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Abstract-The direct and indirect effects of the herbicide atrazine were assessed at the community level under exposure patterns likely to be experienced by stream organisms in Vermont. Both attached algae and benthic invertebrates were collected from a clean site using artificial substrates and established in recirculating 120-L laboratory microcosms. On day 1 of the 14-d experiment, three of six microcosms were treated with 5 g/L atrazine. Concentrations were gradually reduced by replacing a portion of the water in the systems over time so that by day 7, atrazine concentrations were only about 1 g/L. Direct effects of atrazine on algal biomass were assessed by measuring chlorophyll a concentrations in samples of attached algae. No differences in chlorophyll a were found between treatment and control microcosms. Potential indirect effects of reduced algal food availability on benthic invertebrate communities were assessed by comparing assemblages in treatment and control microcosms using both a functional (feeding) group classification and taxonomic composition. There were no significant differences in the functional or taxonomic composition of the benthic invertebrate communities; however, a greater number of insects emerged from the treatment microcosms early in the experiment compared to controls, suggesting some response to atrazine. Further community-level testing is needed to more clearly determine if atrazine is affecting stream organisms.

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