Diurnal and Seasonal Variability of Near-Surface Oxygen in the Strait of Georgia

Dissolved oxygen (O-2) concentrations in waters of the ocean surface mixed layer are generally close to thermodynamic equilibrium with atmospheric concentrations. However, near-surface O-2 levels are also affected by other processes, including primary productivity, and thus measurements of near-surface O-2 can in theory be used to estimate productivity. Here we discuss variations in near-surface O-2 concentrations in the Strait of Georgia by examining a variety of data sets, focusing primarily on ferry-based measurements over a 3-year period (2015-2017). Both diurnal, seasonal, and interannual variations are quantified, and various fluxes into and out of the surface waters are estimated so that the degree to the measured oxygen variations representing biological activity can be assessed. On average, advective and vertical transport makes only a small negative contribution, while other budget terms show a strong seasonal cycle, lowest in winter, peaking in spring and slowly dropping to winter level during summer and autumn. For most of the year, the air-sea flux is the largest term, but the storage term is important during the spring blooms. Diurnal variations of 5-10% saturation in measured oxygen levels must therefore largely reflect diurnal cycles in productivity and respiration. We estimate respiration effects from the rate of decay of O-2 at night, which yields an average respiration rate of 459mmolm(-2)day(-1). This value is almost an order of magnitude larger than the other budget terms we have discussed, suggesting that a future focus on diurnal variations might provide most insight into biological processes in the Strait. U