The question of whether the plankton communities in low-nutrient regions of the ocean, comprising 80% of the global ocean surface area, are net producers or consumers of oxygen and fixed carbon is a key uncertainty in the global carbon cycle1,2. Direct measurements in bottle experiments indicate net oxygen consumption in the sunlit zone3,4,5,6, whereas geochemical evidence suggests that the upper ocean is a net source of oxygen2. One possible resolution to this conflict is that primary production in the gyres is episodic1,2,6 and thus difficult to observe: in this model, oligotrophic regions would be net consumers of oxygen during most of the year, but strong, brief events with high primary production rates might produce enough fixed carbon and dissolved oxygen to yield net production as an average over the annual cycle. Here we examine the balance of oxygen production over three years at sites in the North and South Pacific subtropical gyres using the new technique of oxygen sensors deployed on profiling floats. We find that mixing events during early winter homogenize the upper water column and cause low oxygen concentrations. Oxygen then increases below the mixed layer at a nearly constant rate that is similar to independent measures of net community production. This continuous oxygen increase is consistent with an ecosystem that is a net producer of fixed carbon (net autotrophic) throughout the year, with episodic events not required to sustain positive oxygen production.
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We thank N. Larson for producing the oxygen sensors; D. Swift for his essential contributions to this effort; and the Hawaii Ocean Time-series participants for making dissolved oxygen data available. Research at the University of Washington was supported through the US Argo Program by the National Oceanographic and Atmospheric Administration and by the US Office of Naval Research through the National Ocean Partnership Program. Research at Monterey Bay Aquarium Research Institute was supported by a grant from the David and Lucile Packard Foundation and by the National Science Foundation.
Author Contributions S.C.R. originated the idea of putting oxygen sensors on profiling floats, and directed the construction and deployment of the floats as part of the international Argo project. K.S.J. performed the data analysis. Both authors contributed to the writing of the manuscript.
All float data are available from the global Argo data center at ftp://usgodael.fnmoc.navy.mil/pub/outgoing/argo/.
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Riser, S., Johnson, K. Net production of oxygen in the subtropical ocean. Nature 451, 323–325 (2008). https://doi.org/10.1038/nature06441
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