Probabilistic risk assessment of diuron and prometryn in the Gwydir River catchment, Australia, with the input of a novel bioassay based on algal growth

• Diuron caused more toxicity to Chlorella as judged by growth rates than prometryn.
• ‘Hotspot’ for diuron and prometryn risk was identified in the Gwydir catchment.
• Microplate assays coupled with PRA are useful for rapid assessment of ecotoxicity.

A probabilistic risk assessment of the selected herbicides (diuron and prometryn) in the Gwydir River catchment was conducted, with the input of the EC50 values derived from both literature and a novel bioassay. Laboratory test based on growth of algae exposed to herbicides assayed with a microplate reader was used to examine the toxicity of diuron and prometryn on the growth of Chlorella vulgaris. Both herbicides showed concentration dependent toxicity in inhibiting the growth of Chlorella during the exposure period of 18–72 h. Diuron caused more toxicity as judged by growth rates than prometryn. Thalaba Creek at Merrywinebone was identified as the ‘hotspot’ for diuron and prometryn risk in the Gwydir catchment. The use of microplate assays coupled with probabilistic risk assessment is recommended for rapid assessment of ecotoxicity of indigenous species, allowing identification of locations in river catchments requiring environmental management.