Runoff containing pesticides from agricultural land is recognised as a potential threat to inshore habitats of the Great Barrier Reef (GBR), with the photosystem II (PS) herbicides, including Diuron and Atrazine being the most frequently detected pesticides marine waters of the GBR. These herbicides act by inhibiting photosyntheis in terrestrial weeds and can impact non-target marine species including corals and seagrass in the same way. The potential for pesticides to move from paddock to the reef depends on their individual properties, including their persistence and solubility. Understanding the persistence of herbicides is critical to better parameterize and validate predictive risk models and to better inform the management of application methods and rates of pesticides in the GBR catchments to reduce runoff.
This was the first study to quantify the seawater persistence of priority herbicides detected in the GBR lagoon. Standard Flask Experiment 1A revealed extremely slow degradation of Diuron, Atrazine, Hexazinone, Tebuthiuron, Simazine, and Ametryn in the absence of light and sediments, with all herbicides exhibiting half-lives of between 657 and 1016 days. Experiment 2 was conducted in outdoor ponds with unfiltered coastal water and treatments included the presence and absence of light and sediments. These conditions more closely mimic coastal habitats in the GBR lagoon than flask experiments in incubators. Under the most environmentally relevant conditions (light + sediments) the order of persistence was Metolachlor (half-life, t½ = 32 d) < Atrazine (107 d) < Diuron (139 d) < Hexazinone (201 d) < 2,4-D (288 d) < Tebuthiuron (944 d). The presence of coastal sediments consistently reduced the persistence (1.1 – 6.3-fold) of herbicides, especially in moderate sunlight. Moderate sunlight also reduced the persistence of Diuron, Atrazine, Hexazinone, Tebuthiuron and Metolachlor in the presence of sediments. 2,4-D was a notable exception and was far more persistent in the light. Changes in persistence were most likely due to the influence of light and sediments on microbial populations.
The long persistence of herbicides in seawater in the pond Experiment 2, demonstrates that negligible herbicide degradation is likely in flood plumes lasting for weeks and that herbicides reaching the GBR lagoon may persist there for many months to years. These results help explain the year-round detection of herbicides in the GBR lagoon. This reliable persistence data for priority herbicides detected in the GBR lagoon is now available to managers, regulators and industry to enable (in combination with exposure and toxicity data) more effective and targeted management of these agricultural contaminants and develop more accurate spatial risk assessment models for coral reefs and seagrass beds of the GBR. Further work is needed to quantify the fate of dozens of other herbicides, fungicides and insecticides, as well urban and industrial contaminants detected in the GBR lagoon.