Within waterways and drainage systems aquatic plants can provide important ecosystem services, such as habitat for aquatic fauna, regulating flows and cycling nutrients. However, excessive plant growth can negatively affect waterways by increasing sediment deposition, increasing flood risk, reducing habitat diversity for instream fauna and driving large daily fluctuations in dissolved oxygen and pH levels. Applying herbicides to emergent vegetation is a cost-effective control method commonly employed by waterway managers to control vegetation particularly for flood mitigation. However, studies assessing the effects of direct applications of herbicides for instream vegetation management on other components of the ecosystem are limited.
Within an agricultural drain which discharges to Western Port, Victoria, Australia, we conducted a paired before-after, control-impact study to assess if instream herbicide spraying impacted waterway health. A commercially available glyphosate-based herbicide registered for application in waterways was applied on a single occasion during the regular instream vegetation management program. A variety of waterway health indicators including water and sediment chemistry, aquatic ecology and ecotoxicology were monitored over 6 weeks post spraying to assess impacts to waterway health and effectiveness of herbicide application for vegetation management.
No evidence of significant impacts to Physa acuta or the floating plant Spirodella punctata were observed following the spray application, however, changes were observed within macroinvertebrate communities one day post spraying. By six weeks post spraying macroinvertebrate communities had recovered. On the other hand, herbicide application did not result in significant reductions in vegetation cover during the experimental period.
The study suggests that application of a commercial glyphosate formulation for management of instream emergent vegetation doesn’t cause significant impacts to waterway health in the short-term. The effectiveness of herbicides for instream vegetation management, opportunities to increase the effectiveness of vegetation management and risks to waterway health will be discussed.