Lagrangian Studies of Net Community Production: The Effect of Diel and Multiday Nonsteady State Factors and Vertical Fluxes on O₂/Ar in a Dynamic Upwelling Region

The ratio of dissolved oxygen to argon in seawater is frequently employed to estimate rates of net community production (NCP) in the oceanic mixed layer. The in situ O₂/Ar-based method accounts for many physical factors that influence oxygen concentrations, permitting isolation of the biological oxygen signal produced by the balance of photosynthesis and respiration. However, this technique traditionally relies upon several assumptions when calculating the mixed-layer O₂/Ar budget, most notably the absence of vertical fluxes of O₂/Ar and the principle that the air-sea gas exchange of biological oxygen closely approximates net productivity rates. Employing a Lagrangian study design and leveraging data outputs from a regional physical oceanographic model, we conducted in situ measurements of O₂/Ar in the California Current Ecosystem in spring 2016 and summer 2017 to evaluate these assumptions within a “worst-case” field environment. Quantifying vertical fluxes, incorporating nonsteady state changes in O₂/Ar, and comparing NCP estimates evaluated over several day versus longer timescales, we find differences in NCP metrics calculated over different time intervals to be considerable, also observing significant potential effects from vertical fluxes, particularly advection. Additionally, we observe strong diel variability in O₂/Ar and NCP rates at multiple stations. Our results reemphasize the importance of accounting for vertical fluxes when interpreting O₂/Ar-derived NCP data and the potentially large effect of nonsteady state conditions on NCP evaluated over shorter timescales. In addition, diel cycles in surface O₂/Ar can also bias interpretation of NCP data based on local productivity and the time of day when measurements were made.