Fate of the Mesophotic Coral Ecosystem (MCE) in the Northeastern Gulf of Mexico after the Deepwater Horizon Incident

The Gulf of Mexico, one of the most geomorphologically complex oceanic basin, is also one of the most prolific hydrocarbon reserves in the world. It hosts a varied range of marine communities from shallow reefs and estuarine communities in coastal areas, to deep-sea and chemosynthetic communities in greater depths. In some particular areas in water depth between 50 to 120 m are hosted the mesophotic reef ecosystem, also known as the twilight communities, where sunlight is almost extinguished but still enough to support some photosynthesis. Mesophotic ecosystems are mainly characterized by the presence of both light dependent and independent corals, which are the bioengineering support from a vast variety of invertebrate and vertebrate that compound the mesophotic coral ecosystem. One of these mesophotic coral ecosystem, the Pinnacle Reef Trend, was under the influence of floating oil after the Deepwater Horizon incident released more than 4.1 million barrels of crude oil and 500 T of gas to the environment. Visible injuries in over 400 octocoral and antipatharian colonies were quantified in the aftermath of the DWH oil discharge. Observations were made in September 2011 at water depths of about 65 to 75 m in the Pinnacle Reefs area offshore of Mississippi and Alabama, Gulf of Mexico, using a digital macro camera deployed from an ROV to examine the coral populations of two principal sites: Alabama Alps Reef (AAR) and Roughtongue Reef (RTR). Observed taxa (identifications provisional), listed in order of injury frequency, included the following: Hypnogorgia pendula (Duchassaing & Michelotti, 1864), Bebryce spp., Thesea nivea (Deichmann, 1936), Swiftia exserta (Ellis & Solander, 1786), Antipathes atlantica (Gray, 1857), Stichopathes sp., and Ellisella barbadensis (Duchassaing & Michelotti, 1864). The most conspicuous injuries observed were a biofilm, often with a clumped or flake-like appearance, that covered sea-fan branches. Extreme injuries were characterized by bare skeleton, broken, and missing branches. Comparing the 2011 results to previous photo surveys in the same study sites between 1997 and 1999, we found, in 2011, significantly elevated occurrences of injury covering more than 10% of colony area among taxa with growth forms > 0.5 m. We hypothesize that Tropical Storm Bonnie facilitated and accelerated the mixing process of dispersant-treated hydrocarbons into the water column, resulting in harmful contact with coral colonies at mesophotic depths. Analysis of sediment PAH concentrations at AAR and RTR found levels elevated above pre-DWH discharge values, but orders of magnitude below toxicity thresholds established for fauna in estuarine sediments. The PAH concentrations measured in octocoral and echinoderm tissue samples from AAR and RTR were low compared to detection thresholds (10 - 100 ppb). Our findings indicate that coral injuries observed in 2011 resulted from an acute, isolated event rather than ongoing natural processes. Maximum Entropy Modeling was applied to predict the spatial extent of mesophotic azooxanthellate octocorals and antipatharians within the mesophotic area located between Mississippi (Pinnacle Trend Area) and the mid continental shelf and upper slope of Florida, eastern Gulf of Mexico. Habitat prediction models were generated using geo-referenced, coral-presence records obtained by compiling photographic samples with co-located geophysical data, oceanographic variables, and atmospheric variables. Resulting models were used to predict the extent of suitable habitat in the study area. An independent set of presence-records was used to test the model performance. Results (general and by taxon) predict that suitable areas for MCE exceed 400 km2, which occur along carbonate mounds and paleo-shoreline ridges (hard substrata and high ruggedness) with lower amounts of fine sediments and surrounding waters rich in CDOM and upwelling currents (w). The model results significantly exceeded (>0.5) random output and predicted that ruggedness and CDOM are the most important variables associated with coral habitat. Areas of hard substrate within the study area that were not identified as coral habitat by the model suggest that mesophotic sea fans and sea whips depend as much on the chemical and physical conditions (e.g. currents that transport oxygen and food) as on hard substrata for settlement. Finally, three submarine channels that incise into the continental slope in Northeastern Gulf of Mexico have been identified and described. The slope-source channels, seemingly formed after a mass wasting events, are 65 km long and 2.5 km width on average. They show a transition of their transversal profile from V-shape in the head to U-shape in the toe, high sinuosity in the upper ¾ of the channel, showing accumulation of sediments in the turn-sections of the channels, and accumulation of sediment on the walls due to collapsing of unconsolidated sediments. Sedimentary facies obtained by sediment cores show the evidence of disturbed interval of sedimentation and erosion, with erratic periods of laminar sedimentation, erosion, turbidities, disturbed by coarse fractions of sediment. C14 dating of sediments cores from the inside of the channel suggest that hydrodynamic erosion is continuously occurring inside the channel, exposing older sediments to the surface. However, we do not discard the possibility that mass wasting processes are still occurring in the area. The presence of coarse sediment fractions typically found in the continental shelf, beside the occurrence of turbidite homogenous fractions demonstrate that sediment wasting from the upper slope could be still occur. This could lead to a potential disaster in the area because of the presence of oil industry structures. Massive turbidity flows and submarine slumps have been described to occur in this area and other places