words) Biofilms are sensitive to environmental conditions. Impacts of contaminants on 2 assemblages are often investigated in laboratories or in meso-cosms. Such experiments are rarely representative of effects of contaminants on biofilms under natural conditions. Studies in 4 real field situations, with enough power to detect impacts, are necessary to develop better understanding of the effects of contaminants on ecological processes. Metals are a common 6 estuarine contaminant and can cause

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... sturbances to the assemblages. Using a new technique to experimentally deliver contaminants to micro-algal assemblages, we tested hypotheses about 8 the effects of zinc on micro-algal biofilm growing on settlement panels in subtidal and intertidal habitats. Control panels deployed for one month in each habitat had significantly 10 greater amounts of biofilm than those exposed to zinc. After three months of deployment, results varied with location. The observed effects on the biofilm did not, however, cause 12 significant changes in the fouling assemblages that developed on the panels. 14