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

Abstract

New baseline information is presented on biogeography, abundance and distri-bution of euphausiids from discrete depth samples collected throughout the watercolumn to 3000 m at 42 locations along the Mid-Atlantic Ridge (MAR) betweenIceland and the Azores. Eighteen species were recorded, with Euphausia krohni andThysanoessa longicaudata being most abundant. Eight species had not been recordedin the area previously. The Subpolar Front is a northern boundary to somesouthern species, but not a southern boundary to northern ubiquitous species thatshow submergence. Four major species assemblages were identified and character-ized in terms of spatial distribution and species composition. Numerical abun-dance was highly variable but decreased logarithmically with depth. The greatestaverage abundance and the greatest spatial variation in abundance occurred innorthern waters, over the Reykjanes Ridge. Abundance declined less with depth inthe sub-Arctic Intermediate Water/North Atlantic Current Water than in otherareas and water masses. The near bottom layer (0–200 m from the seabed) hadtypically low density of euphausiids. The MAR is generally deeper than the usualdepth distribution of euphausiids (0–300 m), but many species conduct verticalmigrations to ridge crest depths. Euphausiid distribution and abundance patternsappeared only weakly affected by the MAR.KEYWORDS: zooplankton; Euphausiacea; oceanic; deepwater; biogeography;communities; Mid-Atlantic Ridge