Abstract
Oil spill responders require information on the absolute and relative toxicities of chemical dispersants to relevant receptor species to assess their use in spill response. However, little toxicity data are available for tropical marine species including reef-building corals. In this study, we experimentally assessed the sub-lethal toxicity of five dispersants to larvae of the coral Acropora millepora over three short exposure periods (2, 6 and 24 h) reflecting real-world spill response scenario durations. Inhibition of larval settlement increased rapidly between 2 and 6 h, and was highest at 24 h: EC50 Corexit EC9500A = 4.0 mg l−1; Ardrox 6120 = 4.0 mg l−1; Slickgone LTSW = 2.6 mg L−1; Slickgone NS = 11.1 mg L−1 and Finasol OSR52 = 3.4 mg L−1. Coral larvae were more sensitive to dispersants than most other coral life stages and marine taxa, but the toxic thresholds (EC10s) exceeded most realistic environmental dispersant concentrations. Estimating toxic threshold values for effects of dispersants on coral should benefit the decision-making of oil spill responders by contributing to the development of species sensitivity distributions (SSDs) for dispersant toxicity, and by informing net environmental benefit assessment (NEBA) for dispersant use.
Highlights
Oil spill responders require information on the absolute and relative toxicities of chemical dispersants to relevant receptor species to assess their use in spill response
Coral larval settlement was sensitive to acute exposures to all five individual oil spill dispersants formulations, with the threshold concentrations for toxicity (EC10s) all relatively low in comparison with other coral life stages and marine taxa
The toxicity of each dispersant increased with exposure duration until 6 h, indicating rapid uptake of the toxic components of the formulations and this could be related to the small size and high surface area to volume of the larvae
Summary
Oil spill responders require information on the absolute and relative toxicities of chemical dispersants to relevant receptor species to assess their use in spill response. The greatest application of dispersants to date was in response to the 2010 Deepwater Horizon wellhead blowout in the Gulf of Mexico, where almost 7 million litres of Corexit EC9500A and Corexit EC9527A were used to disperse slicks both at the surface and by injection at the wellhead, 1500 m below the surface[4] The influence of this response on the ecological outcomes for shoreline and deep water ecosystems is debated[3,5], but clearly more studies are needed to test the responses of sensitive biota to oil, dispersed oil and dispersants[6,7]. It is generally recommended to use dispersants when: (i) the oil is physically and chemically “dispersible” (soon after a spill to maximise efficacy); (ii) the environmental conditions (i.e. depth, currents, and wave energy for mixing) are conducive and (iii) when the application is likely to reduce impacts to surface and shore species, taking into account any subsequent impacts to water column and benthic habitats[1]. If chemical dispersal of a spill is considered to provide a net environmental benefit, the comparative efficacy, cost and potential toxicity of dispersants to sensitive “receptor” species should be considered[19]
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