The predominant approach to setting environmental quality guidelines for chemical contaminants to protect aquatic biodiversity involves the use of single-species toxicity tests, which often measure short-term mortality on a limited number of species. It is recognized that the results of these tests may not adequately protect biodiversity in ecosystems; however, it is implicitly assumed that these tests do indicate the relative effects of toxicants in ecosystems. Here, we test this implicit assumption by conducting an experiment in model freshwater ecosystems (mesocosms). We found this relative toxicity assumption was not supported for effects on stream macroinvertebrates comparing two salinity types, sodium bicarbonate (NaHCO 3 ) and salinity with the ionic composition of seawater, which are common contaminants from coal bed effluents and agriculture, respectively. Consequently, extrapolation from single-species toxicity estimates (representing effects on test populations) to the relative effects on natural populations and communities in nature is problematic, at least for NaHCO 3 and salinity with the ionic composition of seawater. Studies should test the implicit assumption of relative effects between other combinations of chemical stressors.

No abstract

No abstract

No abstract

No abstract