Using stable isotopes and quantitative community characteristics to determine a local hydrothermal vent food web

To investigate food web structure in diffuse flow vent environments, an entire macrofaunal community associated with a single aggregation of the tubeworm Ridgeia piscesae was collected from a vent field on the Juan de Fuca Ridge in the NE Pacific. All members of the community were identified and enumerated, and the biomass and stable carbon (delta C-13) and nitrogen (delta N-15) isotope ratios were determined for almost all taxa. Symbiont-bearing invertebrates (primarily R. piscesae) accounted for a vast majority of the biomass of the community, and 3 presumably grazing bacterivorous gastropods dominated the biomass of the consumer fauna. Biomass and abundance of individuals declined from the bacterivore to the scavenger/detritivore to the predator feeding guild. Several species (a folliculinid ciliate, Idas washingtonia, Provanna variabilis) possessed unique stable isotope signatures, suggesting the possibility of symbiotic relationships with autotrophic bacteria. Stable isotope values varied widely between and occasionally within species in the lowest consumer levels suggesting a great diversity of food source C-13 and N-15 composition. Based on the distinct isotopic values of 3 bacterivores, 3 potential pools of isotopically-distinct microbial production were identified. The 4 highest-biomass predatory species (all polynoids) possessed delta C-13 and delta N-15 values consistent with a diet that included the tubeworm R. piscesae, a species comprising 83% of the total biomass in the collection, and the gastropod Depressigyra globulus, a species comprising 10% of the total biomass in the collection. A potential specialist predator (Clypeosectus curvus) on folliculinid ciliates was also identified. Overall the study suggested a dominant pattern of energy transfer from microbial producers to symbiont-bearing siboglinid tubeworms, various bacterivores (gastropods, polychaetes and pycnogonids), and detritivorous polychaetes to predaceous polynoids.