Alternate Trophic Pathways Support Enhanced Bathypelagic Biomass Over a Mid-Ocean Ridge System

A classic paradigm of oceanic ecology is that pelagic animal biomass decreases exponentially with depth. Results of a muti-year study of the distribution and ecology of the pelagic fauna over the northern Mid-Atlantic Ridge (MAR), from Iceland to the Azores, revealed that water column biomass maxima can occur at deep meso- and bathypelagic depths (>750 m). Further, topographic association of the deep-pelagic fauna occurs at some locations. For example, bathypelagic fish abundance and biomass maxima were observed within the benthic boundary layer (<200 m above the bottom) during the 2004 G.O. Sars MAR-ECO expedition. Results of a pelagic food-web model over the MAR suggest that alternate trophic pathways contribute significantly to this deep biomass maxima. Consumption of decapod crustacea and gelatinous zooplankton represented major portions of the total consumption by pelagic fishes. Stable isotope analysis of 63 species, from zooplankton to large benthic predators, suggest short food chains and high trophic efficiency may account for enhanced deep-pelagic biomass.