Corals on oil and gas platforms near the flower garden banks: population characteristics, recruitment, and genetic affinity

Abstract

From the Executive Summary

“ADULT CORAL COMMUNITY STUDY: There are approximately 3,200 oil and gas platforms in the northern Gulf of Mexico (GOM). These platforms provide hard substratum that extends throughout the euphotic zone, in a region where such has been rare in recent geological time. Major exceptions to this are the coral reefs of the Flower Garden Banks (FGB), ~180 km S-SE of Galveston, Texas, which rise up to within 17 m of the sea surface. In this study, we attempted to determine whether extensive scleractinian coral populations have colonized these platforms, quantify them, and determine their population and community characteristics. We also examined the relationship between these variables and distance from the FGB, platform age, and depth. Surveys were performed on 13 oil or gas production platforms down to 33 m depth, encompassing an ellipse around the FGB. Corals occurred in abundance on most of the platforms. Eleven species were found: eight hermatypic scleractinians, two ahermatypic scleractinians, and one hydrozoan coral. The most abundant corals were Madracis decactis, Diploria strigosa, and Tubastraea coccinea. Distance from the FGB was not related to the coral community variables measured, including total coral abundances of the dominant three species in shallow water (< 14 m.), deep water (14-33 m), or both depths combined. Total coral abundance increased significantly with platform age, and the community was best developed on platforms > 12-15 years in age. Abundance of M. decactis increased significantly with platform age in the deeper zone and both depths combined, as did coral species diversity (S). This was not the case with D. strigosa, indicating that it may not be associated with any particular successional sere. Neither was T. coccinea abundance associated with platform age, except in shallow water, where it decreased in abundance with age, indicating that this species may be an opportunistic pioneer species. All corals exhibited a significantly non-uniform depth distribution, with total coral abundance peaking at ~20 m and 28 m depths. The bimodal depth distribution of M. decactis exhibited a disproportionately high peak at depths > 27 m. D. strigosa exhibited peaks at depths of ~10 m and 23 m and was not found at depths > 27 m. T. coccinea exhibited a near-normal distribution, with a single mode at ~17 m depth. Platforms have facilitated expansion of coral populations in the GOM. Such platforms have intrinsic environmental value through the presence of coral populations and this may influence future decisions about their decommissioning (removal).

CORAL SETTLEMENT STUDY: The FGB are two of the very few true coral reefs in the northern GOM. There are other recognized coral reefs in the southern GOM, such as at Tampico, Mexico and the eastern GOM in the Florida Keys. There are also a couple of mesophotic reefs in the northern GOM that boast significant amounts of hermatypic corals and may be considered to be true coral reefs; these include corals at Pulley Ridge in the eastern GOM and McGrail Bank near the shelf edge in the central GOM. Numerous other banks in the northern GOM support significant coral communities that include a variety of corals, sponges, and other reef invertebrates but these are not actively accreting coral reefs. The offshore oil and gas platforms deployed in the northern GOM were the first set of structures in shallow-water in this region since the Holocene, providing substratum for colonization for corals. Because scleractinian corals are sessile epibenthic organisms, the only means of dispersal for them is by larval dispersal and settlement To assess coral recruitment rates on oil and gas platforms around the FGB, terracotta settlement plates were mounted on racks, deployed on, and retrieved from six platforms at depths of 15 and 27 m for a period of one to two years. Plates were analyzed in the laboratory with a dissecting microscope for taxonomic identification, distribution, and abundance.

Density of coral spat settling on plates on the platforms was extraordinarily low when compared with other Caribbean sites or the Great Barrier Reef-averaging < 1/450 cm² -over a period of 10–12 months. This emphasizes the uniqueness and fragility of these artificial reef sites in comparison to natural sites. There was no significant difference between platforms with respect to spat density, suggesting that the distance between the platforms and the E-FGB-their potential larval source, was large enough (> 0.6-1.2 km, as determined by Sammarco and Andrews, 1988) to allow diffusion of larvae, suppressing larval densities. There was a significant difference between coral spat densities on the platforms compared with those directly on the E-FGB (derived from a sister experiment). Only three species of coral spat were found: T. coccinea, Montastraea sp. (most likely Montastraea cavernosa), and Madracis sp. (most likely M. decactis), in order of abundance. This species composition varied substantially from the dominant genera of spat observed on the E-FGB: Agaricia and Porites. The observed recruitment was also unusual in that the dominant recruits matched the dominant adults in the community. Tubastraea and Montastrea spat densities did not vary significantly between platforms or between racks. The platforms in the northern GOM generally exhibit very low levels of coral recruitment for the tropical western Atlantic. Nonetheless, this recruitment has permitted the development of adult coral communities on these platforms and their associated benthic, demersal, and pelagic biota over the past ~30 years. These communities should be considered fragile because of their slow development rate. Mass coral mortality on these platforms would require decades for recovery.

MOLECULAR GENETICS STUDY: At the time of this study, the northern GOM held ~3,600 offshore oil and gas structures. These structures serve as artificial reefs on the continental shelf, where, until their introduction, shallow hard substrata were rare. This newly available substrate has helped to expand scleractinian coral populations in the GOM. Here, we conduct molecular genetic analyses on adult scleractinian corals on the FGB coral reefs (~180 km S-SE of Galveston, Texas [TX]) and on surrounding oil and gas platforms. We have attempted to determine the degree of genetic affinity among the natural populations and those on the surrounding platforms. The three species collected were the most abundant hermatypic scleractinians: M. decactis, D. strigosa, and M. cavernosa. Tissue samples were collected from the E-FGB and W-FGB, and seven platformswithin a 65 km radius, at a depth range of 5-37 m. Genetic variation was assessed using Amplified Fragment Length Polymorphisms (AFLPs). The large number of polymorphic markers generated by AFLPs allowed for the use of standard genetic analysis tools (AMOVA) as well as population allocation techniques (AFLPOP). The AMOVA analyses indicated that the E and W-FGB were genetically homogeneous for M. decactis and D. strigosa populations. M. cavernosa populations, however, were significantly different between banks. In all species, genetic distance (ΦST) increased significantly with geographic distance between populations. In the brooding species M. decactis, this pattern was even stronger when one considered the shortest distance between platforms and the nearest perimeter of the FGB, particularly the nearest FGB, suggesting that the FGB may be a source of larvae for platform populations. The AFLPOP analyses showed that the degree of self-allocation to home sites also increased with inter-site distance. Cross allocations between sites dropped significantly and exponentially in all species within only one to several kilometers of the FGB. M. decactis, a brooder with extended larval release periods and near-immediate settlement competence, shows greater affinity to the FGB with distance than D. strigosa, a broadcaster. This brooder appears to be more effective at colonizing small, nearby target sites and expanding its geographic range at the meso-scale. The low degree of genetic affinity exhibited by all species on the platforms may be attributed to genetic drift/founder effect or relatively small population sizes, although total populations were sampled. In general, genetic affinity decreased with inter-site distance. Young coral populations are highly differentiated at the meso-scale during early stages of community succession, implying that much time and repeated colonization of patchy habitats around larger potential arval sources will be required before genetic equilibrium or homogeneity is reached.”