Vertical distribution (0–1000 m) of macrozooplankton, estimated using the Underwater Video Profiler, in different hydrographic regimes along the northern portion of the Mid-Atlantic Ridge

The vertical distribution (0–1000 m depth) of macrozooplankton along the northern portion of the Mid-Atlantic Ridge (59°58N, 25°53W to 41°29N, 28°19W) was investigated during the MAR-ECO program (June and July 2004) using the Underwater Video Profiler (UVP). Twelve relatively large (>1 cm) groups were selected from the recorded images: sarcodines (with two sub-groups), crustaceans (excluding copepods), chaetognaths, ctenophores (with two sub-groups cydippids and lobates), siphonophores, medusae (with three sub-groups Aeginura grimaldii, Aglantha spp. and all other medusae), appendicularians, and thaliaceans. The numerically dominant groups over the whole area were crustaceans (26%), medusae (20%) and appendicularians (17%). The gelatinous fauna were consistently most numerous at 400–900 m. Appendicularians, ctenophores and Aeginura grimaldii occurred mostly below 300 m (maximum concentrations of 75, 58, and 30 individuals 100 m−3, respectively). Siphonophores, Aglantha spp. and the other medusae were more uniformly distributed in the water column (maxima of 42, 42 and 300 individuals 100 m−3, respectively). The macrozooplankton community below 200 m varied with the spatial distribution of the water masses, suggesting that the Sub-Polar Front restricts the mixing of macrozooplankton communities down to 1000 m depth.