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.

Toxicity of herbicides to cyanobacteria and phytoplankton species of the San Francisco Estuary and Sacramento-San Joaquin River Delta, California, USA

The herbicides glyphosate, imazamox and fluridone are herbicides, with low toxicity towards fish and invertebrates, which are applied to waterways to control invasive aquatic weeds. However, the effects of these herbicides on natural isolates of phytoplankton and cyanobacteria are unknown. Three species of microalgae found in the San Francisco Estuary (SFE)/Sacramento-San Joaquin River Delta (Delta) (Microcystis aeruginosa, Chlamydomonas debaryana, and Thalassiosira pseudonana) were exposed to the three herbicides at a range of concentrations in 96-well plates for 5-8 days. All three algal species were the most sensitive to fluridone, with IC₅₀ of 46.9, 21, and 109 µg L^-1 for M. aeruginosa, T. pseudonana and C. debaryana, respectively. Imazamox inhibited M. aeruginosa and T. pseudonana growth at 3.6 × 10⁴ µg L^-1 or higher, and inhibited C. debaryana growth at 1.0 × 10⁵ µg L^-1 or higher. Glyphosate inhibited growth in all species at ca. 7.0 × 10⁴ µg L^-1 or higher. Fluridone was the only herbicide that inhibited the microalgae at environmentally relevant concentrations in this study and susceptibility to the herbicide depended on the species. Thus, the application of fluridone may affect cyanobacteria and phytoplankton community composition in water bodies where it is applied.

Sub-lethal effects of herbicides penoxsulam, imazamox, fluridone and glyphosate on Delta Smelt (Hypomesus transpacificus)

Concerns regarding non-target toxicity of new herbicides used to control invasive aquatic weeds in the San Francisco Estuary led us to compare sub-lethal toxicity of four herbicides (penoxsulam, imazamox, fluridone, and glyphosate) on an endangered fish species Delta Smelt (Hypomesus transpacificus). We measured 17β-estradiol (E2) and glutathione (GSH) concentrations in liver, and acetylcholinesterase (AChE) activity in brain of female and male fish after 6 h of exposure to each of the four herbicides. Our results indicate that fluridone and glyphosate disrupted the E2 concentration and decreased glutathione concentration in liver, whereas penoxsulam, imazamox, and fluridone inhibited brain AChE activity. E2 concentrations were significantly increased in female and male fish exposed to 0.21 μM of fluridone and in male fish exposed to 0.46, 4.2, and 5300 μM of glyphosate. GSH concentrations decreased in males exposed to fluridone at 2.8 μM and higher, and glyphosate at 4.2 μM. AChE activity was significantly inhibited in both sexes exposed to penoxsulam, imazamox, and fluridone, and more pronounced inhibition was observed in females. The present study demonstrates the potential detrimental effects of these commonly used herbicides on Delta Smelt.