Measuring herbicide (73.3 % glyphosate) exposure response in Halophila ovalis (previously johnsonii) and Halodule...

We evaluated the effects of Roundup QuikPRO™ (73.3 % glyphosate) using real-world herbicide application treatments: (1) overspray (low-dose), (2) powder spill (high-dose), and (3) controls (no-dose). Seagrass and water quality were monitored to observe responses to acute herbicide application. Seagrass shoot densities significantly declined over time in high-dose treatments, whereas seagrass shoot densities in low-dose treatments were comparable to controls. In high-dose treatments, seagrass mortality increased over time, 100 % Halophila ovalis and 81 % Halodule wrightii mortality from day zero to 53. Collectively, glyphosate concentrations were negatively correlated with seagrass shoot densities, and positively correlated with water column nutrients (TN and TP). Based on these results we do not attribute local seagrass declines to low-dose glyphosate exposure, i.e., herbicide overspray events. However, we advise caution against improper herbicide handling, since glyphosate remained detectable in relatively high concentrations (<88 mg/L) after 53 days, resulting in seagrass mortality and increased water column nutrients.