Abstract
Records of backscatter and vertical velocity obtained from moored Acoustic Doppler Current Profilers (ADCP) enabled new insights into the dynamics of deep scattering layers (DSLs) and diel vertical migration (DVM) of mesopelagic biomass between these deep layers and the near-surface photic zone in the southern Norwegian Sea. The DSL exhibits characteristic vertical movement on inter-monthly time scales, which is associated with undulations of the main pycnocline between the warm Atlantic water and the underlying colder water masses. Timing of the DVM is closely linked to the day-night light cycle—decent from the photic zone just before sunrise and ascent immediately after sunset. Seasonal variations are also evident, with the highest DVM activity and lowest depth averaged mean volume backscatter strength (MVBS) during spring. This suggests that both oceanographic and optical conditions are driving the complex dynamics of pelagic and mesopelagic activity in this region. We hypothesize that the increased abundance of calanoid copepods in the near-surface layer during spring increases the motivation for vertical migration of pelagic and mesopelagic species, which therefore can explain the increased DVM activity during this season.
Highlights
A widespread characteristic behavioral pattern of a variety of marine species, including zooplankton, mesopelagic micronekton and planktivorous and piscivorous fish, is their diel vertical migration (DVM) and subsequent distributional change over 24 h (Neilson and Perry, 1990)
We focus on the southern Norwegian Sea, which is dominated by warm and saline Modified North Atlantic Water (MNAW) entering the area from southwest, and cold and less saline East Icelandic Water (EIW) flowing from the northwest (Figure 1A)
Based on two 1 year records of data retrieved from a self-contained Acoustic Doppler Current Profilers (ADCP) moored near the core of the Faroe Current on section N from June 2013–May 2015 (Figure 1), we present the seasonal variation of the vertical velocity and the mean volume backscattering strength (MVBS) at a 20 min resolution
Summary
A widespread characteristic behavioral pattern of a variety of marine species, including zooplankton, mesopelagic micronekton and planktivorous and piscivorous fish, is their diel vertical migration (DVM) and subsequent distributional change over 24 h (Neilson and Perry, 1990). This DVM, conducted by marine taxa in the epipelagic (0–200 m) and the mesopelagic (200– 1,000 m) zones in the ocean (Sutton, 2013; Klevjer et al, 2016) is potentially the largest animal migration on Earth in terms of biomass (Hays, 2003). This is mainly due to realization that active carbon transport which is mediated by zooplankton and nekton vertical migrations has to be parameterized in global biogeochemical models (Bianchi et al, 2013)
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