Effects of Pesticide Contamination from Rice Fields on Stream Benthic Arthropods

The composition of benthic arthropod communities, concentrations of pesticide residues, and river water toxicity for the shrimp Paratya compressa improvisa and the minnow Tanichtys albonubes were surveyed at five stations of the Sawato River before and after transplanting rice plants into paddy fields. The two groups of pesticides reached their peak concentrations at the downstream stations soon and one month after the transplanting, respectively. The first group was composed of malathion, oxadiazon, chlornitrofen and butachlor. The second group was composed of BPMC, molinate, simetryn and thiobencarb. The concentration of each pesticide was usually higher downstream. The toxicity of the river water for the test shrimp increased simultaneously with the pesticide contamination. The abundance of the ephemer-opteran Baetis thermicus decreased with the pesticide contamination. This species was probably damaged by the pesticide contamination.