Variability of macrofaunal community structure in reducing sediments of the Congo deep sea fan: spatial and temporal patterns

The terminal lobes of the Congo deep-sea fan (tropical east Atlantic) are a unique area in the world ocean. These lobes are located at depths of about 5000 m, at a distance of 750 km from the African coast. They are fuelled quasi-continuously by turbidites containing a large proportion of labile organic matter delivered by the Congo River, the second largest river in the world by its freshwater discharge, via a submarine canyon and deep channel incising the margin.
The presently active lobe complex, fuelled since the last 2000 yrs, as well as a now abandoned lobe, were explored during the CongoLobe cruise. At each site, ROV dives revealed characteristic cold-seep habitats such as beds of vesicomyid bivalves and microbial mats, patchily distributed.
The objective of the study was to compare the macrofaunal community structure of different habitats and sites of both the lobe area and the Regab pockmark, a cold-seep site located on the margin in 8 km north of the Congo Channel at a depth of 3200 m.
In common with the pockmark, macrofauna of each habitat and site of the lobe area exhibited high densities and were represented by such typical cold-seep molluscan and polychaete families as Hyalogyrinae, Vesicomyidae, Dorvilleidae, Hesionidae and Ampharetidae. Diversity was the lowest in microbial mats, as usually reported for seeps. Differences in community composition were also observed within the vesicomyid habitat, between the different lobes.

Among the chemical parameters measured (methane, oxygen, hydrogen sulphide in subsurface porewater), the depth of the hydrogen sulphide peak and its concentration closest to the sediment/water interface were the two most contributing factors explaining differences in macrofaunal structure. The species and size of dominant vesicomyid bivalves that vary among habitats and sites and influence chemical gradients by bioturbation and bioirrigation, were also assumed among factors controlling the differences in community structure.

In the lobe area, the observed spatial variability in community structure matches the burying of the sulphide layers and might suggest a temporal evolution of these biogenic habitats, from microbial mats to dense vesicomyid beds dominated by Christineconcha regab in the active lobes and then to sparse vesicomyid beds dominated by Abyssogena southwardae in the disconnected lobes. In the pockmark, bacterial mats and vesicomyid beds seem to follow the same succession pattern along a gradient of sulfidic conditions.

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Distribution and abundance
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CBE5-192
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Elena
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Krylova
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Shirshov Institute of Oceanology, Moscou, Russia
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Christophe
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Rabouille
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aboratoire des Sciences du Climat et de l'Environnement, France