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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 depth3,4,5,6,7. 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.

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Figure 1: Vertical migration as observed with acoustic data.
Figure 2: Observed daytime DVM depths (m) and surface to mesopelagic oxygen gradients (mmol m−3).
Figure 3: Simulated impact of DVM respiration on dissolved oxygen.


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The authors thank J. L. Sarmiento and J. P. Dunne for insightful comments on the project, P. Caldwell for kindly providing the JASADCP data and D. Balachandran for preliminary analysis. D.B. and E.D.G. were financially supported by the Canadian Institute for Advanced Research (CIFAR) Earth System Evolution Program. K.A.S. was financially supported by the Carbon Mitigation Initiative with support from BP and the NOAA Cooperative Institute for Climate Science. D.A.C. acknowledges the support of SSHRC of Canada. Computation resources were provided by the SciNet HPC consortium, the Canada Foundation for Innovation and Compute Canada.

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D.B. led the data and model analysis and wrote the paper. D.B., E.D.G, K.A.S., D.A.C. and C.A.S. contributed to the interpretation of the data and model results. D.B., E.D.G and D.A.C designed and performed the numerical simulations.

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Correspondence to Daniele Bianchi.

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The authors declare no competing financial interests.

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Bianchi, D., Galbraith, E., Carozza, D. et al. Intensification of open-ocean oxygen depletion by vertically migrating animals. Nature Geosci 6, 545–548 (2013).

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