Biological Oxygen Production Across 8000 km of the South Atlantic: Basin Scale Similarity but Mesoscale Variability

Evan M Howard1,2, Rachel HR Stanley1,3, Gwenn Hennon4 and Colleen A Durkin5, (1)WHOI, Woods Hole, MA, United States, (2)Massachusetts Institute of Technology, Earth and Planetary Science, Cambridge, MA, United States, (3)Wellesley College, Chemistry, Wellesley, MA, United States, (4)University of Washington, School of Oceanography, Seattle, WA, United States, (5)Moss Landing Marine Laboratories, Moss Landing, CA, United States
Abstract:
Two important components of the biological carbon pump are (1) gross primary production (GPP), which is equal to total photosynthetic production, and (2) net community production (NCP), which expresses the net carbon drawdown or oxygen production by the biological pump. NCP should be balanced by export of carbon over sufficient scales of time and space. The ratio of net to gross production (NCP/GPP) is similar to the export ratio defined by Laws et al. (2000) and reflects how tightly carbon is being recycled in the ecosystem. We use in situ oxygen tracers (triple oxygen isotope and oxygen:argon ratios) to evaluate meridional trends in surface mixed-layer GPP and NCP across ~8000 km of the tropical and subtropical South Atlantic in March-May 2013. These data encompass biogeographic provinces for which there are few published in situ estimates of phytoplankton production.

NCP is surprisingly uniform along most of the cruise transect with slightly autotrophic values typically ranging from 5 to 10 mmol O2 m-2 d-1. GPP generally follows the same trends as NCP such that the NCP/GPP ratio is typically 0.1 to 0.2. However, notable exceptions to this lack of variability include two stations which have highly enhanced NCP and NCP/GPP ratio compared to the surrounding region. Additionally, the transition region between the subpolar and subtropical gyres has significantly higher NCP than elsewhere along the transect. Particulate organic carbon fluxes from the same cruise indicate that regionally, only about a third of the NCP is balanced by particulate export below the mixed layer.

Overall these results highlight the large-scale similarity of rates of biological production over large regions of the Atlantic Ocean (scale of thousands of kilometers), overlain with high variability on the scale of hundreds of kilometers. This intriguing increase in variability on small scales rather than large ones supports the growing literature that mesoscale and/or submesoscale processes may be key controls on the biological carbon pump.

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