Ecotones at Methane Seeps: Understanding the seepage sphere of trophic influence for carbonate macrofauna

Methane seepage often generates precipitation of carbonate rocks, which host microbes and a diverse fauna. These rocks may promote an interaction between seep and background communities, forming ecotones, and providing hard substrate even after seepage ceases. We analyzed the composition, density and trophic structure of invertebrate assemblages on carbonate rocks at ‘Mound 12’, a methane seep site off the coast of Coast Rica, to assess whether the community and its trophic structure change along seepage gradients. By sampling in situ carbonate rocks with increasing distance from active seepage, we observed a shift in community composition and seep-derived isotopic signatures, although methane-derived carbon was still evident in faunal tissues at transition sites of lesser seepage. Rocks deployed for 7 years at active and transition sites revealed similar fauna and isotopic signatures to the in siturocks, suggesting rapid colonization rates. Wood and bone substrates also deployed for 7 years at different seepage habitats showed similar faunal and isotopic patterns to the experimental rocks. A carbonate transplant experiment assessed effects of seepage and its cessation on community dynamics. Many seep-species maintained their methane-derived carbon signatures when transplanted to sites with lesser seepage over 17 months. Knowledge of seep interactions with background assemblages is crucial for understanding biodiversity at seeps as well as for impact assessment, management, valuation, and for predicting consequences of climate change.