LAUSR

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, permiting isolation of the biological oxygen signal produced by the balance of photosynthesis and respiration. However, this technique traditionaly relies upon several assumptions when calculating the mixed‐layer O2/Ar budget, most notably the absence of verticalfluxes 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 verticalfluxes, incorporating nonsteady state changes in O2/Ar, and comparing NCP estimates evaluated over several day versus longer timescales, wefind diferences in NCP metrics calculated over diferent time intervals to be considerable, also observing significant potential efects from verticalfluxes, particularly advection. Additionaly, we observe strong diel variability in O2/Ar and NCP rates at multiple stations. Our results reemphasize the importance of accounting for verticalfluxes when interpreting O2/Ar‐derived NCP data and the potentialy large efect 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. Plain Language SummaryMarine microbes produce and consume oxygen as a product of a balance between photosynthesis and respiration, thereby causing changes in the concentration of oxygen in the surface ocean. Taking advantage of known relationships between the gas properties of oxygen and argon, researchers can isolate changes in biologicaly produced oxygen from changes that result from physical processes. This calculation relies on several simplifying assumptions. In this study, we test the importance of these assumptions by tracking changes in biological oxygen while folowing several water parcels over the course of 2–4 days. Wefind that accounting for vertical mixing processes and the rate of change in biological oxygen is important when estimating biological oxygen production rates. We alsofind that daily shifts in oxygen as photosynthesis rises through the day and fals to zero at night can influence measurements based on time of colection.