Effects of oil and dispersants on <i>Swiftia exserta</i>, a structure-forming deep-water gorgonian octocoral from mesophotic reefs in the Gulf of Mexico
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One outcome of the Deepwater Horizon (DWH) oil spill was the realization that no well-established toxicity thresholds exist for benthic taxa in deep water (>50 m). Surveys of mesophotic reefs along the Gulf of Mexico Pinnacle Trend in 2011 showed that large octocoral colonies below the oil slick exhibited significantly more injury than in years before the spill. <i>Swiftia exserta</i>, an octocoral species found throughout the West Atlantic at depths of 20-200 meters, was among the injured taxa. In the Gulf of Mexico <i>S. exserta</i> has white polyps, whereas populations in East Florida have red polyps; however, haplotype frequencies of mitochondrial gene <i>mutS</i> suggest no differences between the two populations. Live fragments of <i>S. exserta</i> from East Florida were exposed to varying concentrations of water-accommodated oil fractions (WAFs), Corexit® 9500 dispersant, and chemically-enhanced WAFs (20:1 oil-dispersant mixtures, aka CEWAFs) to determine the vulnerability of <i>S. exserta</i> octocorals to oil and dispersants. Following 96-hour toxicological assays, dispersant-alone and oil-dispersant mixtures were substantially more detrimental to coral health than any of the WAF concentrations tested. Complete mortality was observed within 48 hours for some fragments in the dispersant-alone (nominal 96h LC50 = 51.17 mg/L) and oil-dispersant (nominal 96h LC50 = 46% CEWAF) treatments, while the WAF and control groups remained relatively unaffected. This suggests that oil alone is less detrimental to octocorals than when it is treated with dispersants. This is the first toxicity threshold established for a mesophotic octocoral species subject to the DWH spill, and provides evidence of octocoral sensitivity to oil and dispersants, which should inform scientists and managers in the event of a future oil spill.