FATE AND EFFECTS OF THE INSECTICIDE DURSBAN(R) 4E IN INDOOR ELODEA-DOMINATED AND MACROPHYTE-FREE FRESH-WATER MODEL-ECOSYSTEMS .3. ASPECTS OF ECOSYSTEM FUNCTIONING

The effects of the insecticide Dursban(R) 4E (active ingredient chlorpyrifos) on aspects of water chemistry, community metabolism, and decomposition of particulate organic matter were studied in indoor freshwater microcosms intended to mimic drainage ditches. To several of these systems, a single dose (simulating aerial drift) was applied to achieve nominal chlorpyrifos concentrations of 5 or 35 mug/L. Two experiments were performed; one, in which all model ecosystems were dominated by the macrophyte Elodea nuttallii, and one, using systems devoid of macrophytes. The response of ecosystem functioning is discussed in relation to the shifts in community structure observed. Treatment effects on community metabolism, as signaled by a decrease in dissolved oxygen and pH and an increase in alkalinity and conductivity, could be observed in the high-dose Elodea-dominated microcosms only. In addition, Dursban 4E application in these systems resulted in a more persistent decrease in the decay rate of macrophyte material present in litter bags than in open water microcosms. Concentrations of inorganic nitrogen and orthophosphate in the water column of both types of model ecosystem were hardly affected by insecticide application. It is concluded that the impact of Dursban 4E on ecosystem functioning differed considerably between the two types of model ecosystems, and that functional endpoints were more robust than structural properties.