A common inquiry from aquaculture producers and pond owners is what impact a mistaken aerial herbicide application will have on their fish. These calls frequently follow a fish kill which has occurred after a suspected overspray. With few exceptions, the most used row-crop herbicides will be harmless to fish. What is often then asked is could the herbicides have negatively impacted the phytoplankton bloom, causing it to crash, leading to a fish kill due to low dissolved oxygen.
To investigate the potential for an herbicide overspray to negatively impact an algal bloom, extension crop scientists were consulted for a list of the most common aerially applied herbicide active ingredients (AI). A representative label was selected for each AI to determine the maximum amount of chemical that would potentially be applied to the pond, to the depth of one foot (AI ppm/acre*ft). This value was then compared to the EC50 for a representative green algae Raphidocelis subcapitata. These results were then compared with known algicides.
The results of this investigation indicate that some of the most common aerially applied herbicides have the potential to negatively impact planktonic algal blooms. Due to the nature of algae and ponds, while an herbicide overspray might negatively impact algae, this may not necessarily cause a low dissolved oxygen fish kill, but it cannot be categorically dismissed.