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Periphyton communities are an ecologically important source of information when conducting experimental stream ecotoxicological studies . Experimental stream studies often utilize unreplicated systems in order to understand dose-response phenomena . The objective of this study was to examine the robustness of such a dose-response investigation . Autotrophic and heterotrophic periphyton were evaluated in replicated control experimental streams with open recruitment (once-through flow design) . Measurements of population and community structure and community function over an eleven week colonization period were made on unglazed clay tile and cobble substrata . Several measures were significantly different during the study ; however, most of these were a result of initial stochastic colonization events or associated with relatively rare algal populations . During the latter half of the study autotrophic and heterotrophic community measurements on tile and cobble substrata displayed similar trends in both control streams . Population and community metrics were significantly different for approximately 5% of the 400 measurements made after the initial sampling . These results strongly suggest that the driving forces behind structuring aquatic population and community responses will be test chemical exposure and not spuriously developed stream ecosystems with individual trajectories . Recruitment in these open systems is evenly distributed across the replicated experimental stream ecosystems .

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