Isotopic evidence for chemosynthesis‐based nutrition of macrobenthos: The lightness of being at Pacific methane seeps

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Abstract
The importance of chemosynthetic nutritional pathways was examined for macrofaunal invertebrates (≫300 µm) from methane seeps in the Gulf of Alaska (4,413–4,443 m), on the Oregon margin (590 m), and on the northern California slope [Eel River margin] (520 m) by use of natural abundance stable isotopic data. Seep macrofauna exhibited lighter Δ13C and Δ15N values than those in nonseep sediments, but isotopic signatures varied among seep sites. Macrofaunal isotopic signatures indicated chemosynthetically fixed carbon sources with a significant contribution from methane‐derived carbon (MDC) in macrofauna from sediments of pogonophoran fields (average Δ13C, 246.44‰, 32%–51% MDC) and Calyptogena phaseoliformis beds (average Δ13C, 240.89‰, 12%–40% MDC) in the Gulf of Alaska and in microbial mat sediments on the Oregon margin (average Δ13C, 243.80‰, 20%–44% MDC). Lesser influence of MDC was noted in macrofauna from sediments of Calyptogena pacifica beds on the Oregon (average Δ13C, 233.38‰, 0%–27% MDC) and California ( Δ13C, 225.10‰, 0%‐22% MDC) margins and from California microbial mat sediments ( Δ13C, 222.23‰, 0%–5% MDC). Although most macrofauna appeared to be heterotrophic, light Δ15N and Δ13C values together provided evidence for chemoautotrophic symbioses in selected taxa. Carbon isotopic signatures were consistent with consumption of methane‐oxidizing archaea by some dorvilleid polychaetes (Δ13C, 290.62‰ and 273.80‰) and with grazing on filamentous sulfur bacteria by gastropods and polychaetes from the Oregon and California seeps. The importance of chemosynthetic trophic pathways varies regionally and among microhabitats, taxonomic groups, and feeding guilds.

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