Habitat heterogeneity influences cold‐seep macrofaunal communities within and among seeps along the Norwegian margin. Part 1: macrofaunal community structure

Håkon Mosby mud volcano (HMMV) is one of the most active and most studied seep sites in European waters. Many authors have described its thermal activity, dynamic of mud flows, and geochemical and microbial processes. It is characterised by a concentric zonation of successive biogenic habitats related to an activity and geochemical gradient from its centre to its periphery. Around the central area covered by mud flows, white and grey microbial mats occur among areas of bare sediment, whereas siboglinid tubeworm fields of Sclerolinum contortum and/or Oligobrachia haakonmosbiensis colonise the peripheral areas. The meiofaunal community is known to be structured among habitats, but the macrofauna has rarely been investigated and has never been sampled in situ. As part of the European project HERMES, using the ROVs Victor 6000 and Quest 4000, we sampled quantitatively the different habitats of the volcano for macrofauna sensus lato, retained on a 250‐ or 500‐μm sieve. We also sampled a newly discovered pockmark on Storegga slide (cne 5.6) and two pockmarks (G11, G12) in the Nyegga area. Macrofauna was identified and counted from phylum to family level. Our results on HMMV showed a gradient of increasing density and diversity from the volcano centre (1–3 taxa; 260 ind·m^?2) to the peripheral siboglinid fields (8–14 taxa, 93,000 ind·m^?2), with an intermediate situation for microbial mats. For macrofauna ≥500 μm, non‐siboglinid polychaetes dominated the communities of the central mud volcano area, white mats and S. contortum fields (83, 89 and 37% of the total, respectively), whereas gastropods dominated grey mats and O. haakonmosbiensis fields (89 and 44% of the total, respectively). Polychaete families followed the same pattern of diversity according to habitats within HMMV. Of 23 polychaete families identified, only one occurred in the centre, and three in the microbial mats. Capitellidae and Dorvilleidae (typical of organically and sulphide‐enriched areas) occurred at remarkably high densities in white microbial mats and in O. haakonmosbiensis fields. The S. contortum fields were the most diverse habitat with 12 polychaete families. The 250‐μm fraction showed similar taxa dominating the habitats, but taking meiofauna into account, nematodes became the major taxon in white mats and in S. contortum fields, where they were particularly large in size, whereas copepods dominated in other habitats. Meiofauna and macrofauna did not show the same patterns of density according to habitats. Using principal components analysis the habitats at HMMV were clearly distinct, and clustered according to dominant species of siboglinids and type of microbial mats. Pockmarks at Nyegga showed a similar concentric pattern of habitats around fluid sources as on the volcano, which seemed similarly to influence macrofauna composition, but at a much smaller scale. Total taxa and polychaete diversity are high in the S. contortum fields in these pockmarks as well. Regional‐scale comparisons including HMMV and Storegga suggested a higher influence of habitat‐type than seep‐site on the community structure.