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U.S. Environmental Protection Agency, Environmental Research Laboratory, 6201 Congdon Boulevard, Duluth, MN 55804; U.S. Environmental Protection Agency, Environmental Research Laboratory, 6201 Congdon Boulevard, Duluth, MN 55804; U.S. Environmental Protection Agency, Environmental Research Laboratory, 6201 Congdon Boulevard, Duluth, MN 55804; Monticello Ecological Research Station, University of Minnesota, P.O. Box 500, Monticello, MN 55155-3898

Replicated experimental wetland channels were exposed to increasing doses of atrazine at concentrations typical of midwestern surface waters following spring runoff (0-75 µg/l). Mean measured atrazine concentrations 230 m downstream of inputs ranged from 78 to 96% of concentrations at the head of channels (residence time = 43-62 hrs). Endpoints measured included: nutrient levels, periphyton biomass, aufwuchs net or gross productivity and respiration, growth of selected emergent, submerged, and floating macrophytes, and survival and growth of Rana pipiens (15, 25 µg/l) and Pimephales promelas (50, 75 µg/l). Periphyton colonization also was measured with or without nutrient enrichment. Only aufwuchs and wild rice (Zizania aquatica) were significantly affected by atrazine at levels tested. Aufwuchs net and gross productivity and/or respiration were significantly depressed by incubation in treated water as compared to control water at all concentrations tested (P <0.05), while wild rice senesced prematurely at treatments of 50 or 75 µg/l atrazine. Periphyton developed resistance to atrazine at the 50 µg/l exposure regime.