Pollution-induced community tolerance as a measure of species interaction in toxicity assessment.
The assessment of the environmental hazards posed by chemical pollutants typically results from single-species tests that are extrapolated to ecosystems. The aim of the present study was to compare this type of extrapolation for a herbicide with the chronic effects that may be observed at a community level and to evaluate currently applied risk assessment strategies for their ability to predict chemical effects on complex communities. Freshwater periphyton communities, grown in indoor aquaria, were exposed to the pollutant diuron for 3 months. Acute toxic effects of diuron were detected as photosynthesis inhibition using quenching analysis of chla-fluorescence. Chronic effects of the herbicide were observed in terms of changes in biomass and algal class composition as well as pollution-induced community tolerance (PICT). The PICT concept is based on a chemical exerting selection pressure on a community and therefore eliminating sensitive species. As a result, the measured community tolerance increases. Short-term effects of diuron arise from 4-9 µg L-1 as half-maximal effect concentration (EC50). It is further shown that diuron concentrations down to 0.08 µg L-1 caused chronic effects in two independent microcosm studies. The observed threshold concentration of 0.08 µg L-1 still caused changes in biomass and class composition as well as an increased community tolerance. The determined EC50 values increased by a factor of 2-3 in diuron-exposed periphyton communities. This threshold value could not be predicted by advanced extrapolation methods such as species sensitivity distribution or acute-to-chronic effect ratios. Synthesis and applications. The chronic community-level effects of the pollutant diuron were not predictable from single-species tests. However, regulations such as the EC Water Framework Directive or the EC-REACH process (Registrations, Evaluation and Authorisation of Chemicals) rely on this type of information. The management of chemicals in the environment should be based upon higher-tier assessment tools. Species interaction, detectable and quantifiable by the PICT methodology, may serve as a prognostic tool in chemical hazard assessment when extrapolating effects from single-species tests to community level.