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Quantifying changes to infaunal communities associated with several deep-sea coral habitats in the Gulf of Mexico and their potential recovery from the DWH oil spill

December 31, 2019

Extensive information is available about infaunal soft-sediment communities in the Gulf of Mexico (Gulf) (Pequegnat et al. 1990, Rowe and Kennicutt II 2009, Wei et al. 2010), particularly from the large-scale sampling effort of the Deep Gulf of Mexico Benthos (DGOMB) project in the early 2000s (Rowe and Kennicutt II 2009). Infaunal soft-sediment communities in the northern Gulf differ by geographic location and depth (Rowe and Kennicutt II 2009, Wei et al. 2010). Density decreases with depth, while taxa diversity exhibits a mid-depth (1,100-1,300 m) maximum (Rowe and Kennicutt II 2009). Community composition is influenced by both geographic location and depth, with zones (as defined by Wei et al. 2010) encompassing specific depth ranges, ranging from 635 to 3,314 m, and separated into east and west components. These zones were correlated to detrital particulate organic carbon (POC) export flux, primarily from the Mississippi River (Wei et al. 2010), where POC flux decreases with depth (Biggs et al. 2008). The flux of POC has also been found to be higher in the northeast Gulf than the northwest (Biggs et al. 2008), and consequently, biomass of infaunal communities is positively correlated with sedimentorganic carbon content (Morse and Beazley 2008).

Most of the deep Gulf is composed of soft-sediment environments, but the relative flat seafloor is
punctuated in areas with other heterogeneous habitats, including chemosynthetic environments and deepsea coral habitats. Deep-sea corals create a complex three-dimensional structure that enhances local biodiversity, supporting diverse and abundant fish and invertebrate communities (Mortensen et al. 1995, Costello et al. 2005, Henry and Roberts 2007, Ross and Quattrini 2007, Buhl-Mortensen et al. 2010). In recent years, knowledge of the sphere of influence of deep-sea corals has expanded, with evidence that coral habitats also influence surrounding sediments (Mienis et al. 2012, Demopoulos et al. 2014, Fisher et al. 2014, Demopoulos et al. 2016, Bourque and Demopoulos 2018). Deep-sea corals are capable of altering their associated biotic and abiotic environment, thus serving as ecosystem engineers (e.g., Jones et al. 1994). The depositional environment and associated hydrodynamic regime around coral habitats differ from the extensive expanses of soft-sediments that dominate the sea floor (e.g., Mienis et al. 2009a. 2009a, Mienis et al. 2009b, Mienis et al. 2012), with the three-dimensional structure of the coral causing turbulent flows that enhance sediment accumulation adjacent to coral structures. In the northern Gulf, deep-sea corals generally occur on mounds of authigenic carbonate (Schroeder 2002) where elevation above the benthic boundary layer into higher velocity laminar flows allows for increased availability of food resources (Buhl-Mortensen and Mortensen 2005). The different hydrodynamics around corals likely affects the sediment geochemistry and in turn infaunal community structure and function (Demopoulos et al. 2014).

Ecosystem-based research on Gulf infaunal communities has primarily focused on soft-sediment
environments. Initial research on deep-sea coral-associated infaunal communities focused on Lophelia pertusa (e.g., Demopoulos et al. 2014), and more recent studies focused on octocorals (Fisher et al. 2014, Demopoulos et al. 2016, Bourque and Demopoulos 2018) and comparisons among coral habitat types (Bourque and Demopoulos 2018). Coral-adjacent sediment communities are distinctly different from nearby background soft-sediment (Demopoulos et al. 2014, Bourque and Demopoulos 2018), with a sphere of influence estimated to be between 14 and 100 m (Demopoulos et al. 2014, Bourque and Demopoulos 2018). The coral type (e.g., L. pertusa, Madrepora oculata, octocorals) also influences sediment communities, with L. pertusa habitats distinct from both M. oculata and octocoral habitats (Bourque and Demopoulos 2018). Differences among coral communities are influenced by depth,