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

Thor A Klevjer,Webjørn Melle,Tor Knutsen,Dag Lorents Aksnes

doi:10.1016/j.dsr2.2020.104811

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 primarily attributable to optical conditions there, as high nighttime light levels appeared to exclude the micronekton from the epipelagic zone.

Highlights

The connectivity between the upper and the lower ocean is strongly influenced by patterns of vertical biomass structure and involve animals performing interzonal diel vertical migration (DVM)
Some studies have concluded that the contribution of mesopelagic organisms to overall vertical transport of carbon is relatively small (Longhurst and Glen Harrison, 1989), while other studies suggest that the fish-mediated carbon flux to the deep sea is potentially huge (Davison et al, 2013; Irigoien et al, 2014; Klevjer et al, 2016)
In NS, ICS and IRS temperatures increased towards the surface, whereas LS was characterized by fresher and colder tem peratures towards the surface (Fig. 2)

Summary

Introduction

Some studies have concluded that the contribution of mesopelagic organisms to overall vertical transport of carbon is relatively small (Longhurst and Glen Harrison, 1989), while other studies suggest that the fish-mediated carbon flux to the deep sea is potentially huge (Davison et al, 2013; Irigoien et al, 2014; Klevjer et al, 2016). While a large portion of this uncertainty is caused by uncertainties in mesopelagic biomasses (Kaartvedt et al, 2012; Irigoien et al, 2014; Proud et al, 2018), increased knowledge of large-scale patterns in migratory behaviour (Klevjer et al, 2016) and amplitudes (Bianchi et al, 2013) are required to determine the fish-mediated carbon flux

Methods

Results

Conclusion