Community structure and feeding ecology of meiofauna associated with methane seepage 1 at the Darwin mud volcano ( Gulf of Cádiz )

We sampled the Darwin mud volcano (MV) for meiofaunal community and 14 trophic structure in relation to pore-water geochemistry along a 10-m transect from a seep 15 site on the rim of the crater towards the MV slope. Pore-water profiles indicated 16 considerable variation in upward methane (CH4) flow among sediment cores taken along the 17 transect, with highest flux in the seep sediment core, gradually decreasing along the 18 transect, to no CH4 flux in the core taken at a 5 m distance. Low sulphate concentrations and 19 high levels of total alkalinity and sulphide (H2S) suggested that anaerobic oxidation of 20 methane (AOM) occurred close to the sediment surface in the seep sediment core. High H2S 21 2 levels had a genus and species-specific impact on meiofaunal densities. Nematode genus 22 composition varied gradually between sediment cores, with the genus Sabatieria dominating 23 almost all sediment cores. However, genus diversity increased with increasing distance from 24 the seep site. These limited data suggest that the community structure of seep meiofauna is 25 highly dependent on local (a)biotic habitat characteristics, and a typical seep meiofauna 26 community cannot be delineated. Stable isotope values suggested the nematode diet up to 27 10 m from the seep site included thiotrophic carbon. The thicker hemipelagic sediment layer 28 (photosynthetic carbon), the increased trophic diversity, and the heavier nematode δ 13 C 29 farther from the seep site suggest a decrease in thiotrophy and an increase in 30 photosynthetic carbon in the nematode diet. 31