Modelling oxygen and argon to improve estimation of net community productivity in a coastal upwelling zone using ∆O2/Ar




Teeter, Lianna

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Under steady state conditions where the rate of biological oxygen production is balanced by oxygen evasion to the atmosphere, net community production (NCP) can be estimated from mixed layer oxygen/argon measurements. This method is effective in the open ocean but not in coastal zones where upwelling of low oxygen water violates the simple steady state assumption. Since these upwelling regions are highly productive, excluding them can lead to significant underestimations of global productivity. Here, I use a quasi-2D version of the Regional Ocean Modelling System (ROMS), including oxygen and argon as prognostic variables, to model the relationship between NCP and the sea-to-air flux of biological oxygen in a coastal upwelling system. The relationship between the sea-to-air flux of biological oxygen and NCP is poorest near the shore during upwelling favourable winds when waters that are undersaturated in oxygen reach the surface and depress the oxygen/argon ratio. I averaged NCP temporally and spatially over the residence time with respect to gas exchange and the Lagrangian motion of a water parcel. I found that the maximum distance travelled (∼25 km) over this time period indicated a distance from the upwelling plume at which much of the the low oxygen signal is erased. When the sea-to-air flux of biological oxygen was below 20 mmol m−2 day−1, NCP was usually also found in that range. Above that range the sea-to-air flux of biological oxygen is a lower bound for NCP. NCP occurring below the mixed layer can affect the sea-to-air flux of biological oxygen either by entrainment or diffusion into the mixed layer causing an overestimation of NCP, but this process had a minimal effect on most of my model data. Removing values with mixed layers deeper than 25 m improves the estimation, although further studies may reveal that this depth should be adjusted based on mean wind forcing.



oxygen, argon, oxygen/argon, NCP, net community productivity, productivity, primary productivity, dissolved gasses, modelling, ROMS