A biomonitoring test was used to assess whether river waters maintained sufficient water quality to support aquatic life. Water was collected from mountain streams around Mt. Tanigawa in Gunma Prefecture, Japan from May to October 2010 and water quality was assayed. Water from some streams was acidic and was found to have low water hardness. Acute immobilization test using Daphnia magna confirmed that acute chemical toxicity was present in some of the streams. This acute chemical toxicity remained strong until June and then decreased slightly. Multivariate statistical analysis of water quality parameters and immobilization demonstrated that acidic water could produce immobilization of D. magna. The acute toxicity of hydrogen ions H⁺ was examined in parallel with effects of water hardness using D. magna as a test species. Water samples with pH of 6.0 or lower were toxic to D. magna; however, low water hardness did not affect survival. These results indicate that immobilization of D. magna was attributable to the low pH levels observed in mountain stream waters early in the season and that the low pH of those streams could have been caused by acid deposition, based on seasonal water characteristics and a low conductivity. We also discuss reasons why acidification of the stream waters occurred only in some of the mountain streams in the area.

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