Investigation of concentrations of paddy herbicides and their transformation products in the Sakura River, Japan, and toxicity of the compounds to a diatom and a green alga.

Four paddy herbicides and their transformation products (TPs) were monitored in the Sakura River, Japan, during the rice growing seasons of 2009 and 2010. Toxicity tests to an attached diatom, Mayamaea atomus, and a green alga, Pseudokirchneriella subcapitata, were also conducted. Clomeprop propionic acid, which forms from the degrading herbicide, was detected in the river water at much higher concentrations than the parent compound (the maximum concentration of the TP and the parent compound; 0.829–0.925 μg/L and 0.039–0.073 μg/L, respectively). The toxicity of the TPs to the diatom and green alga was relatively low; the 72-h median effective concentration (EC50) value > 1,470 μg/L; for each compound, the maximum concentration in the river did not exceed the EC50 value.