Aquatic Invertebrates

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Fish and aquatic invertebrates are often more sensitive to herbicides than terrestrial animals because of their physiology or the increased exposure potential that may result from herbicide movement into aquatic sites. Aquatic species can be exposed to herbicides through direct spray, spray drift, spills or surface runoff. Though few commonly-used herbicide active ingredients are highly acutely toxic to aquatic organisms, toxic effects can result from the exposure to other ingredients in formulated products, such as surfactants.

Treatment of aquatic vegetation presents its own challenges, since decomposing plant matter can reduce the dissolved oxygen content enough to suffocate aquatic life in the treated area; this potential should be considered when treating large areas of lakes and ponds.


Reading the chart: For each bar, the labeled central value is the most likely estimate. The right end of the bar assumes worst-case conditions for all underlying variables; the left end of the bar assumes best-case conditions. Mitigation is advised if risk enters the red zone.

Taxa: Aquatic invertebrates.

Assumptions: Terrestrial application of herbicide at half of the maximum rate on a representative product’s label (see Table 4-1); 10-acre treatment with no buffer zone between treatment area and water body.

Likelihood: Buffer zones are required on many water ways (salmonid streams, for instance) and are common practice when using herbicides not approved for aquatic use. Dry season applications in California will result in a long interval before a rain event, resulting in lower residues for runoff.

Mitigation: Use low-volume applications and reduce the amount applied per acre. Use buffer zones (see Bakke (2001) to help gauge effective buffer distances). Make applications during the dry season to avoid runoff. For applications near waterways, use herbicide formulations intended for use in aquatic systems.

Risk calculated as a function of:

  • The inherent acute toxicity of the herbicide to aquatic invertebrates
  • Herbicide characteristics that affect transport through soil to water (water solubility, ability to adsorb to soil)
  • Soil type; and
  • The application rate.

Herbicide degradation is not considered, as the estimate is for runoff occurring soon after the application. Except for glyphosate with the POEA surfactant, risks in this chart do not account for potential toxicity of any surfactants that are part of the product formulation or added to spray mixtures.

Methodology and sources: See Appendix B and the main page on this website, where you can access a spreadsheet for adjusting application rates and other variables.

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