Comparing macroinvertebrate assemblages at organic-contaminated river sites with different zinc concentrations: Metal-sensitive taxa may already be absent.

We investigated responses of macroinvertebrates to different zinc concentrations in urban rivers contaminated with organic matter in a regional-scale monitoring survey and a smaller-scale field study. The present study was designed to test our prediction that total zinc concentrations of ∼60 μg/L (twice the Japanese environmental quality standard) do not lead to significant reductions in richness or abundance of macroinvertebrates in organic-contaminated rivers (biochemical oxygen demand of >3 mg/L). At the organic-contaminated sites in both surveys, very few species were present, and metal-sensitive heptageniid and ephemerellid mayflies were generally absent. In the regional-scale study, total zinc concentrations of up to 70 μg/L resulted in little reduction in macroinvertebrate richness. In the local-scale study, macroinvertebrate richness and abundance were not greatly reduced at the polluted downstream site with a total zinc concentration of 48 μg/L. Results from both surveys support our prediction. Therefore, an important implication of this study is that macroinvertebrate taxa that are susceptible to metal pollution should be sparse or absent in organic-contaminated rivers, so the impacts of metals such as zinc may be limited owing to the species-poor communities. Further research is required to evaluate the importance of reduced zinc bioavailability associated with increased organic matter and water hardness to the species-poor communities in organic-contaminated rivers.