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Long-term history of chemosynthetic molluscan assemblages at Gulf of Mexico hydrocarbons seep sites
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Deep-sea chemosynthetic communities living on the Gulf of Mexico continental slope off Louisiana in areas of hydrocarbon seepage were discovered less than ten years ago. These communities are dominated by large, chemosynthetic bivalves and tubeworms found in high densities. The dominant species include mytilid mussels and thyasirid, vesicomyid and lucinid clams. The mussels, Bathymodiolus sp., contain methanotrophic symbionts, whereas, the clams, Thyasira oleophila, Lucinoma sp., L. atlantis, Vesicomya cordata, and Calyptogena ponderosa, harbor sulfur-oxidizing symbionts. Seep assemblages from three sites, GB-386, GB-425, and GC-234, were sampled by piston core, in order to determine the long-term history of these assemblages from their preserved death assemblages in the sediments. Shell material found in the cores was highly fragmented. A computer program was developed in order to reconstruct the original population from a set of fragments. The program was designed to minimize the amount of time necessary to analyze each fragment, by using readily measured parameters, such as shell length, width, weight, and thickness, in the reconstruction. To test the program, four species of bivalves, with similar shell size and shape to the dominant chemosynthetic species, were chosen. Overall, numerical abundance and individual shell size were reconstructed with reasonable accuracy. The program was then applied to the mussel, thyasirid, and lucinid fragments found in the piston cores. The method of Powell and Stanton (1985) was used to estimate the amount of paleoproduction (biomass) and paleoingestion (calories) contained in the cores. Downcore distributions of species composition, paleoproduction, paleoingestion, tier and guild structure, and size-frequency distributions were analyzed to give insight into each seep sites' long-term history. Mussels were the dominant species in the four GC-234 cores, while thyasirids were dominant in the two GB-425 cores. Lucinids were only dominant in the single GB-386 core. Chemosynthetic assemblages were not very common in the sedimentary record. Overall, the assemblages were only present 21.6% of the time. The absence of chemosynthetic assemblages was due to either poor preservation of the previously living populations, or environmental conditions precluded juveniles from settling at these sites. Habitat optimality varied between near optimal and sub-optimal for the mussels and thyasirids, whereas it was always sub-optimal for lucinids. Species composition and tier and guild structure rarely changed significantly over the length of the cores. Individual seep sites are able to support large populations of a single chemosynthetic species over a period of several thousand years. Replacement of one chemosynthetic species by a significant population of another was not observed in the cores, but replacement of a chemosynthetic species by a non-chemosynthetic species was common. Over a period of several hundred years, chemosynthetic fauna are relatively persistent at a seep site. Persistence on a scale of 500 to 1,000 years was less common. Local extinction followed by recolonization of the site by the same species was commonly observed on this scale. This suggests that the chemosynthetic fauna of the Gulf of Mexico hydrocarbon seeps are resilient over geologically long time spans, and that disturbances will not have a geologically long-term effect on seep assemblages, although they may have on time scales of human life spans.
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Warren, Kenneth Anderson (1995). Long-term history of chemosynthetic molluscan assemblages at Gulf of Mexico hydrocarbons seep sites. Master's thesis, Texas A&M University. Available electronically from
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