Abstract
Diverse invertebrate communities inhabiting carbonate rocks at methane seeps may rely on a mix of local chemosynthetic production and sinking photosynthetic organic matter, with relative importance shaped by environmental conditions. We investigate the contribution of chemosynthetic carbon to macrofaunal diets at six Southern California methane seeps (275–1020 m depth) and the influence of seep site, seepage habitat, water depth, and oxygen on trophic structure, using stable isotope analyses, Bayesian mixing models, and generalized additive models. Macrofauna exhibit wide isotopic ranges and methane‐derived carbon (MDC) contributions, with several species deriving > 80% of their diet from chemosynthetic sources, including pyropeltid limpets, provannid snails, and dorvilleid and nereidid polychaetes. Negative δ 15 N values and high MDC in a trichobranchid polychaete and sponge are consistent with N 2 fixation or incorporation of N‐fixing microbial biomass. Taxa historically not considered as ‘seep fauna’ (e.g., bryozoans and cnidarians) also incorporate substantial chemosynthetic input, challenging their classification as ‘background’ taxa. Community‐wide trophic structure varied strongly by seep habitat, with active seepage habitats showing higher chemosynthetic reliance, trophic diversity, and isotope ranges. Seep production export into surrounding systems was greater at deeper seeps, suggesting a stronger trophic sphere of seep influence where photosynthetic input is more limited. This increased reliance on chemosynthesis with depth appears to reverse the typical positive δ 15 N‐depth relationship observed in non‐chemosynthetic fauna. Low oxygen seems to favor a restricted number of species with specialized trophic niches. Macrofauna isotopic variability across seep sites likely reflects differences in fluid flux regime and geochemistry. Our study demonstrates that chemosynthetic production plays a broad role in supporting the deep‐sea communities off Southern California and reveals complex, and potentially novel, trophic interactions at methane seeps. These findings underscore the importance of incorporating trophic diversity and associated functional heterogeneity in conservation planning, especially as environmental changes reshape deep‐sea ecosystems, warranting more nuanced protection frameworks.