Herbicide persistence and toxicity in the tropical marine environment

Abstract

Agricultural herbicides are ubiquitous in the nearshore water bodies of the World Heritage listed Great Barrier Reef (GBR). The main transport mechanism for herbicides is via offsite migration during intense monsoonal rainfall events of the wet season. However, some herbicides can be detected year round, indicating potentially long persistence. Despite the high mobility and potential toxicity of herbicides, there is very little information on the degradation of herbicides in the marine environment. This study was comprised of 4 major components: (i) flask experiments to test herbicide degradation over 60 days; (ii) an extended flask experiment for 365 days including light and temperature treatments; (iii) an outdoor open tank experiment for 365 days with light and sediment treatments; and (iv) experiments to examine the potential toxicity of transformation products. The flask degradation experiments were designed to mimic natural conditions including low herbicide concentrations, relevant temperatures and light and the inclusion of natural microbial communities. Very little degradation was recorded over the standard 60 d period (Experiment 1). The second experiment over 365 d revealed half-lives of PSII herbicides ametryn, atrazine, diuron, hexazinone and tebuthiuron were consistently greater than a year. 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-lives were recorded for non-PSII herbicides: 47 d for glyphosate; 88 d for 2,4-D and 281 d for metolachlor. The presence of low 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 environmental relevance of the degradation experiments were further improved in non-standard 365 d long experiments conducted in large open tanks, under different light scenarios and in the presence and absence of coastal sediments. All PSII herbicides were persistent under control conditions (dark, no sediments) with half-lives of 300 d for atrazine, 499 d diuron, 1994 d hexazinone, 1766 d tebuthiuron, while the non-PSII herbicides were less persistent 147 d metolachlor and 59 d for 2,4-D. The degradation of all herbicides was 2 – 10 fold more rapid in the presence of a diurnal light cycle and coastal sediments; whereas, 2,4-D degraded more slowly in the presence of light. Despite the more rapid degradation observed for most herbicides in the presence of light and sediments, the half-lives remained greater than 100 d for the photosystem II herbicides. Finally, to assess the potential contribution of toxicity by transformation products of four priority PSII herbicides, I directly compared the acute toxicity of partially degraded herbicides (including transformation products) with their parent herbicide to: (i) coral symbionts (Symbiodinium sp.) and (ii) the green algae Dunaliella sp. (iii) prawn (Penaeus monodon) larvae. Concentration dependent effects on photosynthetic efficiency (∆F/Fm’) by all parent herbicides was observed in both phototrophic test species. The toxicity in all degraded solutions could be accounted for by the measured concentrations of the parent compound apart from diuron. The increased potency of the degraded diuron solution may be due to transformation products; however, it was unlikely to be caused by the most well-known breakdown product 3,4-DCA, which did not inhibit ∆F/Fm’ in Symbiodinium sp. at concentrations detected in the degraded mixture. Parent herbicides affected prawn larval metamorphosis only at unrealistically high concentrations (≥ 1000 µg l-1). In contrast, larval prawn metamorphosis was sensitive to the transformation products of atrazine, with both DIA and DEA, significantly inhibiting metamorphosis at 3.5 and 3.8 µg l-1, respectively. These transformation products may have contributed to inhibition observed in the degraded atrazine mixtures. 3,4-DCA caused significant inhibition in metamorphosis at 188 µg l-1 and was likewise more toxic to prawn larvae than its parent herbicide diuron. Reliable data on the persistence of chemicals is important for the development of fate models applied to the spatial and seasonal distribution of these chemicals and associated the much needed risk assessments for priority herbicides found in the GBR. The experiments found that: (i) PSII herbicides were more persistent in seawater than common non-PSII herbicides in the presence of natural microbial communities; (ii) environmental conditions including temperature, light and sediments has strong influences on degradation rates, probably due to differences in microbial communities and (iii) some but not all degradation mixtures can be more toxic to phototrophic and non-phototrophic species than expected from the contributions of the parent herbicides. These experiments generated some of the most relevant and reliable data available on the persistence of high priority herbicides but more information is needed to improve our understanding of the potential persistence of emerging pesticides. The potential contribution of herbicide transformation products to total toxicity highlights the need to test the sensitivity of a broader range of non-target species that may be exposed in natural waters, including the GBR.