Examining Complex Vertical Movements of Mesopelagic Scattering Layers: From Taxonomic-Based Migration Decisions to Global Biological Fluxes

Abstract

Diel vertical migrations (DVM) of mesopelagic animals, which organize in depth-discrete acoustic scattering layers (SLs) linking the surface and deep ocean, represent the largest mass movement of animals on the planet. DVMs are recognized to play a pivotal role in structuring ecological and physicochemical processes in oceanic ecosystems. Mass animal migrations reflect adaptive decisions made by organisms in response to spatiotemporal variations in resources, conferring foraging or reproductive advantages while reducing predation risk. However, there exists little data describing fine-scale behavioral patterns of mesopelagic migrants. Here, we describe the migration patterns of SLs using acoustic data collected in the Gulf of Mexico and examine the characteristics of the migrating layers connecting SLs during DVMs. Our results highlight the complex dynamics of these vertical migrations and reveal that DMVs, during both descending and ascending phases, are comprised of multiple threads differing in speed, length, and taxonomic composition; suggesting that different groups of mesopelagic organisms rely on different adaptive migration strategies. Predictions from an acoustic-based carbon flux model parameterized with DEEPEND catch data indicate that taxonomic categories greatly differ in their contribution to the vertical transport of carbon, with differences reaching several orders of magnitude, implying important biogeochemical consequences. Our study helps to better understand the dynamics of DMVs and their role in trophic interactions, vertical connectivity of food-webs, and Gulf of Mexico biological pump