Sponge Communities in Mesophotic Reefs of the Gulf of Mexico Before and After the Deepwater Horizon Oil Discharge

Mesophotic reefs across the outer continental shelf of the northeastern Gulf of Mexico were examined for possible impacts of the Macondo well’s Deepwater Horizon 2010 oil spill. For weeks following the spill, Alabama Alps Reef and Roughtongue Reef were situated 60-88 m under floating oil, with Alabama Alps closer to the spill and under oil for 20 more days. ROVs surveyed the reefs in 2011, 2014, and from 1997 to 1999. Sponges were present, but they are difficult to identify with taxonomic precision from photographic evidence. The sponges were visually quantified using still images captured from ROV video transects and the average number of sponges per photo for each site was calculated along with morphological forms. Following the spill, the number of sponges at Alabama Alps Reef notably declined while those at Roughtongue Reef increased. Both sites experienced growth by 2014, though Roughtongue Reef’s sponge population increased much more dramatically. Encrusting morphologies overwhelmingly dominated populations until 2014. These changes in morphology reinforce the numerical reduction in the number of sponge individuals and represent probable sublethal impacts of the oil discharge. Predation and disease, among other mortality factors, possibly influenced the changes within the populations.