A biomonitoring test was used to assess whether rivers and streams maintained sufficient water quality to support aquatic life. Daphnia magna were exposed to the water samples collected from 17 sites of 13 rivers and streams in Tokyo and its suburbs and the acute response of D. magna was monitored.
The mobility of the organism was acutely inhibited in the water of six rivers and streams tested. Discharge to these sites was from paddy field, and was a significant proportion of the total flow of the streams and rivers. Gas chromatography-mass spectroscopy of the samples revealed the presence of some pesticides in the water samples. Determination of the toxicity of organophosphate insecticides such as fenitrothion and diazinon to D. magna showed 50% inhibition concentration of mobility at 48-hr exposure (48 h-EC₅₀) of 0.10 to 1.0 μg/l. The concentrations of these insecticides in the water samples correlated well with the toxicity measurements of each sample. These insecticide concentrations accounted for most of the toxicity.
This biomonitoring test using D. magna was highly sensitive and simple. The significance of detecting these insecticides at acutely toxic concentrations to D. magna in some waters of the streams and rivers indicates that they have an adverse effect on some aquatic lives such as crustacean and benthic insects.