Award: OCE-1737080

     The net flux of biologically-produced organic carbon from the euphotic zone of the ocean (the net community production, NCP, or biological organic carbon pump) controls the CO₂ content of the atmosphere and oxygen content of the ocean on decadal to centennial time scales.  The global NCP is of the same magnitude as the flux of anthropogenic CO₂ from land to the atmosphere by fossil fuel burning.  In order to predict the influence of growing atmospheric anthropogenic CO₂ on climate and ocean oxygen concentration one must understand the value of ocean NCP fluxes.  Remote sensing measurements of ocean color via satellite indicate that waters in the Eastern Equatorial Pacific are much more productive than they are in the subtropical regions to the north and south.  Even though biological fluxes in this region of the ocean are thought to be strong, there have been few measurements to verify this because long-term, time-series measurements of metabolic tracers are difficult to do.  However, relatively new methods of in situ oxygen measurements on profiling floats are accurate enough to capture the effects of seasonal biological changes of oxygen.  Our goal in this study was to determine the NCP in the Equatorial Pacific using time dependent measurements of oxygen which are stoichiometrically related to changes in organic carbon.  These data are interpreted in terms of NCP fluxes by simple O₂ mass balance and a three dimensional circulation ocean model.  

     We deployed 16 profiling floats with well-calibrated O₂ sensors and a lifetime of ~4 years in the region between 3 and 6 degrees latitude in both north and southern hemispheres.  The floats travel a lot in this area because of ocean circulation, but near one-dimensionality can be assumed because trajectories are strongly zonal with much smaller meridional transport.  By capturing latitudinal and temporal changes in oxygen we are able to show from both simple O₂ mass balance and three-dimensional model results that net biological O₂ production was consistent with NCP values of 4 ± 1 mol C m^-2 yr^-1 in the latitude range of 3-6 degrees and then decreases to about 1 mol C m^-2 yr^-1 in the subtropical gyres.  It was not possible to determine NCP on the Equator (± 2 degrees latitude) using oxygen mass balance because oxygen flux values are overwhelmed by physical processes of upwelling and equatorial eddies in this location.  Three-dimensional ocean model results within two degrees of the Equator suggest NCP values that are greater than twice those between 3 and 6 degrees latitude, indicating the strength of biological processes on the Equator, where it is difficult to measure this flux. 

Last Modified: 01/21/2024
Modified by: Steven R Emerson