The response of Halodule wrightii and Halophila johnsonii to acute and chronic glyphosate stress

Seagrasses are essential habitats for many species and provide critical ecosystem functions. Globally seagrasses are declining at an alarming rate. Locally, seagrass loss is concerning because of the magnitude of habitat loss as well as the uncertainty surrounding the mechanisms driving seagrass decline. We exposed two prevalent seagrasses in the IRL to the terrestrial herbicide Roundup QuikPROTM (active ingredient 73% glyphosate) and quantified seagrass response over 53 days in a closed tank system. Direct application of 112 mg glyphosate (15mL of 7.5 g/L) onto Halodule wrightii and Halophila johnsonii, simulating overspray during low tide, yielded persistent concentrations of 2.58 mg/L glyphosate after 13 days. Direct application had acute effects, but survivorship after 53 days was equivalent to Control tanks. Indirect application of 125 mg/L glyphosate, with persistent concentrations of 83.4 mg/L glyphosate after 13 days, resulted in complete H. johnsonii mortality and nearly complete mortality of H. wrightii. Despite high-water solubility, glyphosate poses a measurable threat to seagrasses which warrants further study.