Spatial patterns of tissue stable isotope contents give insight into the nutritional sources for seep communities on the Gulf of Mexico lower slope

In this study, we present the first thorough trophic characterization of cold seep macrofaunal communities on the Gulf of Mexico lower continental slope (>1000 m depth). We analyzed tissue δ¹³C, d15N, and δ³⁴S of vestimentiferan tubeworms, bathymodiolin mussels, vesicomyid clams, and their associated macrofaunal communities from discrete collections made across the entire lower slope. Over half of macrofauna associated with mussels and about half associated with vestimentiferans had δ¹³C values below -45%. We also observed high spatial variability in the δ¹³C values of entire local communities, and the δ¹³C of associated fauna were significantly correlated with the δ¹³C compositions of the symbiotic species from the same location. These data indicate widespread incorporation of methane-derived carbon in mussel and vestimentiferan communities. This finding was particularly surprising in communities associated with older vestimentiferans, given the low rates of seepage observed in similar communities on the upper slope. On average, d15N values in mussels and their associates were significantly more depleted and more variable than vestimentiferans, clams, and their associates, and there was a significant linear relationship between tissue d15N values of mussels and their associated communities. The tissue δ³⁴S values in macrofauna associated with vestimentiferans were more variable and significantly more depleted than mussel associates (δ³⁴S = -16.8 to +19.1% for vestimentiferan associates and δ³⁴S = -3.1 to +20.8% for mussel associates), consistent with higher isotopic fractionation during sulfate reduction in vestimentiferan habitats and a potentially higher nutritional contribution of sulfide-derived organic sulfur in vestimentiferan communities.