Pesticide occurrence in a subtropical estuary, Australia: Complementary sampling methods.

Monitoring pesticide run-off in the aquatic environment is ecologically important. Effective methods are required to detect the wide range of possible pesticides that enter estuaries from the surrounding catchment. Here, we investigate the occurrence of pesticides in the Richmond River estuary, Australia, and compare the effectiveness of using oysters and Chemcatcher® passive sampling devices against composite water samples. Samples were collected from six sites during two sampling periods: from January to March 2020 (4 weekly composite water samples and oyster collections) and from February to March 2021 (8 twice weekly composite water samples and Chemcatcher® deployment). Samples were analysed for up to 174 pesticides. A total of 21 pesticides were detected across all sites using all methods. The number of pesticides and mixture of pesticides detected in the 2020 sampling was higher in oyster samples than in water samples. In 2021, Chemcatcher® samplers detected more pesticides than in water samples. Herbicides were the most common in all samples. Insecticides and most fungicides were detected only in oysters and Chemcatcher®. Overall, the use of three complementary sampling approaches demonstrated a high level of pesticide input into the Richmond River estuary, highlighting the usefulness of oysters as biomonitors for some pesticides.