Benthic megafauna on steep slopes at the Northern Mid-Atlantic Ridge

The role of small-scale (<10 km) habitat availability in structuring deep-sea hard substratum assemblages is poorly understood. Epibenthic megafauna and substratum availability were studied on steep slopes at the Mid-Atlantic Ridge from May to July 2010 northwest, northeast, southwest and southeast of the Charlie-Gibbs Fracture Zone (CGFZ; 48-54 degrees N) at between 2095 and 2601 m depth. Megafauna were six times denser north of the CGFZ compared with the south and differences in density were almost entirely driven by sessile fauna. There was no significant difference in habitat availability amongst sites. Rocky substratum made up 48% of the total area surveyed, with individual transects having between 0% and 82% rock. Assemblage structures were different amongst all superstations. The north was dominated by demospongids and hexactinellids, whereas the southern superstations were dominated by antho-zoans and hexactinellids. Differences in megafaunal assemblages north and south of the CGFZ primarily reflected variations in demospongid and antho-zoan species composition. With 213-1825 individuals.ha(-1), and 7-24 species per superstation, hexactinellids were the most species-rich (36 species) and cosmopolitan taxa at the study site, supporting observations elsewhere along the ridge and in the CGFZ. The absence of significant differences in substrata availability suggested alternative drivers for density or percentage cover. The amount of hard substratum available only limited sessile megafauna density at one transect that was entirely covered with sediments. Species richness was highest for areas with intermediate values of substratum coverage (35-43% rock).