Abstract

Hard-bottom ‘mesophotic’ reefs along the ‘40-fathom’ (73 m) shelf edge in the northern Gulf of Mexico were investigated for potential effects of the Deepwater Horizon (DWH) oil spill from the Macondo well in April 2010. Alabama Alps Reef, Roughtongue Reef, and Yellowtail Reef were near the well, situated 60–88 m below floating oil discharged during the DWH spill for several weeks and subject to dispersant applications. In contrast, Coral Trees Reef and Madison Swanson South Reef were far from the DWH spill site and below the slick for less than a week or not at all, respectively. The reefs were surveyed by ROV in 2010, 2011, and 2014 and compared to similar surveys conducted one and two decades earlier. Large gorgonian octocorals were present at all sites in moderate abundance including Swiftia exserta, Hypnogorgia pendula, Thesea spp., and Placogorgia spp. The gorgonians were assessed for health and condition in a before-after-control-impact (BACI) research design using still images captured from ROV video transects. Injury was modeled as a categorical response to proximity and time using logistic regression. Condition of gorgonians at sites near Macondo well declined significantly post-spill. Before the spill, injury was observed for 4–9 % of large gorgonians. After the spill, injury was observed in 38–50 % of large gorgonians. Odds of injury for sites near Macondo were 10.8 times higher post-spill, but unchanged at far sites. The majority of marked injured colonies in 2011 declined further in condition by 2014. Marked healthy colonies generally remained healthy. Background stresses to corals, including fishing activity, fishing debris, and coral predation, were noted during surveys, but do not appear to account for the decline in condition at study sites near Macondo well.

Highlights

  • Communicated by Ecology Editor Dr Stuart A

  • Alabama Alps Reef, Roughtongue Reef, and Yellowtail Reef were near the well, situated 60–88 m below floating oil discharged during the Deepwater Horizon (DWH) spill for several weeks and subject to dispersant applications

  • This study asks: is there evidence of injury to gorgonian octocorals on mesophotic reefs near DWH? What types of injury were observed and to what degree? How does condition of gorgonians compare between sites near and far? Was there a change in condition to gorgonians before and after the DWH oil spill? We provide this information to assess potential injury to biota in the mesophotic zone relative to the DWH oil spill in the Gulf of Mexico

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Summary

Introduction

Communicated by Ecology Editor Dr Stuart A. The Deepwater Horizon oil spill released 4.3 million barrels of crude oil near 1500 m depth in the Gulf of Mexico over a period of 87 d in the spring of 2010 from the Macondo well MC 252 (McNutt et al 2011). The biological footprint of the DWH spill on infaunal sediments has been calculated as 148 km (Montagna et al 2013), but recent studies indicate that the chemical footprint of impacted areas may be substantially larger than previously anticipated (Valentine et al 2014). There is a potential that the biological footprint of the spill, including injury to corals, could increase with increasing search effort (Fisher et al 2014)

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