An aquatic, multicomponent microcosm simulating a northern prairie wetland was used to assess the potential effects of six extensively used agricultural pesticides on this important wildlife habitat. Using a nested experimental design, 16 4-liter aquatic microcosms were treated with three concentrations of each of the pesticides carbofuran, fonofos, phorate, atrazine, treflan and trial-late. The microcosm units were incubated for 30 d in an environmental chamber, with a 16-h light:8-h dark cycle, maintained at 20°C. Specific limnological, biological and toxicological parameters were monitored over time by observing the interactions of water, animals, sediment and plants with the pesticides. The laboratory protocol was designed as an initial, rapid, economical screening test to determine the effect, but not the fate, of chemical contaminants in terms of toxicity, impaired productivity and community biochemical functions.
Static acute toxicity tests with Daphnia magna and Chironomus riparius suggested that carbofuran, fonofos, phorate and triallate were very toxic to aquatic invertebrates. For D. magna the 48-h EC50 values were 48, 15, 19 and 57 μg/L, respectively. Invertebrate viability tests indicated rapid changes in the toxicological persistence of these pesticides after microcosm interaction. Populations of D. magna were established in the 10 μg/L test concentration of carbofuran, phorate, triallate and fonofos at 1, 1, 14 and 28 d, respectively. Preexposure of the wetland sediments to either triallate or fonofos did not appear to change the relative toxicological persistence of each compound in the water column. Changes in pH, alkalinity, conductivity, dissolved oxygen, total nitrogen and total phosphorus were also observed with different pesticide treatments. Atrazine significantly reduced gross primary productivity and inhibited algal and macrophytic growth. In general, there was no evidence of significant inhibition of microbial functions in the water or hydrosoil of the treated microcosms. The respiratory electron transfer system, phosphatase activity, oxygen consumption and mineralization of dissolved organic carbon were not significantly impacted by any of these pesticides in hydrosoils. However, the impact of atrazine, fonofos and triallate on invertebrates and plants in the microcosm—both key elements in wetland productivity—would suggest that caution be used in application of these pesticides in or near wetland habitats.