Different Biogeochemical Regimes Structure Faunal Communities in Seep Habitats in the Western Atlantic

Visible types of chemosynthetic seep habitats (e.g. microbial mats, active venting, megafauna) often represent different chemical environments related to variation in the flow of methane through sediments and rates of methane oxidation. Seep faunal communities are known to be structured by the underlying sediment geochemistry, where high levels of hydrogen sulfide and low oxygen concentrations often result in communities comprised of taxa tolerant of local conditions. However, fluid fluxes and methane concentrations vary on both small and large spatial and temporal scales, suggesting that the broadly described seep habitats encompass a wide range of microhabitats that have the potential to support distinct communities. We investigated microbial mat, active methane bubbling, and nearby non-seep sediments at newly discovered shallow seeps at Pea Island in the western Atlantic (299-469 m) in 2018 and 2019. Sediments were assessed for macrofaunal abundance, diversity, and community structure, and multiple sediment geochemical and physical properties, including hydrogen sulfide concentration, stable isotopic composition, particle size, and organic content. Each microbial mat was composed of distinct infaunal communities characterized by either high densities of Capitellidae or Tubificinae annelids with overall low diversity. Although both these annelids are known to be tolerant of high sulfide concentrations, these results suggest underlying differences in the localized geochemical and physical environments. Clarifying the relationship between different seep microhabitats and their associated local sediment geochemistry will enhance our understanding of the geochemical controls on diversity and community composition and the broad-scale interrelationship among individual seep habitats.