What determines the vertical structuring of pelagic ecosystems in the global ocean?
Offshore pelagic ecosystems are composed of vertically and functionally distinct epipelagic, migrant and resident mesopelagic communities. While this vertical structure plays a key role in carbon sequestration and in supporting important fisheries, there is still no consensus on the respective contribution of the environmental factors (light, oxygen) and processes controlling it at both global and regional scale. Here we combine mechanistic modelling and acoustic observations from the worldwide Malaspina scientific campaign to show that, while underwater light intensity is the primary factor controlling the vertical distribution and migration of pelagic organisms globally, oxygen plays a critical role in limiting the depth of migratory communities and the abundance of mesopelagic communities in Oxygen Minimum Zones. Furthermore, we show that a faithful reproduction of acoustic observations in some regions of the global ocean (southern Indian Ocean, western Pacific) cannot be achieved without separating migratory and resident mesopelagic communities into deep and shallow groups. By proposing a unified mechanistic model and an archetypical ecosystem structure constrained by comprehensive acoustic observations, this study provides a consistent understanding of the vertical structure and function of global pelagic ecosystems and paves the way for more reliable estimates of their climate-induced variability and change.