Biogeography of the Global Ocean’s Mesopelagic Zone

Abstract

Summary 11 12The global ocean's near-surface can be partitioned into distinct provinces on the basis of regional 13 primary productivity and oceanography [1]. This ecological geography provides a valuable 14 framework for understanding spatial variability in ecosystem function, but has relevance only part and holds potentially huge fish resources [3][4][5]. It is, however, hidden from satellite observation, and 18 a lack of globally-consistent data has prevented development of a global-scale understanding. 19Acoustic Deep Scattering Layers (DSLs) are prominent features of the mesopelagic. These vertically-20 narrow (tens to hundreds of m) but horizontally-extensive layers (continuous for tens to thousands 21 of km) comprise communities of fish and zooplankton, and are readily detectable using 22 echosounders. We have compiled a database of DSL characteristics globally. We show that DSL and 23 acoustic backscattering intensity (a measure of biomass) can be modelled accurately using just 24 surface primary production, temperature and wind-stress. Spatial variability in these environmental 25 factors leads to a natural partition of the mesopelagic into ten distinct classes. These classes demark 26 a more complex biogeography than the latitudinally-banded schemes that have been proposed 27 before [6,7]. Knowledge of how environmental factors influence the mesopelagic enables future 28 change to be explored: we predict that by 2100 there will be widespread homogenisation of 29 mesopelagic communities, and that mesopelagic biomass could increase by c. 17%. The biomass 30 increase requires increased trophic efficiency, which could arise because of ocean warming and DSL 31shallowing. 32 33

Global biogeochemical provinces of the mesopelagic zone

“…This depth range does not account for spatial environmental or biological heterogeneity encountered over the global ocean and does not cover the full distribution of environmental or ecological features of deep‐pelagic ecosystems. For instance, vertical ranges of deep oxygen minimum zones, oceanic equatorial upwelling zones, deep nutrient pumping, and subtropical and subpolar frontal systems are not fully covered by the 200–1,000 m range (Stramma, Johnson, Sprintall, & Mohrholz, ; Falkowski, Barber, & Smetacek, ; Proud et al., ; Sutton et al., [see Figure ]). In this context, we have proposed to revisit the depth range used to partition the water column using an empirical methodology that considers the vertical environmental variability.…”

“…This vertical change in environmental conditions also drives changes in marine community assemblage structure (Reygondeau et al, 2012;Robinson et al, 2010;Steinberg, Cope, Wilson, & Kobari, 2008;Stemmann et al, 2008). However, some recent studies have characterized local biotic and/or abiotic features to identify ecological deep-water provinces at a macroscale (Proud et al, 2017;Steinberg et al, 2008;Sutton et al, 2017).…”

A Multidisciplinary Approach for Generating Globally Consistent Data on Mesophotic, Deep-Pelagic, and Bathyal Biological Communities

“…The plethora of different sampling methods required to obtain a comprehensive assessment of biodiversity in the pelagic zone requires prioritizing the taxa. Growing databases of fisheries acoustic data (Proud et al, 2017) and net sample data (Sutton et al, 2017) suggest biogeographic structure in the deep pelagic. These biogeographic patterns are not the same as those observed in surface water (e.g., Longhurst, 2007), which is hardly surprising given ocean currents, the sinking of surface production, and the potential connectivity of meso pelagic populations.…”

“…These biogeographic patterns are not the same as those observed in surface water (e.g., Longhurst, 2007), which is hardly surprising given ocean currents, the sinking of surface production, and the potential connectivity of meso pelagic populations. Nonetheless, temperature and wind stress can accurately predict depth and backscattering intensity (a proxy for biomass) of deep-scattering layers (Proud et al, 2017).…”

An exploratory study of heterotrophic protists of the mesopelagic Mediterranean Sea

“…With regard to the biological component, the taxonomic composition of fish and metazoan fauna has been described as relatively impoverished in terms of species richness (Sutton et al, 2017). However, mesopelagic fish biomass appears to be comparatively high based on the characteristics of the deepscattering layer, from which fish biomass has been modelled (Proud et al, 2017). With regard to microbes, the metabolic activity of prokaryotes appears to be higher for mesopelagic Mediterranean populations compared with those found at similar depths in other systems, related possibly to the relatively warm temperature of the deep waters, 13 C (Luna et al, 2012).…”