Fine-scale depth structure of pelagic communities throughout the global ocean based on acoustic sound scattering layers

Abstract

Most biomass in the mesopelagic zone (200-1000 m) comprises zooplankton and fish aggregated in layers known as sound scattering layers (SSLs; they scatter sound and are detectable using echosounders). Some of these animals migrate vertically to and from the near surface on a daily cycle (diel vertical migration; DVM), transporting carbon between the surface and the deep ocean (biological carbon pump; BCP). To gain insight to potential global variability in the contribution of SSLs to the BCP, and to pelagic ecology generally (SSLs are likely prey fields for numerous predators), we report here regional-scale (90000 km2) community depth structure based on the fine-scale (10s of m) vertical distribution of SSLs. We extracted SSLs from a near-global dataset of 38 kHz echosounder observations and constructed local (300 km by 300 km) SSL depth and echo intensity (a proxy for biomass) probability distributions. The probability distributions fell into six spatially coherent regional-scale SSL probability distributions (RSPDs). All but one RSPD exhibited clear DVM, and all RSPDs included stable night-time resident deep scattering layers (DSLs; SSLs deeper 2 than 200 m). Analysis of DSL number and stability (probability of observation at depth) revealed 2 distinct DSL types: 1.) Single-Shallow DSL (single DSL at c. 500 m), and 2.) Double-Deep DSL (two DSLs at c. 600 and m). By including consideration of this fine-scale depth structure in biogeographic partitions and ecosystem models, we will better understand the role of mesopelagic communities in pelagic food-webs and consequences for them of climate change.

Swimbladder morphology masks Southern Ocean mesopelagic fish biomass

“…590 m per cycle) [33], to enable them to forage on abundant near-surface zooplankton at night, while avoiding shallow-water predators during daytime [34]. However, at extreme polar latitudes, DVM is apparently reduced relative to lower latitude habitats [35]. A key underlying factor could be a poleward shift in the light environment, which is known to be an important stimulus of DVM behaviour [36].…”

Energy-Rich Mesopelagic Fishes Revealed as a Critical Prey Resource for a Deep-Diving Predator Using Quantitative Fatty Acid Signature Analysis

“…The estimated diets in this study, with seals consuming both migrating and non-migrating fishes, are also consistent with the documented foraging behavior of female northern elephant seals observed with time depth recorders. Female seals display a marked day/night pattern in their foraging dives (mean day: ∼620 m, mean night: ∼450 m), as well as a diurnal bimodal dive distribution (modes: 385 m and 641 m, Robinson et al, 2012;Naito et al, 2017), which mirrors the vertical distribution of the deep scattering layers elephant seals forage upon (Frost and McCrone, 1979;Klevjer et al, 2016;Proud et al, 2018). Migrating mesopelagic fishes, like myctophids, rise from depths of ∼300-700 m during the day to < 20 m at night (Watanabe et al, 1999;Brodeur and Yamamura, 2005).…”

Mesopelagic Sound Scattering Layers of the High Arctic: Seasonal Variations in Biomass, Species Assemblage, and Trophic Relationships

“…Sound scattering layers (SSL) appear on active acoustic echograms as extensive echoes above background noise resulting from aggregations of organisms in the water-column (Proud et al, 2015). Mesopelagic SSL composed of a mixture of macrozooplankton and small pelagic fish are ubiquitous in the world's oceans (Irigoien et al, 2014;Proud et al, 2018). The global biomass of mesopelagic fish is estimated to 10 billion metric tons (Irigoien et al, 2014;St.…”