Figure 3.

Utilities and uncertainties of our semi-analytical diagnostic framework in OceMar and RiOMar regimes. (a) Known OceMars and RiOMars, with upward and downward arrows indicating sources and sinks of atmospheric CO₂, respectively. (b) Comparison between predicted pCO₂ based on different nutrient species and observed pCO₂ in the South China Sea basin (red, two seasons) and the Arabian Sea basin (pink, five seasons). The diagonal line indicates a perfect fit between the diagnosed and observed values, and vertical bars represent one standard deviation (1 SD) of the diagnosed pCO₂ values, which are largely due to the spatial variability of the observations. The consistency between CO₂ source or sink diagnoses based on NO₃ or PO₄ demonstrates insensitivity to the choice of nutrient species. (c) As in (b), but for the Pearl River plume (green, two seasons); in both seasons, our framework identifies an addition of P that is not accounted for by our observations but needs to be considered to bring the predicted pCO₂ to that based on NO₃. The added P is likely transformed from dissolved organic phosphorus (DOP). (d) As in (b), but for the Amazon River plume (blue, one season), which our framework diagnoses as a source, opposite to the observed sink. Under the Redfield stoichiometry, even with unaccounted-for N or P added, our framework still predicts a sea surface pCO₂ (blue) higher than the observed air pCO₂ (grey line) but, under a non-Redfield stoichiometry, the same added nutrients lead to a predicted pCO₂ (orange) that is close to or within the uncertainty range due to spatial variability (grey shadow) of the observed average sea surface pCO₂ (black line).