Cold-seep assemblages on a giant pockmark off West Africa: spatial patterns and environmental control

A giant pockmark colonised by dense cold-seep assemblages near 3160 m depth along the Congo-Angola margin has been surveyed by the ROV Victor 6000. The quantitative distribution of chemosynthetic communities was mapped along the dive tracks from a video study using GIS and image mosaicking. Several types of faunal assemblages, either dominated by bivalves of the families Mytilidae (Bathmodiolus sp.) Vesicom idae (Calyptogena sp., 'Vesicomya' aff. chuni), or by Siboglinidae polychaetes (Escarpia southwardae) were mapped over the 800-m diameter pockmark area and sampled for fauna, water and sediment. The isotopic analyses (delta C-13) of tissues from symbiont-bearing species were within the range typical of nutrition via symbiosis using methane for mussels and sulphide for vesicomyids and siboglinids. The living chemosynthetic communities were distributed on a SW-NE axis, corresponding to the expression at the sediment surface of a main buried channel providing fluids to the pockmark. The site was characterised by a more active central part in a depression with abundant carbonate concretions where high-density clusters of siboglinids and mytilids dominate. Large fields of dead and live vesicomyids with a lower mean density were observed in the external areas. The mean coverage of each of the three symbiotic taxa in these two contrasted areas was estimated from mosaic analysis and was up to 30% in the central area dominated by E. southwardae bushes (23%). Symbiont-bearing species distribution was consistent with methane concentrations in seawater that were generally higher in mytilid beds than in the vicinity of siboglinids and vesicomyids. A Principal Component Analysis performed on environmental factors at the ten sampling sites revealed that 37% of the observed variance in the distribution of symbiont-bearing species may be explained by variation in both methane and oxygen concentrations, while a Canonical Redundancy Analysis selected methane concentration as the only variable which explains symbiont-bearing species densities. This spatial distribution of chemosynthetic species at the pockmark scale may reflect temporal patterns of succession of both substrate and fauna, and may be related to different individual pockmarks visible on the microbathymetry mapped using ROV data.