Abstract
Effects of overall herbicide toxicity on primary production were investigated by an algal growth test using Selenastrum capricornutum (Chlorophyceae) with water samples collected from a sampling station in the R. Kokai from April to August, 1991. Bacteria were eliminated from the water samples before the test, and extra N (33 mg⋅l-1) and P (5.38 mg⋅l-1) were added. The concentrations of 8 kinds of herbicide were analysed simultaneously. Growth of Selenastrum was inhibited significantly in the water samples from early May to late June by several of the herbicides. The most serious inhibition in mid-May was attributed to the joint toxicity of butachlor and pretilachlor. The sequential growth inhibition from the end of May to late June was attributed mostly to simetryn, although combined effects of several other herbicides were slightly recognizable at the end of May.
Periphyton which formed on the artificial substrates was also affected, judging from the number of species as well as the rate of accumulation, possibly by several kinds of herbicide from mid-May to mid-June, when the growth of Selenastrum was inhibited in the river water samples. These results suggest that algal production in rivers may be affected by herbicides at least in the period from May to June.
