Temporal shift in nutrient input to cold-seep food webs revealed by stable-isotope signatures of associated communities

A pattern of succession has been demonstrated at Gulf of Mexico cold seeps proceeding from a high-biomass endemic community dominated by grazers to a diverse community of endemic and nonendemic species in multiple trophic levels to a low-biomass community comprising primarily nonendemic predatory species. We test the hypothesis that these shifts in community structure are due to a decline in the availability and reliance on local chemosynthetic productivity. Isotopic signatures were measured in 134 individuals from 34 different species in three tubeworm aggregations in different stages of succession. The average stable isotope delta values of the fauna were more positive in the older aggregations, suggesting a decreased reliance on local chemosynthetic productivity. These trends were also apparent in the species that were present in all three aggregations and for the grazer trophic group as a whole. The proportion of biomass in the lower trophic levels declined in the older aggregation. Food webs were reconstructed on the basis of variable trophic shifts and included the error in stable isotope measurements. The majority of potential trophic links supported by carbon and nitrogen isotopes alone were rejected when sulfur stable isotope values were included. Food webs consisted of relatively few trophic links overall, with the most complex food web in the more diverse community of the middle aggregation. These results suggest that many of the species in the tubeworm aggregations are generalists, and these species may feed on prey outside the aggregation, especially in the older aggregation, where few trophic links could be detected.