Vertical distribution and migration of mesopelagic scatterers in four north Atlantic basins

Abstract

We studied vertical distribution and diel vertical migration (DVM) behaviour of mesopelagic acoustic scattering layers in relation to environmental conditions in the Norwegian Sea, the Iceland Sea, the Irminger Sea, and the Labrador Sea. Distinct mesopelagic scattering layers were found in all basins, but the daytime depth of the layers varied between basins. The results suggested that daytime vertical distribution across the four basins are strongly influenced by optical conditions. DVM occurred in all basins, and since daytime vertical distribution was influenced by optical conditions, it affected the amplitude of vertical connectivity. We used the proportion of the acoustic backscatter that migrated vertically into the epipelagic zone as a proxy for active vertical flux to the mesopelagic. The proportion of micronekton backscatter participating in the vertical migrations varied between the basins, with the highest and lowest vertical connectivity in the Norwegian Sea and the Iceland Sea respectively. We conclude that a more than 8-fold reduction in backscatter flux in the Iceland Sea was attributable to optical conditions there.