Macrofaunal community structure on the gulf of mexico continental slope: the role of disturbance and habitat heterogeneity at local and regional scales
Ammons, Archie Wood
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The ecological forces that drive community structure of deep-sea benthic communities are poorly understood, yet such communities rival in biological complexity those of coral reefs or rainforests. Using components of the recently concluded DGoMB project, local and regional-scale structure of benthic macrofaunal communities were examined at thirty two locations throughout the continental slope of the northern Gulf of Mexico. Controlling factors associated with sediment disturbance, food supply, and faunal competition between functional ecological groups were evaluated for correlative and relational patterns. A higher order taxonomic sufficiency approach was used to calculate both alpha and beta diversity. The results of this study indicate that macrofaunal communities are very patchy, having wide variations in abundance at within-site, adjacent-site, and across-basin scales, yet all sample areas possess a large richness of higher taxa. Declining abundance was noted with increasing water depth and reduced particulate organic carbon levels. Upper-slope submarine canyons possess some of the highest abundances. Less mobile macrofauna, such as poriferans, bivalves, and scaphopods, dominate slope communities above the 500 meter contour. Sediments exhibiting intense megafaunal bioturbation inhibit abundances of sedentary macrofaunal taxa, but such mixing is positively associated with increased abundances of polychaetes and ambulatory crustaceans, including peracarids, harpacticoids, and ostracods. Prominent sediment mixing was noted at most sites, including portions of the Sigsbee Abyssal Plain. The western Gulf of Mexico was less biologically active than the eastern Gulf of Mexico, which possesses two extensive submarine canyons that appear to act as regional nutrient traps. I conclude that the physiographic complexity of the northern Gulf of Mexico continental slope influences macrofaunal community structure. Biological disturbance, in the form of sediment mixing, is widespread throughout most slope depths, and the benthic environment is food-limited. It appears that disequilibrium-type ecological processes predominate in this area, supporting similar findings by previous studies in other regions of the ocean, usually at far smaller scales and none representative at the basin-level. Use of higher order taxonomy in lieu of genus or species-level faunal identifications for diversity measurements was inadequate for detecting spatial patterns or environmental responses.