Transport bias in ΔO2/Ar-based NCP in dynamic coastal regions

Net community production (NCP) represents the amount of biologically-produced organic carbon that is available to be exported out of the surface ocean and is often estimated using in-situ measurements of the O2/Ar ratio in the surface mixed layer. However, physical processes that introduce low-oxygen waters into the surface ocean can significantly bias NCP estimates. Furthermore, without an additional measure of export across the euphotic depth, it is difficult to determine whether discrepancies between NCP and export across deeper horizons are due to differences in the location of biological production and export, or the result of physical biases.

In a recent paper in JMS, my colleague John Fleming and I used an advection-diffusion-reaction model of DOC consumption beneath the euphotic depth in the Southern California Bight to show that ΔO2/Ar-based NCP agreed well with DOC-based euphotic zone export when corrected for physical biases, but was significantly different than NCP estimated without a correction for physical transport because the transport terms accounted for ~84% of the ΔO2/Ar signal.

Many of the world’s most productive regions, like coastal California, are physically dynamic and thus the application of the ΔO2/Ar-based approach to estimate NCP can be limited in these regions unless the transport signal is well constrained to mitigate this bias.