Biological Productivity and Air-Sea Gas Exchange in a Coastal Upwelling Zone from Oxygen Isotopes and Noble Gases

We have quantified biological productivity and air-sea gas exchange during a six-day quasi-Lagrangian experiment in Monterey Bay, CA, using in situ gas tracers. Net community productivity was quantified from continuous mixed layer measurements of the O₂/Ar ratio (using an underway mass spectrometer) and vertical profiles of O₂ concentration and O₂/Ar. Gross primary productivity was quantified from over 150 discrete measurements of the triple oxygen isotope composition of O₂. A potential disadvantage of these methods, particularly in upwelling regions, is that inaccurate parameterization of the physically driven fluxes of O₂ can bias productivity estimates. To detect upwelling and mixing events and to quantify air-sea gas exchange we collected measurements of the five stable noble gases throughout the cruise. To account for vertical fluxes of the gases, we used profiles of diapycnal diffusivity quantified from ~6-hourly microstructure shear probe profiles. We developed a vertical model and used it to evaluate the skill of four published gas exchange parameterizations in simulating the observed mixed layer noble gas distributions. After adjusting the modeled mixed layer gas distributions for an upwelling event partway through the cruise, we found that an air sea gas exchange parameterization that explicitly represented bubble processes and was grounded in inert gas data gave the best fit for the less soluble gases He and Ne. We then used the best-fit gas exchange parameterization and our vertical model in conjunction with the O₂/Ar and O₂ isotope data, in order to quantify rates of net community and gross primary production in this coastal system. Additionally, we observed diurnal changes in the gas tracers (and therefore diurnal changes in net community and gross primary production) throughout the cruise. The amplitude of these oscillations increased following the upwelling event, suggesting that rates of photosynthesis and respiration both increased. We have also compared our in situ gas tracer-based estimates of productivity with concurrent sediment trap fluxes of particulate organic carbon and nitrogen, and incubation-based measurements of the uptake of ¹⁵NH₄⁺ (regenerated production), ¹⁵NO₃^- (new production), and ¹⁴C-DIC (net primary production).