Geographic expansion of hermatypic and ahermatypic corals in the Gulf of Mexico, and implications for dispersal and recruitment

The {\textgreater}3000 oil/gas platforms in the northern Gulf of Mexico (GOM) provide shallow hard-substratum in a soft-bottom dominated ecosystem. Excepting the Flower Garden Banks (FGB) and deeper mesophotic banks, no shallow hard substrate has been available offshore since the Holocene. Platforms have facilitated coral geographic expansion in this region. We determined the distribution, abundance, and species diversity patterns on 42 platforms in this region, at {\textless}= 37 m depth, along four cross-shelf transects: 1) S-SE from Corpus Christi, Texas; 2) S from Lake Sabine, Texas; 3) S from Terrebonne Bay, Louisiana; and 4) S-SW from Mobile, Alabama. Nine hermatypic, two ahermatypic, and one hydrozoan coral were found: hermatypes-Madracis decactis. Diploria strigosa, Montastraea cavernosa, Porites astreoides, Madracis formosa, Colpophyllia natans, Stephanocoenia intercepta, Stephanocoenia michelinii, and Millepora alcicornis (Hydrozoa); ahermatypes-Tubastraea cocci flea. Oculina diffusa, and Phyllangia americana. Pattern-seeking analyses identified four community types: one with no hermatypic corals (inner and mid-shelf); one high diversity set of platforms, dominated by abundant Madracis decactis, occurring with four other species per platform (outer shelf); and a third dominated by Madracis decactis, in low abundance, co-dominated by D. strigosa and Millepora alcicornis (outer shelf). Species diversity was highest around the Flower Garden Banks. Hermatypic density and D. strigosa (broadcaster) in particular, was maximum around the FGB, extending northwards. Madracis decactis (brooder) densities peaked at the shelf edge, off Terrebonne Bay, Louisiana. Montastraea cavernosa densities were bimodal, peaking near the FGB and east of the Mississippi River. The FGB appear to be the source of hermatypic corals for most platform populations. Differences between the distribution patterns of Madracis decactis and D. strigosa, Montastraea cavernosa, and O. diffusa suggest that the brooder effects longer recruitment distances by averaging larval dispersal over a broad range of hydrographic conditions throughout the year, while the broadcaster utilizes only one set of conditions over the same period. Averaging respective dispersal strategies over decades may have produced the observed disparate distribution patterns. Ahermatypic coral density exceeded hermatypic density by 10,000 fold, peaking south of Terrebonne Bay, Louisiana and Mobile, Alabama, due to T. coccinea. Ahermatypic diversity was highest off Matagorda Island, Texas, decreasing from west to east. Multi-variate pattern-seeking analysis (PATN) identified four ahermatypic community types: one dominated by O. diffusa and T. coccinea in low abundances (shelf-wide distribution); one characterized by the absence of ahermatypic corals (generally near-shore); one heavily dominated by T. coccinea, and also by O. diffusa in low abundances (mid-shelf to shelf edge); and one dominated by T. coccinea, co-dominated by both P. americana and O. diffusa in low abundances (shelf-edge). T. coccinea and P. americana appear to be derived from the southern GOM off Mexico or the Caribbean. Unlike shrinking coral populations in other parts of the world, corals have expanded their range substantially in the northern Gulf of Mexico, in areas previously devoid or near devoid of corals, facilitated by thousands of oil/gas platforms deployed for decades throughout the region. It is also possible that coral populations on these platforms may act as potential larval sources for the FGB in the event of a mass coral mortality there. (C) 2012 Elsevier B.V. All rights reserved.