• Hexazinone;
  • Ecotoxicity;
  • Periphyton;
  • Invertebrates;
  • Streams


Experiments were conducted in laboratory test chambers and outdoor stream channels to measure the effects of hexazinone on stream periphyton and macroinvertebrate communities. All experimental units were treated at the nominal concentration of 2.7 mg/L, which represents the maximum expected environmental concentration in water from forest vegetation management applications. In laboratory flow-through tests, a 12-h exposure of stream periphyton on natural substrates to hexazinone resulted in a significant reduction in net photosynthetic activity, but no significant effects on periphyton biomass (chlorophyl-a concentration). Recovery from toxic effects was rapid, with no significant differences in photosynthetic activity among treated and control units within 3 h after the hexazinone concentrations were cleared. In stream channels treated for 12 h with hexazinone, there were no significant effects on periphyton biomass and no significant differences in invertebrate drift, compared to control channels. There were also no indications of adverse treatment effects on the invertebrate communities in the stream channels 14 d after the application. The treated and control channels differed in invertebrate community structure, as determined by a stepwise discriminant analysis [Wilks' lambda F equivalent (3,2) = 127.6, p = 0.007], and this was supported by a two-way ANOVA which showed a significant difference in abundance of macroinvertebrates [F(1, 56) = 10.5, p = 0.002] between treated and control channels, averaged over all taxa. However, subsequent pairwise t tests, when adjusted for multiple comparisons, were unable to identify significant differences between treatment levels for any of the 14 taxa with sufficient data for analysis. Although not significant, the comparison of means showed that abundance of most taxa was higher in the treated channels than in the controls.