1. School of Oceanography, University of Washington, Seattle, Washington 98195, USA
2. Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA

Correspondence to: Stephen C. Riser¹Kenneth S. Johnson² Correspondence and requests for materials should be addressed to S.C.R. (Email: riser@ocean.washington.edu) or K.S.J. (Email: johnson@mbari.org).

Abstract

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 cycle^{1, 2}. Direct measurements in bottle experiments indicate net oxygen consumption in the sunlit zone^{3, 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 episodic^{1, 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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