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

Diel vertical migrations (DVM) of mesopelagic animals, organized in depth-discrete acoustic scattering layers (SLs) between the surface to the deep ocean, represent the largest mass movement of animalson 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 is a paucity of data to elucidate the fine-scale behavioral patterns of mesopelagic migrants during DVMs. Here, we studied the migration patterns of SLs using acoustic data collected during DEEPEND cruises in the Gulf of Mexico (GoM). We examined the characteristics of the migrating layers (timing, speed) connecting SLs during DVMs, and applied acoustic models to classify the dominant scatterers of the migrating mesopelagic nekton community to quantify their contributions to the ascending and descending threads. 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 GoM biological pump