MEPS 208:21-39 (2000)  -  doi:10.3354/meps208021

Do methane seeps support distinct macrofaunal assemblages? Observations on community structure and nutrition from the northern California slope and shelf

Lisa A. Levin1,*, David W. James1, Christopher M. Martin1, Anthony E. Rathburn1, Leslie H. Harris2, Robert H. Michener3

1Marine Life Research Group, Scripps Institution of Oceanography, La Jolla, California 92093-0218, USA
2Los Angeles County Museum of Natural History, 900 Exposition Boulevard, Los Angeles, California 90007, USA
3Boston University Stable Isotope Laboratory, Department of Biology, 5 Cummington St., Boston, Massachusetts 02215, USA

ABSTRACT: Although the conspicuous epifauna of reducing environments are known to exhibit strong morphological, physiological, and nutritional adaptations for life in these habitats, it is less clear whether infaunal organisms do so as well. We examined metazoan macrofauna from methane-seep sediments on the northern California slope (500 to 525 m depth) and from seep and non-seep sediments at 3 locations on the shelf (31 to 53 m depth) to determine whether the community structure and nutritional sources of seep infauna were distinct from those in non-seep, margin sediments. Seep macrofauna consisted mainly of normal slope and shelf species found in productive settings. Several macrofaunal taxa, such as Capitella sp., Diastylopsis dawsoni, and Synidotea angulata, exhibited a preference for seeps. Other taxa, such as the amphipods Rhepoxynius abronius and R. daboius, avoided seeps. Species richness of shelf macrofauna, evaluated by rarefaction and diversity indices (H¹ and J¹), generally did not differ in seep and non-seep sediments. Similarly, stable isotopic composition (δ13C, δ15N) of active seep and non-seep macrofauna did not differ at the 3 shelf sites. Stable isotopic analyses of calcareous material confirmed the presence of methane-influenced pore waters at the slope study site. At one slope clam bed, macrofaunal δ13C signatures were lower and δ15N values were higher than at another clam bed, inactive slope sediments and shelf sites. However, only 1 of 14 macrofaunal taxa (a dorvilleid polychaete) exhibited isotopic evidence of chemosynthetic nutritional sources. At these sites, seep influence on the ecology of continental margin infauna appears spatially limited and relatively subtle. At their current level of activity, the northern California slope and shelf seeps appear to function as ephemeral, small-scale disturbances that are not sufficiently persistent to allow chemosynthesis-based trophic specialization by most infauna. Rather, we suggest that many of the infauna inhabiting these seep sediments are shelf and slope species preadapted to organic-rich, reducing environments.

KEY WORDS: Macrofauna · Macrobenthos · Methane cold seep · Sulfide · Stable isotope · δ13C · δ15N · Chemoautotrophy · Diversity · Eel River margin

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