Spatial and Temporal Patterns of Benthic Biodiversity in the Gulf of Mexico

Despite being a known benthic biological hotspot and environmental ecotone, the DeSoto Canyon has hardly been characterized for its benthic diversity and community structure. Moreover, it is a known deposition and impact zone due to hydrocarbon pollution from the Deepwater Horizon oil spill (DwH). As part of a project to evaluate the impact of the DwH and characterize the spatial and temporal ecological patterns, macrofauna (seafloor-inhabiting organisms 300 - 500 μm) were sampled within the canyon and on the adjacent slope. Five stations within the canyon, sampled 2012 – 2014, and two stations on the adjacent open slope sampled between 2013 – 2014, were used for analysis of interannual dynamics, and temporal variability in the face of the DwH. Elevated abundance was observed at the start of the time-series for overall macrofauna and deposit feeder abundance. However, diversity metrics showed no difference within stations among timepoints. Community and feeding guild structure varied by station, as expected, but showed no statistical difference among time points within a station. Cluster analyses of structure showed grouping more by station than by time point, but some temporal variability was noted in temporal trajectory overlays. Metrics designed for measuring oil contamination impact and overall community stress including the benthic polychaete/amphipod ratio, feeding guild abundance, macrofaunal indicators designed from the DwH, and community dispersion, generally exhibited a paucity of evidence of impact, both yearly and with site-to-site comparisons. This suggests low to moderate levels of impact in the canyon consistent with the low deposition of hydrocarbons in the canyon documented by previous studies, timing of sampling, and quick recovery of canyon foraminifera. Taken together these results suggest relatively low levels of temporal variability within the DeSoto Canyon, with little evidence of the influence of oil on these sites within the studied time range. To examine spatial variability, macrofauna from 13 stations along the canyon wall and axis, and on the adjacent slope, were sampled along with sediment, terrain, and water mass parameters. Within the canyon, abundance and species richness decreased with depth, while evenness increased. Cluster analysis identified three depth-related groups: stations at 464 – 485 m, 669 – 1834 m, and > 2000 m, within the canyon that conformed to previously established bathymetric boundaries for the broader Gulf of Mexico. Distance-based linear modeling (DISTLM) identified community structure within the canyon most related to water parameters, especially fluorometry and oxygen saturation, with some models also including sediment organic carbon or topographical slope. Canyon wall abundances were higher than those of the canyon axis (center) or adjacent slope. Community structure differed between all three habitat types. Ordination of community structure suggests a longitudinal pattern that potentially tracks with increasing sea-surface chlorophyll that occurs in the eastward direction across the northern GOM. As indicated by univariate tests and Canyon and slope differences may result from seasonal water masses entrained by canyon topography characterized by high salinity, oxygen saturation, fluorometry, and turbidity. Higher fluorescence and turbidity in the canyon did not translate into higher sediment organic matter. Current flushing along canyon wall channels and the canyon axis may explain the low organic matter. Differences in abundance and structure between the canyon wall and axis may result from microhabitat heterogeneity due to potential hydrocarbon seepage, organically enriched sediment deposits along channels, or remnant influence from the DwH spill. The larger Gulf of Mexico is a recognized biogeographic province, but few studies have characterized the inter-regional variation of the benthos. The Biodiversity of the Gulf of Mexico (BioGoMx) database, which contains occurrence information of extant species in the GOM, allows for the analysis of benthic mollusc diversity and distribution across the entire basin. For analyses, the GOM was split in 4 geographic sectors (NE, NW, SE, and SW) and 6 depth classes (inshore, upper shelf, lower shelf, upper slope, lower slope, and abyssal plain) for a total of 24 geographic-depth polygons. The northern GOM contained higher species richness than the south, the east more than the west. Species richness decreased with depth with maxima occurring on the upper shelf. Bivalves and gastropods dominated each geographic sector and depth class, together comprising >90% of the molluscan species richness. Assemblages were structured by depth more than by geographic sector. GOM molluscs fell into 3 broad depth-based assemblages: the inshore and continental shelf, the continental slope, and the abyssal plain species combined with the western lower slope. Geographically, taxonomic distinctness analysis indicated most NE depths fell below average distinctness and by depth, polygons above and below the continental shelf break were frequently distinct. Cluster analysis based on taxonomic dissimilarity agreed with the analyses based on the species occurrence data. Mollusc feeding strategies largely followed estimated proportions for the larger Atlantic. Carnivory and suspension feeding were the most common with grazing, herbivory, and parasitism following behind. Chemosymbiotic species were also prevalent due to the widespread occurrence of cold seep habitats. Further taxonomic research and more sampling are needed to determine patterns at finer scales. Overall, the GOM remains an ecologically distinct basin on temporal scales, and on spatial scales both large and small. Further research is required to substantiate the lack of overall temporal change in benthic fauna and the distinct spatial uniqueness and heterogeneity that takes place in local ecosystems such as marine canyons and across the entire basin. The patterns observed also offer a baseline of comparison for future studies, and vital information for policymakers and stakeholders, especially in the face of ongoing resource extraction (e.g., fossil fuels) that occurs in the deep Gulf of Mexico.