Effects of seven forestry management herbicides on Myriophyllum sibiricum, as compared with other nontarget aquatic organisms

Historically, studies on the nontarget aquatic effects of forest-use herbicides focused largely on fish and invertebrates despite the ecological importance of aquatic plants. Regulatory requirements and the aquatic phytotoxicity database need to be improved to reflect the importance of aquatic plants. Based on a review of the scientific literature and results from laboratory testing with the submersed macrophyte Myriophyllum sibiricum Komarov, the effects of 2,4 dichlorophenoxyacetic acid (2,4-D), glyphosate, hexazinone, imazapyr, metsulfuron methyl, sulfometuron methyl, and triclopyr on nontarget aquatic organisms were compared. Laboratory results indicate that M. sibiricum is sensitive to herbicide effects, especially those that impact root growth. For most herbicides, the inhibitory concentrations 25 and 50 (IC(25) and IC(50), concentrations that inhibit an endpoint parameter by 25 and 50%, respectively) for shoot and root growth were below the expected environmental concentrations (EEC). For example, the IC(50) values for root dry mass were 0.000 12 and 0.000 22 mg active ingredient/L for sulfometuron methyl and metsulfuron methyl, respectively, concentrations that were approximately 3100 and 1700 times below the EEC for these compounds. Interspecies comparisons, conducted during this study, demonstrated that M. sibiricum was generally equally or more sensitive to these herbicides than other aquatic plant species (i.e., floating macrophytes and algae) and, in some cases, more sensitive than fish, zooplankton, and other invertebrates. For example, available data demonstrated that 2,4-D, imazapyr, and the sulfonylureas investigated were more toxic to rooted and floating macrophytes than to other aquatic organisms, including algae. Because of the high sensitivity and ecological significance of aquatic macrophytes, a greater emphasis should be placed on evaluating aquatic phytotoxicity in future ecotoxicological research.