Assessment of herbicide effects on algal production in the Kokai River (Japan) using a model stream and Selenastrum bioassay.

The effects of herbicides on algal production in the Kokai River were assessed using a Selenastrum growth inhibition test and by an investigation of the periphyton formed in a channel carrying river water. Herbicides in the water were analysed at the same intervals as the algal growth test (three times per week) from April to August 1992. Growth of Selenastrum in the river water samples was inhibited severely in mid-May, mostly by the joint effects of butachlor and pretilachlor and from early to late June mostly by simetryn alone. The influence of six other kinds of herbicide was low or not detectable. Selenastrum growth ratios (SGR) in the river water samples were correlated with fluctuations in SG-IP (Selenastrum growth inhibition potential), the sum of the growth inhibition ratios by the respective herbicides at the concentrations detected in the river. However, the expected growth inhibition (SG-IP) was usually larger than the observed value (SGR); the reasons for this are discussed.The rate of accumulation of periphyton, which formed on a substrate in the channel, decreased from early May and stayed at relatively low levels until early June. The low accumulation rate from mid-June to mid-July, when growth of Selenastrum had almost recovered, can be attributed to grazing by invertebrates. These results suggested that algal production in the Kokai River is affected by the single or joint action of herbicides, at least in the period from early May to early June.