ABUNDANCE, BIOMASS AND SPATIAL-DISTRIBUTION PATTERN OF BRITTLE STARS (ECHINODERMATA, OPHIUROIDEA) ON THE KOLBEINSEY RIDGE NORTH OF ICELAND

Species composition, abundance, biomass and spatial distribution pattern of brittle star assemblages of the Kolbeinsey Ridge, north of Iceland, were investigated by analysing seafloor photographs and epibenthic sled catches. Sampling was conducted in July 1992 along a 34 km long cross-ridge transect at 67-degrees-55'N in depths ranging from 830 to 1100 m. Five brittle star species were found. Only one species, Ophiocten gracilis, occurred with densities of more than 1 ind m-2. Both ridge slopes were characterized by soft bottom habitats, the ophiuroid distribution, however, revealed differences between slopes. Generally, brittle star densities were significantly higher on the eastern slope. There, the highest mean abundance per station of O. gracilis was 497 ind m-2. Except for one station, a distinct patchiness on the 100 m-scale was obvious in the spatial distribution of O. gracilis. Small settling stages with disc diameters < 1 mm accounted for up to 98% of the population of O. gracilis near the ridge top, but their abundance share decreased with depth to only 6% on the eastern slope foot. Biomasses of O. gracilis were extrapolated by combining abundances estimated from photographic counts with a size-weight relationship and size frequencies established from sled catches. The highest value (120 mg AFDW m-2) was found on the deep eastern slope where large O. gracilis with disc diameters > 4 mm were most abundant. Our findings reflect a cross-ridge gradient in terms of hydrographic regime and, hence, probably pattern of food supply for the benthos.