02 Aug 2021

02 Aug 2021

Review status: a revised version of this preprint is currently under review for the journal BG.

Derivation of seawater pCO2 from net community production identifies the South Atlantic Ocean as a CO2 source

Daniel Ford1,2, Gavin H. Tilstone1, Jamie D. Shutler2, and Vassilis Kitidis1 Daniel Ford et al.
  • 1Plymouth Marine Laboratory, Plymouth, UK
  • 2College of Life and Environmental Sciences, University of Exeter, UK

Abstract. A key step in assessing the global carbon budget is the determination of the partial pressure of CO2 in seawater (pCO2 (sw)). Spatially complete observational fields of pCO2 (sw)  are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of pCO2 (sw) using satellite observations. Within these schemes, satellite chlorophyll a (Chl a) is often used as a proxy for the biological drawdown or release of CO2. Chl a does not however quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP).

In this study, pCO2 (sw) over the South Atlantic Ocean is estimated using a feed forward neural network (FNN) scheme and either satellite derived NCP, net primary production (NPP) or Chl a to compare which biological proxy is the most accurate. Estimates of pCO2 (sw)  using NCP, NPP or Chl a were similar, but NCP was more accurate for the Amazon Plume and upwelling regions, which were not fully reproduced when using Chl a or NPP. Reducing the uncertainties in the satellite biological parameters to estimate pCO2 (sw), illustrated further improvement and greater differences for NCP compared to NPP or Chl a. Using NCP to estimate pCO2 (sw) showed that the South Atlantic Ocean is a CO2 source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region becomes a sink for CO2. These results highlight that using NCP improved the accuracy of estimating pCO2 (sw), and changes the South Atlantic Ocean from a CO2 sink to a source. Reducing the uncertainties in NCP derived from satellite parameters will further improve our ability to quantify the global ocean CO2 sink.

Daniel Ford et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-171', Jonathan Sharp, 19 Aug 2021
    • AC1: 'Reply on RC1', Daniel Ford, 05 Oct 2021
  • RC2: 'Comment on bg-2021-171', Anonymous Referee #2, 31 Aug 2021
    • AC1: 'Reply on RC1', Daniel Ford, 05 Oct 2021

Daniel Ford et al.

Daniel Ford et al.


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Short summary
This study identifies the most accurate biological proxy for the estimation of seawater pCO2 fields, which are key to assessing the ocean carbon sink. Our analysis shows that the net community production (NCP), the balance between photosynthesis and respiration, was more accurate than chlorophyll-a within a neural network scheme. The improved pCO2 estimates, based on NCP, identified the South Atlantic Ocean as a net CO2 source, compared to a CO2 sink using chlorophyll-a.