Abundance patterns and species assemblages of euphausiids associated with the Mid-Atlantic Ridge, North Atlantic

New baseline information is presented on biogeography, abundance and distribution of euphausiids from discrete depth samples collected throughout the water column to 3000 m at 42 locations along the Mid-Atlantic Ridge (MAR) between Iceland and the Azores. Eighteen species were recorded, with Euphausia krohni and Thysanoessa longicaudata being most abundant. Eight species had not been recorded in the area previously. The Subpolar Front is a northern boundary to some southern species, but not a southern boundary to northern ubiquitous species that show submergence. Four major species assemblages were identified and characterized in terms of spatial distribution and species composition. Numerical abundance was highly variable but decreased logarithmically with depth. The greatest average abundance and the greatest spatial variation in abundance occurred in northern waters, over the Reykjanes Ridge. Abundance declined less with depth in the sub-Arctic Intermediate Water/North Atlantic Current Water than in other areas and water masses. The near bottom layer (0-200 m from the seabed) had typically low density of euphausiids. The MAR is generally deeper than the usual depth distribution of euphausiids (0-300 m), but many species conduct vertical migrations to ridge crest depths. Euphausiid distribution and abundance patterns appeared only weakly affected by the MAR.