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

Abstract

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.

Keywords:

• Fluridone
• Imazamox
• Glyphosate
• Microcystis
• Chlamydomonas
• Thalassiosira

Additional information

Funding

This study was supported by the California Department of Fish and Wildlife from the Water Quality, Supply, and Infrastructure Improvement Act of 2014 (Proposition 1, CWC §79707[g], Agreement No.: P1696041) (Program managers: Adam Ballard and Jason Roeh) and the California State Water Resources Control Board (Agreement No.: 16-046-150) (Program manager: Daniel McClure). We are also thankful to Perez J, Clare-Botor N, Flores M, Carr D, Diamond T, Pedersen A at the Aquatic Health Program, University of California, Davis, for maintaining algal cultures and for supporting the exposure tests.