roups of microinvertebrates (Cladocera, Cyclopoida and Calanoida Copepoda, Rotifera) were limited, with few significant density changes observed durin g the experiment. Canonical correspondence analysis (CCA) was used to analy se the structure of the microinvertebrate community at the species level. P ercent cover of enclosure bottom by submersed macrophytes and alkalinity we re the only significant environmental variables; 10 environmental variables in the CCA accounted for 90 % of the variance in the microinvertebrate spe cies data. Differential mortality of arthropod microinvertebrates resulted from chlorpyrifos addition; calanoid copepods were more tolerant than clado cerans and cyclopoid copepods. An increase in the proportional abundance of small planktonic rotifers was observed after insecticide treatment, probab ly due to decreased competition with cladocerans and reduced cyclopoid cope pod predation. Inorganic nutrient addition did not substantially alter the microinvertebrate community structure.