Environmental drivers and dynamics of macroinvertebrate communities on carbonates at Southern California methane seeps

Methane seeps are recognized for hosting diverse communities and playing a crucial role in global carbon cycling. This study characterizes macroinvertebrate (> 300 μm) communities on carbonates at six methane seeps in the Southern California Borderland (USA) (370-1020 metres), addressing questions about their community structure and response to water depth, oxygen concentrations, and seepage activity. Overall, sites with less active seepage were dominated by ophiuroids, isopods, amphipods, and cnidarians. Active seep sites supported provannid snails, pyropeltid limpets, and dorvilleid, cirratulid, and lacydoniid polychaetes. Mensurative studies revealed that macrofaunal density and diversity were lower with increasing depth, decreasing oxygen, and increasing seepage. Experiments at Del Mar Seep, which involved transferring carbonates from an active seep setting to an inactive setting, showed that after 25 months the macrofaunal communities had fewer species dependent on seep-associated microbes and resembled non-seep fauna. Colonization experiments at Lasuen Knoll Seep, in which defaunated carbonates were deployed for 23 months at active and inactive settings, revealed that seepage allows for higher density and homogenizes communities. Southern California methane seeps are highly heterogenous, strongly influenced by overlying oxygen concentration, and respond rapidly to changes in seepage. Such information can inform regional conservation and management policies.