Pesticide and methylmercury fluxes to a marine protected region of Australia influenced by agricultural expansion

Estuarine environments provide diverse ecological services, including carbon burial and the sequestration of pollutants. Yet, urban expansion and anthropogenic activities can impact the ability of estuarine systems to retain and store pollutants, with unknown consequences for marine communities. Here, we present dated sediment cores from within the Solitarily Island Marine Park, a marine life sanctuary in Australia subject to runoff from a highly impacted catchment. We reveal historical accumulation rates of trace metals and pesticides likely associated with specific types of agricultural activities, including banana and blueberry production. Propiconazole and tebuconazole, fungicides highly reactive in marine habitats, were recorded in the sediment profile located at the site nearest the freshwater source. Furthermore, mercury content revealed elevated levels in the most recent sediments, up to 0.13 mg kg^-1 in 1999, while methylmercury (MeHg) was detected ranging from 0.1 mg kg^-1 in 2017 to 0.2 mg kg^-1 in more recent sediments. Additionally, arsenic accumulation rates are shown here to have significantly increased from 19.1 mg m^-2 year^-1 in 1930 to 259.6 mg m^-2 year^-1 in 2020. These substances were found to be related to organic matter which was determined to be mostly of terrestrially derived sources, probably related to historical catchment deforestation, as indicated through δ¹³C and C:N molar ratios. This study highlights the importance in monitoring and evaluating agriculture-derived pollutant discharge to protected coastal regions and provides an initial dataset that may be used to monitor pollutant runoff in catchments impacted by expanding agricultural activities throughout Australia, and around the globe.