| Summary: | Hydrocarbon seeps serve as oases of local primary production in the otherwise food-poor deep sea. Animals that have adapted to handle the toxic chemistry in these environments thrive and form dense high-biomass communities. In this thesis, stable isotopes were used to explore food web interactions in several seep-associated communities in the Gulf of Mexico. We used collection techniques that capture the entire local community associated with the target foundation species and subsampled from this community for stable isotope analysis. Tissue stable isotope values in the reef-building cold-water coral Lophelia pertusa refuted a long-standing hypothesis that cold-water corals rely on primary production from seeps around which they occur. They instead are probably taking advantage of the abundant carbonate rock substrate found at seeps. The communities associated with the corals also showed no indication of nutritional input from seeps. Bathymodiolin mussel tissue δ13C values showed that biogenic methane dominates the methane at lower-slope seeps and extremely low δ15N values in one collection of Bathymodiolus childressi point toward an unusually abundant or isotopically depleted nitrogen source. Very low δ13C values in the vestimentiferan tubeworms Escarpia laminata and Lamellibrachia sp. 1 support uptake of porewater dissolved inorganic carbon through their buried roots. Consistent differences in δ15N between the co-occurring E. laminata and Lamellibrachia sp. 1 provide evidence for partitioning of inorganic nitrogen sources, a finding that has significant implications for the ecology and evolution of this taxonomic group. Extremely variable δ34S values in vestimentiferans provide evidence that waste sulfate excreted from vestimentiferan roots is reduced to sulfide by microbial consortia in the vicinity of the individual root, ensuring a sulfide supply to the vestimentiferans symbionts. Tissue δ13C, δ15N, and δ34S values in heterotrophic animals collected with vestimentiferans, mussels, and vesicomyid clams showed that the majority of animals derive a significant portion of their nutrition from seep primary production. The δ13C values in the heterotrophs tracked the δ13C of their associated symbiotic fauna, which varied substantially between collection locations. This indicates that most of the animals feed locally and that the δ13C signature of the local hydrocarbon pool affect the δ13C values of the entire community. The δ15N values of heterotrophs collected with mussels were significantly more depleted than those collected with vestimentiferans both overall and within sites. This was consistent with an approximately 8 depletion in mussel tissue δ15N values compared to vestimentiferan δ15N values within the same sites. Together, these data indicate a difference in the isotope compositions of the inorganic nitrogen sources in the two microhabitats. The lack of isotope data for inorganic nitrogen at seeps prohibits further speculation into the underlying cause of this difference.
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