A chemosynthetic ecotone - 'chemotone' - surrounds deep-sea methane seeps

Ecotones, zones of transition between distinct organismal communities, have been described as ‘biodiversity hotspots’ from a myriad of environments, yet have not been extensively studied in the deep ocean. Deep-sea methane seeps host highly productive communities, fuelled predominantly by chemical energy via chemosynthesis, with influence that extends out into background habitats, possibly through the formation of an ecotone, or in this case a ‘chemotone’. To investigate this, we analysed the characteristics of macrofaunal assemblages (density, biomass, diversity, species composition, and trait characteristics), collected using HOV Alvin from sediments surrounding five Costa Rican methane seeps. Geochemical variables (HS, DIC, δ¹³C of DIC) provided a clear distinction between active and transition (potential chemotone) samples, but transition and background samples were not statistically distinguishable. We found evidence for a chemotone separating active and background habitats, with biomass 1/4 that of active habitat, but 7.6 x greater than background habitat, and a distinct species composition, including species from both active and background habitats, as well as numerous habitat endemic species (particularly peracarid crustacean and sabellid polychaete species). However, faunal densities and trait composition were not statistically distinguishable between transition and background samples. Species richness and trait diversity metrics were not elevated in transition samples relative to active or background samples. Species diversity and evenness were greater in transition samples than in active samples, but were not distinguishable from background samples. The chemotone surrounding seeps boosts beta diversity, and may be driven by the gradient in food availability from seeps towards predominantly photosynthetic-based background communities. Conservation efforts may need to include measures to protect the chemotone surrounding deep-sea methane seeps.