Derivation of seawater p CO 2 from net community production identifies the South Atlantic Ocean as a CO 2 source

• Published in: Biogeosciences
• Main Authors: D. J. Ford, G. H. Tilstone, J. D. Shutler, V. Kitidis
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2022
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; South Atlantic Ocean
• Online Access: https://doi.org/10.5194/bg-19-93-2022; https://doaj.org/article/8bc734bd79e24c3bb9ceb801385d02ef

A key step in assessing the global carbon budget is the determination of the partial pressure of CO 2 in seawater ( p CO 2 (sw) ). Spatially complete observational fields of p CO 2 (sw) are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of p CO 2 (sw) using satellite observations. As part of this process, satellite chlorophyll a (Chl a ) is often used as a proxy for the biological drawdown or release of CO 2 . Chl a does not, however, quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP). In this study, p CO 2 (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 produces the most accurate fields of p CO 2 (sw) . Estimates of p CO 2 (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. A perturbation analysis assessed the potential maximum reduction in p CO 2 (sw) uncertainties that could be achieved by reducing the uncertainties in the satellite biological parameters. This illustrated further improvement using NCP compared to NPP or Chl a . Using NCP to estimate p CO 2 (sw) showed that the South Atlantic Ocean is a CO 2 source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region appears to be a sink for CO 2 . These results highlight that using NCP improved the accuracy of estimating p CO 2 (sw) and changes the South Atlantic Ocean from a CO 2 sink to a source. Reducing the uncertainties in NCP derived from satellite parameters will ultimately improve our understanding and confidence in quantification of the global ocean as a CO 2 sink.