The effects of zinc on the structure and functioning of a freshwater community : A microcosm experiment

Résumé

A major problem with risk assessment of chemicals is the extrapolation of laboratory single-species toxicity tests, which oversimplify the actual field situation by ignoring species interactions, to natural communities. The authors tested if the bioavailability-normalized 5% hazardous concentration (HC5) estimated from chronic planktonic single-species toxicity data (HC5_plankton) for zinc (Zn) is protective for a plankton community and investigated the direct and indirect effects of Zn (at HC5 and HC50) on a freshwater community's structure and function. Microcosms were exposed to 3 different Zn concentrations (background, HC5_plankton=75μgZn/L and HC50_plankton=300μgZn/L) for 5wk. The planktonic groups revealed a consistent no-observed-effect concentration for the community of 75μgZn/L, similar to or higher than the HC5_plankton, thus suggesting its protectiveness in the present study. At 300μgZn/L a significant reduction in cladocerans resulted in increases of rotifer, ciliate, and phytoplankton abundance. In addition, the phytoplankton community shifted in dominance from grazing-resistant to edible species. Contrary to the species sensitivity distribution (SSD) prediction, which identified phytoplankton as the most sensitive group, only the total chlorophyll and the abundance of 2 phytoplankton species were adversely affected at 300μgZn/L. Thus, although the HC5 estimated from the bioavailability-normalized SSD was overall protective for the plankton community, the SSD was not able to correctly predict the species sensitivity ranking within their community context at the HC50.