Decline in condition of gorgonian octocorals on mesophotic reefs in the northern Gulf of Mexico: before and after the Deepwater Horizon oil spill

Coral Reefs (2016) 35:77–90 DOI 10.1007/s00338-015-1363-2 REPORT Decline in condition of gorgonian octocorals on mesophotic reefs in the northern Gulf of Mexico: before and after the Deepwater Horizon oil spill Peter J. Etnoyer1 • Leslie N. Wickes2 • Mauricio Silva3 • J. D. Dubick2 • Len Balthis1 • Enrique Salgado2 • Ian R. MacDonald3 Received: 6 February 2015 / Accepted: 5 October 2015 / Published online: 20 October 2015 Ó The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Hard-bottom ‘mesophotic’ reefs along the ‘40fathom’ (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 beforeafter-control-impact (BACI) research design using still images captured from ROV video transects. Injury was modeled as a categorical response to proximity and time Communicated by Ecology Editor Dr. Stuart A. Sandin Electronic supplementary material The online version of this article (doi:10.1007/s00338-015-1363-2) contains supplementary material, which is available to authorized users. & Peter J. Etnoyer 1 NOAA Center for Coastal Environmental Health and Biomolecular Research, 219 Fort Johnson Rd., Charleston, SC 29412, USA 2 JHT, Inc., 2710 Discovery Dr., Suite 100, Orlando, FL 32826, USA 3 Florida State University, Tallahassee, FL 32306, USA 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. Keywords Octocoral
Gorgonian
Gulf of Mexico
Mesophotic zone
Oil spill
Health assessment Introduction 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 oil formed a large subsurface plume and produced a large surface slick that was visible by airplane and satellite for nearly 90 d (NOAA 2014). The biological footprint of the DWH spill on infaunal sediments has been calculated as 148 km2 (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). 123 78 The surface oil slick from the DWH blowout persisted in the northern Gulf of Mexico for several weeks over a series of deep-water rocky reefs called the Pinnacle Trend (Fig. 1). Fishermen refer to these as the ‘broken grounds’ (Gittings et al. 1992). The rocky plateaus occur along the 60- to 90-m depth contours between the Mississippi Delta and Pensacola, FL. Recreational and commercial fishermen recognize these ‘40-fathom’ reefs as good fishing habitat for snapper and grouper, conferring social and economic value on these natural resources (Prytherch 1983; ScottDenton et al. 2011). There are nine named reefs in the Pinnacle Trend (Gardner et al. 2000). Among the largest of these are Alabama Alps Reef (AAR), Roughtongue Reef (RTR), and Yellowtail Reef (YTR), which were situated below the spill for 19–39 d (NOAA 2014). AAR (39 d), RTR (19 d), and YTR (19 d) are 57–109 km from the well. The reefs have been surveyed by remotely operated vehicle (ROV) since 1989. Two other reefs about 200 km to the east of the Macondo wellhead, Madison Swanson South Reef and Coral Trees Reefs, were below the slick for 0–3 d, and surveyed by ROV since 1997. Gorgonian octocorals (sea fans), black corals, and scleractinian corals occur on all of these reefs, providing structure and refuge for small demersal fishes that are prey to larger fishes (Weaver et al. 2002). The corals contribute substantially to biodiversity in high relief areas on the reefs (Gittings et al. 1992). Suspension feeding corals are vulnerable to oil and dispersants (NOAA 2010) so these animals were considered as part of the Natural Resource Damage Assessment (NRDA) for DWH. Injury to gorgonian octocorals has been demonstrated adjacent to the wellhead, and this injury has been linked to DWH (White et al. 2012) for sites up to 22 km from the Macondo well (Fisher et al. 2014). Additional findings have linked Fig. 1 A map showing the surface oil slick generated by the Deepwater Horizon blowout at Macondo wellhead (gray triangle), overlaid with the location of four mesophotic reefs assessed in this study (yellow circles)—Alabama Alps Reef (AAR), Roughtongue Reef (RTR), Coral Trees Reef (CTR), and Madison Swanson South Reef (MSSR). Yellowtail Reef is adjacent to RTR 123 Coral Reefs (2016) 35:77–90 numerous injured coral colonies observed in the Pinnacle Trend to exposure to DWH oil (Silva et al. 2015). Deep-water gorgonian octocorals (Alcyonacea) and black corals (Antipatharia) are common and conspicuous in the 60–90 m ‘mesophotic’ depth range in the northern Gulf of Mexico, from Texas to Florida (Bayer 1961; Rezak et al. 1985; Cairns and Bayer 2009). The gorgonians are important components of deep-reef habitat because they provide structural complexity, substrate, and refuge for fish and invertebrates (Krieger and Wing 2002; Buhl-Mortensen and Mortensen 2005). The soft coral colonies grow on rocky, current-swept habitat, and prefer high relief features. Gorgonian octocorals on the Pinnacle Trend reefs are predominantly heterotrophic suspension feeders. Their branches grow in a flabellate, reticulated (sea fan) shape with an orientation perpendicular to prevailing currents in order to maximize particle flux and rates of capture for the polyps along the br (...truncated)