Deepwater coral distribution and abundance on active offshore oil and gas platforms and decommissioned Rigs-to-Reefs platforms

Abstract

From the Executive Summary:

“Thousands of oil platforms in the northern Gulf of Mexico (GOM) provide hard substratumwhich has not been present in GOM shallow water for thousands of years. These platforms, acting as numerous artificial islands throughout the region, have facilitated the geographic expansion of Caribbean reef coral populations. The U.S. DOI Bureau of Ocean Energy Management (BOEM) has provided for the “reefing” of -platforms as artificial reefs to create additional hard substratum habitat in this region, with the ultimate objective being to promote fisheries development. We assessed the effectiveness of these Rigs-to-Reef structures in promoting coral community development on the toppled platforms in deep water using ROV reconnaissance (max. depth ~110 m). We compared coral community development on five toppled platforms with two active, standing production platforms.

The corals found on the platforms were the hermatypic/zooxanthellate Madracis decactis and several ahermatypic/azooxanthellate: Tubastraea coccinea, Oculina diffusa, and Phyllangia americana. Comparisons of standing and toppled platforms show no significant difference in total coral density. This was due primarily to varying, non-correlated, opposing species-specific changes in abundance between the two types of structures. That is, M. decactis and T. coccinea densities were significantly higher on toppled Rigs-to-Reefs structures. In the latter case, the corals were found on the “bottom” portion of the legs that had been up-ended during the toppling process, presumably having settled and grown there after toppling. On the other hand, P. americana was more abundant on standing platforms than on the toppled ones. The densities of O. diffusa were found to be equivalent on both types of structures. With respect to depth distribution, when all corals were considered together, they were distributed more deeply on standing platforms than on toppled Rigs-to-Reefs structures. This was particularly true in O. diffusa, P. americana, and T. coccinea. There was no significant difference between depth distributions between the two types of structures in M. decactis. All corals of this species were found to occur in depths of < 50 m. This was most likely due to this species being zooxanthellate and requiring light for survival and growth. It is believed that the density reduction of all coral colonies on the Rigs-to-Reefs structures in deeper water may be due to physical disturbance upon removal of the platform and additional disturbance during deployment of the structures at their new site.

The above data suggest that Rigs-to-Reefs structures do function as a substrate for coral settlement. The probability of continued growth of the corals, however, varies by species specifically between structure types. Toppling did not appear to enhance development of hermatypic coral populations, increase coral abundances in general, or create a three-dimensional reef-like habitat (as observed by ROV) which could promote demersal fish community development.”