Macrophytes are an important component in freshwater systems, providing a range of ecosystem services. However, invasive alien aquatic plants cause significant socio-economic and environmental impacts. This is especially a problem in urban catchments due to the proximity of human vectors that aid dispersal of alien species and the environmental degradation that promotes nuisance growth of aquatic vegetation. Management of invasive aquatic plants often risks significant damage to native vegetation, potentially affecting the ecological integrity of freshwater systems. While aquatic plants differ widely in their response to herbicides, information on sensitivity of native macrophytes to herbicides is sparse. This hampers management of invasive macrophytes as it is difficult to predict unintended effects on native vegetation.
We evaluated the sensitivity of common native Australian macrophytes to two herbicides registered for use in freshwater, flumioxazin and florpyrauxifen-benzyl, in outdoor mesocosms and subsequently in field trials in lakes and wetlands. We found that sensitivity of the tested wetland plants differed substantially between plant species and the two herbicides. For example, native reed species that typically grow along the shoreline were mostly resistant to the two tested herbicides while target species were highly sensitive to the chemicals. The native floating attached macrophyte, water-snowflake, was hardly affected by flumioxazin, but highly sensitive to florpyrauxifen-benzyl. With this background knowledge, we were able to selectively remove invasive macrophytes from lakes and wetlands, while leaving the native vegetation intact. The removal of invasive macrophytes also triggered germination of native macrophytes from the natural seed bank and assisted restoration of the native plant community.
Our work shows that herbicides, when carefully selected and at appropriate doses, can be highly specific in removing invasive target species while protecting native macrophytes. Also, native species that are resistant to aquatic herbicides should be selected for future wetland restoration projects as this will simplify future invasive species management. Lastly, invasive macrophytes such as salvinia, often find refuge among emergent shoreline vegetation, necessitating ongoing control efforts. Selecting the correct herbicide will enable targeting remnant invasives hiding in shoreline vegetation to be targeted without sacrificing native vegetation.