The Effect of Herbicide Formulations and Herbicide-Adjuvant Mixtures on Aquatic Food Web Species of the Sacramento-San Joaquin Delta, California, USA.

Herbicides have recently been designated as one of the most frequently detected chemical types in aquatic systems. We evaluated the effect of the herbicide formulations Clearcast® (active ingredient imazamox) and Galleon® SC (active ingredient penoxsulam), in conjunction with the adjuvant Agri-dex®, on the diatom Thalassiosira pseudonana and the copepod Eurytemora affinis and whether their application in invasive aquatic weed control has acutely negative effects on important delta food web species. Herbicide-adjuvant mixtures were more toxic than herbicide formulations alone. For the diatom, 50% inhibition concentration (IC50) were 84.4 mg/L for Clearcast, >100 mg/L for Galleon SC, and 38.5 mg/L for Agri-dex. In the herbicide mixture diatom tests, IC50s decreased to 2.8 mg/L for Clearcast + 64 mg/L Agri-dex and to 6.6 mg/L for Galleon SC + 41 mg/L Agri-dex. In the copepod tests, 50% effect concentrations (EC50s) were 45.4 mg/L for Agri-dex, 31.2 mg/L for Galleon SC, and >100 mg/L for Clearcast. When tested together, EC50s were reduced to 24.1 for Galleon SC and 67.6 mg/L for Clearcast in the presence of 50 mg/L Agri-dex. Environmentally relevant herbicide-adjuvant mixture ratios were at the no-observed-effect level. Mixture interactions between herbicides and adjuvants indicate the potential for increased toxicity in herbicide formulations and tank mixes, especially in consideration of the unlisted, proprietary ingredients which are included in herbicide formulations, making predicting nontarget effects challenging. Environ Toxicol Chem 2020;39:1375-1381. © 2020 SETAC.