The balance of plankton respiration and photosynthesis in the open oceans

Abstract

Approximately half of plant production occurs in the oceans. As oceans are open systems, a degree of imbalance between biological production and consumption can, in principle, be sustained by the import or export of organic material. Deficits in the overall oceanic budget of organic matter must be made up by import from terrestrial, freshwater and estuarine ecosystems, mainly as river-borne material. As these inputs occur at the periphery of the ocean, their contribution is largely restricted to continental-shelf waters1. But it has been calculated2 — using discrete in vitro observations — that in environments where net carbon fixation rates are low, respiration exceeds photosynthesis, therefore leaving the system with an organic carbon deficit. Such areas would include the central oligotrophic parts of the oceans, and it is difficult to envisage that such imbalances in these remote areas could be sustained by organic-matter import. Here I use an analysis of depth-integrated measures of production and respiration from five open-ocean regions to show that, in the upper 100 m of the water column, biological production generally exceeds consumption. This excess is sufficient to sustain estimated organic-matter export out of these surface waters, consistent with the conclusion from simple mass-balance calculations1 that the open oceans as a whole are not substantially out of organic carbon balance. There is no evidence of the large regional imbalances observed previously2. I conclude that the form of data analysis is critical.