Herbicide Persistence in Seawater Simulation Experiments

Abstract

Herbicides are detected year-round in marine waters, including those of the World Heritage listed Great Barrier Reef (GBR). The few previous studies that have investigated herbicide persistence in seawater generally reported half-lives in the order of months, and several studies were too short to detect significant degradation. Here we investigated the persistence of eight herbicides commonly detected in the GBR or its catchments in standard OECD simulation flask experiments, but with the aim to mimic natural conditions similar to those found on the GBR (i.e., relatively low herbicide concentrations, typical temperatures, light and microbial communities). Very little degradation was recorded over the standard 60 d period (Experiment 1) so a second experiment was extended to 365 d. Half-lives of PSII herbicides ametryn, atrazine, diuron, hexazinone and tebuthiuron were consistently greater than a year, indicating high persistence. The detection of atrazine and diuron metabolites and longer persistence in mercuric chloride-treated seawater confirmed that biodegradation contributed to the breakdown of herbicides. The shortest half-life recorded was 88 d for growth-regulating herbicide 2,4-D at 31°C in the dark, while the fatty acid-inhibitor metolachlor exhibited a minimum half-life of 281 d. The presence of moderate light and elevated temperatures affected the persistence of most of the herbicides; however, the scale and direction of the differences were not predictable and were likely due to changes in microbial community composition. The persistence estimates here represent some of the first appropriate data for application in risk assessments for herbicide exposure in tropical marine systems. The long persistence of herbicides identified in the present study helps explain detection of herbicides in nearshore waters of the GBR year round. Little degradation of these herbicides would be expected during the wet season with runoff and associated flood plumes transporting a high proportion of the original herbicide from rivers into the GBR lagoon.

Degradation of Herbicides in the Tropical Marine Environment: Influence of Light and Sediment

“…The persistence of contaminants in the environment is governed by the rates of multiple processes including hydrolysis, light/ UV driven photolysis and microbial degradation (metabolism) [21–23]. Microbial degradation is considered the dominant route of degradation for most PSII herbicides in aquatic systems [22, 24, 25] and our recent standard flask study indicates very slow degradation of diuron, atrazine, hexazinone and tebuthiuron in seawater, with evidence of both hydrolysis and microbial metabolism of these herbicides [26]. …”

“…There is a relatively large number of in situ and laboratory studies on the persistence of PSII herbicides in freshwater aquatic systems (see [27]); however, few laboratory studies have examined persistence in seawater (summarised in [26]). The long seawater persistence of PSII herbicides observed in standard flask studies (half-lives >500 d) [26] indicates slower degradation than in freshwater, and may be influenced by a range of factors including salinity, pH and alternative organic carbon food sources and concentrations of other nutrients, which in turn affect microbial degradation and hydrolysis.…”

“…The long seawater persistence of PSII herbicides observed in standard flask studies (half-lives >500 d) [26] indicates slower degradation than in freshwater, and may be influenced by a range of factors including salinity, pH and alternative organic carbon food sources and concentrations of other nutrients, which in turn affect microbial degradation and hydrolysis. Most previous studies on PSII herbicide persistence in seawater were not conducted for long enough to calculate reliable estimates of half-lives and several of these studies applied very high initial concentrations of herbicides (e.g.…”

“…Most previous studies on PSII herbicide persistence in seawater were not conducted for long enough to calculate reliable estimates of half-lives and several of these studies applied very high initial concentrations of herbicides (e.g. 5000 μg l -1 ) which may affect degradation rates by artificially influencing natural microbial communities and/or may be toxic to some components of the communities [26]. Our previous degradation studies applied the most natural conditions practical in a standard flask environment [26, 28], including low contaminant concentrations (~10 μg l -1 ), native microbial communities, no artificial nutrients and tests were conducted over 12 months under different light and temperature conditions [26, 29].…”

“…5000 μg l -1 ) which may affect degradation rates by artificially influencing natural microbial communities and/or may be toxic to some components of the communities [26]. Our previous degradation studies applied the most natural conditions practical in a standard flask environment [26, 28], including low contaminant concentrations (~10 μg l -1 ), native microbial communities, no artificial nutrients and tests were conducted over 12 months under different light and temperature conditions [26, 29]. The application of low light increased the degradation rates of diuron, tebuthiuron and the non-PSII herbicide glyphosate, and slowed the degradation of the non-PSII herbicide 2,4-D [26, 29].…”