Long-term effects of copper on the structure of freshwater periphyton communities and their tolerance to copper, zinc, nickel and silver

A community adapted to elevated ambient levels of a particular pollutant is expected, compared to a non-exposed community, to display an increased tolerance to that pollutant. The potential of tolerance measurements as a method to detect metal-induced structural impacts at the community level is poorly known. Particularly, the determination of increased tolerance to various metals may confound conclusions related to the causes of the impact. In this study the effects of long-term copper exposure on the community structure of freshwater periphyton, and the short-term community tolerance of photosynthesis to copper, zinc, nickel and silver were determined. Using an outdoor flow-through aquaria system, we carried out long-term exposure of freshwater periphyton communities to copper (0, 0.05, 0.1, 0.5, 1 and 5 mu M copper). After 12 weeks we examined how the copper exposure affected the taxonomic composition, photosynthesis rate and tolerance thereof to copper, zinc, nickel and silver. Effects included changes in the distribution of algal classes from a community dominated by Cyanophyceae to one dominated by Chlorophyta. The relative abundance of Oocystis nephrocytioides increased from less than 1% in the control aquaria to 56% in the 5 mu M copper treatments. Except at the highest copper exposure, communities did not significantly differ in their photosynthesis rate, although the short-term tolerance of photosynthesis to metals was affected by the copper treatments. Significant increases in tolerance to copper were found in communities previously exposed to greater than or equal to 0.1 mu M copper concentrations. Communities exposed to copper also displayed an increased co-tolerance to zinc, nickel and silver. These observations suggest that copper-induced structural impacts on periphyton communities can be evidenced as an increased tolerance to copper. However, because of the occurrence of co-tolerance, the identification of the metals that have induced the structural and tolerance changes may require metal determinations in organisms. (C) 2000 Elsevier Science B.V. All rights reserved.