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

Abstract

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 (Bathymodiolus sp.) or Vesicomyidae (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 (d 13 C) 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.

Cold Seep Epifaunal Communities on the Hikurangi Margin, New Zealand: Composition, Succession, and Vulnerability to Human Activities

“…For instance, while the well-oxygenated New Zealand seep sites [16] are characteristically populated by mixed communities of tubeworms, clams, mussels, and sponges, those on the Californian and Oregon margins are dominated by bacterial mats and vesicomyid clams within the oxygen minimum zone [44] but by tubeworms and clams in oxygenated areas [45]. In the Atlantic, seeps in the Gulf of Mexico and on the equatorial West African margin are dominated by tubeworms and mussels, although clams may also be present [39], [42]. Perhaps more significantly, the distinctive ampharetid beds appear to be ubiquitous at the New Zealand sites.…”

Biogeochemistry of a low-activity cold seep in the Larsen B area, western Weddell Sea, Antarctica

“…4d). The geochemical measurements indicate very low methane fluxes and oxidation rates at the Larsen B cold seep system, similar to some cold seep systems in the Gulf of Cadiz which do not host thiotrophic mats or chemosynthetic bivalves, but sporadically populations of siboglinid tubeworms (Niemann et al, 2006a). In comparison, methane concentrations in surface sediments at active seeps supporting thiotrophic microbial mats or dense clam populations (e.g., Haakon Mosby Mud Volcano, the Gulf of Mexico, Hydrate Ridge) are in the millimolar range, and sulphate is consumed in the first few centimetres of the sediment, leading to high sulphide concentrations >4 mM (Boetius et al, 2000;Torres et al, 2002;Joye et al, 2004;de Beer et al, 2006;Niemann et al, 2006b).…”

“…Usually, ANME-1 and/or ANME-2 sequences are present in cold seep sediments and comprise the dominant group of anaerobic methanotrophs, while ANME-3 has been detected only rarely in these systems (Knittel et al, 2005;Omoregie et al, 2008Omoregie et al, , 2009Knittel and Boetius, 2009;Roussel et al, 2009). So far, only one other cold seep, the Arctic Haakon Mosby Mud Volcano, has been found where ANME-3 is the dominant ANME type (Niemann et al, 2006b;. This raises the question whether ANME-3 is best adapted to the ice-cold temperature conditions of Arctic or Antarctic waters or if additional factors select for this particular group in these environments.…”

“…So far, three archaeal groups of marine, anaerobic methanotrophs (ANME-1, -2 and -3) related to the methanogenic Methanosarcinales and Methanomicrobiales were discovered (Hinrichs et al, 1999;Boetius et al, 2000;Orphan et al, 2001Orphan et al, , 2002Knittel et al, 2005;Niemann et al, 2006b;Lösekann et al, 2007). The (presumed) sulphate-reducing partner bacteria are members of the Desulfosarcina/Desulfococcus (ANME-1 and -2) or the Desulfobulbus branch (ANME-3) of the δ-proteobacteria (Boetius et al, 2000;Orphan et al, 2001;Knittel et al, 2003;Niemann et al, 2006b;Lösekann et al, 2007). In addition to the sulphate-dependent mode of AOM, recent studies demonstrated that alternative oxidants are utilised as terminal electron acceptors during AOM.…”

An overview of chemosynthetic symbioses in bivalves from the North Atlantic and Mediterranean Sea

“…Other areas have been less intensively investigated, such as deep hydrothermal vents in the Caribbean Sea (Piccard vent field, Mid-Cayman Ridge). Cold seeps are beginning to be explored in the Barbados prism, the east American margin (Blake Ridge), the Sea of Marmara, the eastern Mediterranean (Olimpi mud field, Anaximander mounds, Nile deep-sea fan), the Gulf of Cadiz (various mud volcanoes), the Norwegian margin (Haakon Mosby mud volcano and Storegga Slide), the Gulf of Guinea (Guiness, Regab, Kouilou), the Arctic margin (Loki's castle), the Caribbean Sea off Colombia, and the Laurentian deep-sea fan (Connelly et al, 2012;Cosel and Olu, 2009;Van Dover et al, 2003;Gebruk et al, 2003;Gracia et al, 2012;Olu et al, 1996;Olu-LeRoy et al, 2007a;Sahling et al, 2008.…”