Intensification of open-ocean oxygen depletion by vertically migrating animals

Throughout the ocean, countless small animals swim to depth in the daytime, presumably to seek refuge from large predators1,2. These animals return to the surface at night to feed1,2. This substantial diel vertical migration can result in the transfer of significant amounts of carbon and nutrients from the surface to depth3–7. However, its consequences on ocean chemistry at the global scale have remained uncertain8,9. Here, we determine the depths of these diel migrations in the global ocean using a global array of backscatter data from acoustic Doppler current profilers, collected between 1990 and 2011. We show that the depth of diel migration follows coherent large-scale patterns. We find that migration depth is greater where subsurface oxygen concentrations are high, such that seawater oxygen concentration is the best single predictor of migration depth at the global scale. In oxygen minimum zone areas, migratory animals generally descend as far as the upper margins of the low-oxygen waters. Using an ocean biogeochemical model coupled to a general circulation model, we show that by focusing oxygen consumption in poorly ventilated regions of the upper ocean, diel vertical migration intensifies oxygen depletion in the upper margin of oxygen minimum zones. We suggest that future changes in the extent of oxygen minimum zones could alter the migratory depths of marine organisms, with consequences for marine biogeochemistry, food webs and fisheries. The concentrations of dissolved oxygen in the ocean are determined by the interplay of the three-dimensional circulation and a wide range of ecosystem processes. The complexity of the marine ecosystem is overwhelmingly evident from the vast diversity of oceanic life, but this complexity must be greatly idealized in any attempt to quantify its role in ocean biogeochemistry. Among the typical simplifications of ocean biogeochemistry models, the respiration of organicmatter in the subsurface is generally treated as the work of ubiquitous microbes feeding on organic particles that settle passively through the water column, ignoring the potential role of animals in the transport and respiration of organic matter. However, the biomass of animals is large (>2.5 Pg; ref. 10), comparable to that of phytoplankton, and their motility makes their effect potentially very different from that of passive microbes. Although animals exhibit a daunting array of behaviours in the marine environment, many of which remain poorly understood, a commonobserved behaviour is the diel verticalmigration (DVM). The pervasive occurrence of DVM has long been recognized by the oceanographic community. Increased night-time catches near the surface became evident in oceanographic surveys of the early twentieth century and provided the first documented evidence of DVM (ref. 11). Extensive acoustic surveys subsequently revealed sound scattering layers that migrated on a diurnal basis between the surface andmid-depths1,12, and it became clear thatmesopelagic