Distribution, identity, and possible processes sustaining meso- and bathypelagic scattering layers on the northern Mid-Atlantic Ridge

As an element of comprehensive exploratory studies of the poorly known pelagic ecosystem associated with the Mid-Atlantic Ridge, acoustic data collected by R.V. G.O. Sars during the MAR-ECO expedition in June–July 2004 were used to describe vertical and geographical distributions of meso- and bathypelagic scattering layers. Scattering layers were observed in the entire study area and at all depths to 3000 m with a split-beam echosounder using the 18-kHz drop-keel mounted transducer. The spatial variation in density of surface to 1500 m area backscattering strength, and that of individually identifiable vertical layers, is described in relation to bathymetry and large-scale hydrographical features such as the Sub-polar Front. The inhabitants of scattering layers were identified by exploratory midwater trawling. A major pattern was the elevated backscattering associated with waters around the Charlie-Gibbs Fracture Zone (CGFZ) and the associated Sub-polar Front at and south of 52°N. In this section of the Mid-Atlantic Ridge, the interaction between topographical features, circulation and primary production appears to create favorable conditions for many taxa across all trophic levels. By comparison, the density was lower both to the south of the CGFZ towards the Azores and in particular northwards towards Iceland. Along the north–south gradient a peak in near-surface algal biomass was also found near the CGFZ, with lower values further north and south. We hypothesize that fish productivity is bottom-up regulated, i.e., limited by primary production within the ridge-associated system, supplemented by advection of allochthonous material, especially in the south.