Benthic macrofaunal community structure in the Norwegian Trench, deep skagerrak

Benthic communities were studied at 15 stations along two transects at between 144 and 682 m depth in the Norwegian Trench. Communities of the deep accumulation bottoms generally exceeding 400 m depth, and 65 nautic miles apart, had a high similarity in species-dominance composition (Bray-Curtis index). The faunal composition on the slopes also showed high similarities, but was less homogeneous. The average number of species per 0.1 m2 was significantly higher at the shallow-slope stations (mean 28.6) than at the deep-trench stations (mean 19.8). Among all stations abundance varied between 455 and 6660 ind·m−2 and biomass was generally low, <40 g wet wt·m−2 (excluding some large individuals). Dominant faunal groups were polychaetes and molluscs followed by crustaceans. The tube-building polychaete Spiochaetopterus bergensis was numerically dominant at all deep-trench stations, where the bivalves Thyasira eumyaria and Kelliella miliaris were also abundant. Sediment characteristics, transport and accumulation rates are discussed as structuring factors for the benthic communities.