Disentangling the effects of low pH and metal toxicity on macroinvertebrate diversity.

One of the primary goals of biological assessment of rivers is to identify whether contaminants or other stressors limit the ecological potential of running waters. Quantitative relationships between species richness and environmental gradients are useful to better understand biodiversity patterns. However, describing such relationships for a single environmental gradient is not straightforward, because species richness is typically dependent on a multitude of environmental gradients. Many studies have focused on the effects of pH and high metals concentration on freshwater macroinvertebrate community but, due to data limitation and the lack of evaluation tools, the ecological effects of metals mixture in streams are less studied and still unclear. We address an old question: is it the low pH or the metals that are deleterious for stream ecosystems? With new tools, we can achieve improved understanding of the true importance of the two stressors. The accumulation of metals in organisms was predicted using an equilibrium speciation model developed by the US Geological Survey to help understand how the changes in water quality influence the ecology of these systems. Because most metal-contaminated streams were affected by mixtures of metals (Al, Cd, Cu, Pb, Zn), we compared a concentration addition and an independent action approach of aqueous metal toxicity. Our study quantified the limiting effects of pH and chronic metal mixtures toxicity for macroinvertebrate community richness. We verified that current environmental quality standards for metals are protective of aquatic biodiversity and proved that pH has a direct limiting effect on richness and it does not only act via modifying the availability and the toxicity of metals. These questions were applied to a dataset spanning two continents with diverse geological features and ecosystems, providing a broad basis for understanding how physico-chemical conditions limit global freshwater biodiversity.