Does a sum of toxic units exceeding 1 imply adverse impacts on macroinvertebrate assemblages? A field study in a northern Japanese river receiving treated mine discharge


In ecological risk assessment, sum-of-toxic-unit approaches based on measured water quality factors such as trace metals are used to infer ecological impacts in the environment. However, it is uncertain whether the use of such approaches yields accurate risk predictions. To address this issue, we investigated and compared (1) water quality, including trace metals, and (2) benthic macroinvertebrate communities in a northern Japanese river receiving treated discharge from an abandoned mine and in a nearby reference river. As a sum-of-toxic-unit approach, we employed a cumulative criterion unit (CCU), namely, the sum of the ratios of the dissolved concentrations of a metal (Cu, Zn, Cd, or Pb) divided by the US Environmental Protection Agency hardness-adjusted environmental water quality criterion for that metal. Compared with the reference sites, at the metal-contaminated sites, the richness, abundance, and structure of macroinvertebrate communities were little affected, with CCUs of 1.7 to 7.4, suggesting that CCU values exceeding 1 do not always indicate marked adverse impacts on these metrics. Further study is still required to derive a more compelling conclusion on the generally applicable relationships between CCUs and ecological impacts on river invertebrates. This would lead to better ecological risk assessments based on sum-of-toxic-unit approaches.

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We are grateful to Susumu Norota and Kazuto Ohmori of Hokkaido Research Organization for their help in study site selection, and Shosaku Kashiwada and Daiki Kitamura of Toyo University for their help in metal analysis. The paper does not necessarily reflect the policies or views of any government agencies. Useful comments by anonymous reviewers are greatly appreciated.


Preparation of this manuscript was supported partly by the Environment Research and Technology Development Fund (5RF-1801) of the Environmental Restoration and Conservation Agency of Japan.

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Correspondence to Yuichi Iwasaki.

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Iwasaki, Y., Fujisawa, M., Ogino, T. et al. Does a sum of toxic units exceeding 1 imply adverse impacts on macroinvertebrate assemblages? A field study in a northern Japanese river receiving treated mine discharge. Environ Monit Assess 192, 83 (2020).

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  • Heavy metals
  • Aquatic insects
  • Invertebrates
  • Species richness
  • Streams
  • Ecological impacts