Annual flux of dissolved organic carbon from the euphotic zone in the northwestern Sargasso Sea

Abstract

THE export of biogenic carbon from the upper ocean is responsible for maintaining the vertical gradient of dissolved inorganic carbon and thus indirectly for regulating the level of atmospheric CO2 (ref. 1). Large, rapidly sinking particles are thought to dominate this export2, and this sinking flux has been thought to balance new production3. Recent measurements of particle export4—6 and estimates of new production7—9 have questioned this picture, however. Here we report measurements of dissolved organic carbon (DOC) off Bermuda, which provide strong support for the idea10—15 that this component of oceanic carbon is also an important and dynamic part of the ocean carbon cycle. We find that DOC accumulates in the early spring owing to increased primary production, and is partially consumed in the summer and autumn. The DOC that escapes remineralization is exported from the surface ocean the following winter, and we estimate this export to be equal to or greater than the measured particle flux, allowing us to close the annual vertical carbon budget for this site to within a factor of two. Our observations should be applicable to other temperate, sub-polar and continental-shelf regions of the world ocean which exhibit convective mixing and vernal restratification.