Characterizing the deep sound scattering layer with a moored upward-looking broadband split-beam echosounder

Abstract

The deep scattering layer (DSL) is a ubiquitous feature of the global ocean. It consists of a large community of mesopelagic organisms which links the marine food web and has recently garnered much interest from commercial fisheries. Such biological communities are inherently coupled with oceanic physical processes such as mesoscale eddies, internal waves and boundary currents. However, very little is known about the physical consequences and biological responses in this coupled system. To address these questions, an upward-looking broadband split-beam echosounder (36kHz – 44 kHz) was moored at ∼580 m depth (bottom depth ∼2790 m) in the New England slope waters in July 2021. The time series analysis focuses on (1) volume scattering strength and spectrum shape and (2) target strength, layer density and compositions, and individual behavior in three dimensions. We demonstrate the seasonal variability in the biomass, distribution, and vertical migration patterns of the deep scattering layer, and quantify the temporal and spatial variability of the DSL under the influence of multiple oceanic physical processes, including warm core rings, semidiurnal internal tides, and near-inertial waves.