Biomass and Mass-balance Isotope Content of Seep Populations on the Upper Slope Gulf of Mexico, Determined from Archived Samples.

摘要

Methods are needed to assess the health of chemosynthetic communities so that anthropogenic impacts can be separated from natural changes associated with succession. The work reported successfully demonstrated the utility of analyses of stable isotopes ?13C, ?15N, and ?34S for the purpose of such an assessment. The specimens analyzed were contained in a LSU archive of material collected during BOEM-supported surveys in 1989–92 and NOAA-supported collecting in 2006. Two sites of know differences were examined. Bush Hill is at the junction of Green Canyon block 184/5 and Brine Pool NR-1 in Green Canyon 233. The former is a topographic high developed along a major fault. The latter is interpreted as a mud volcano located in a region of slope failure. The species analyzed were the foundation species of mussel Bathymodiolus childressi (tissue analyses = 253; shell analyses = 154) and an associated snail Bathynerita naticoidea (tissue analyses =247). The former depends on methanotrophic microbial symbionts in its gills for its primary nutrition although filter feeding is possible. The latter is fully heterotrophic and grazes on the surface of the mussel shell consuming any available detritus. Bush Hill mussels and snails differ greatly from the Brine Pool NR-1 in a manner consistent with thermogenic methane at Bush Hill and biogenic methane at the Brine Pool. As a carbon source, methane alone cannot explain an equally dramatic difference in ?15N and ?34S values. The link among ?15N, ?13C, and ?34S remains poorly understood but can be explained on the basis of four trophic resources being consumed. Bathymodiolus childressi is a mixotrophic rather than strictly chemoautotrophic foundation species. The four trophic resources modeled indicated that the mussel populations at both seeps consume methane through two symbiotic pathways and detritus through two additional pathways. The dominant detrital source appeared to be sulfide oxidizing microbes. The isotope compositions of all the sources except phytoplankton detritus are seep-specific. Sufficient methane-derived respiratory carbon was found in the shells for dead shell assemblages to be used for assessment of methane metabolism. Future research based on shells from dead seep communities may be able to trace the time course of the loss of the methane resource. Recommendations for future research center around the management benefit of being able to establish the condition of the major foundation species. For mussel population, the existence of and the manner of feeding on the thiotrophic detritus can be undertaken in the field and in the laboratory. Application of stable isotope analyses to dead shell material should be undertaken to gain an understanding of the causes of extinction for the many dead-shell assemblages encountered during the exploration for seep systems.