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 chlorp yrifos) on aspects of water chemistry, community metabolism, and decom position of particulate organic matter were studied in indoor freshwat er microcosms intended to mimic drainage ditches. To several of these systems, a single dose (simulating aerial drift) was applied to achiev e nominal chlorpyrifos concentrations of 5 or 35 mug/L. Two experiment s were performed; one, in which all model ecosystems were dominated by the macrophyte Elodea nuttallii, and one, using systems devoid of mac rophytes. The response of ecosystem functioning is discussed in relati on to the shifts in community structure observed. Treatment effects on community metabolism, as signaled by a decrease in dissolved oxygen a nd pH and an increase in alkalinity and conductivity, could be observe d in the high-dose Elodea-dominated microcosms only. In addition, Durs ban 4E application in these systems resulted in a more persistent decr ease in the decay rate of macrophyte material present in litter bags t han in open water microcosms. Concentrations of inorganic nitrogen and orthophosphate in the water column of both types of model ecosystem w ere hardly affected by insecticide application. It is concluded that t he impact of Dursban 4E on ecosystem functioning differed considerably between the two types of model ecosystems, and that functional endpoi nts were more robust than structural properties.