Seasonal Variability of Benthic Metabolism on the central Oregon Shelf

Strong seasonal patterns in bottom-water oxygen concentrations are observed on the Oregon shelf. Hypoxic bottom-water conditions (<60 µmol kg^-1) occur during the summer months and are associated with upwelling and a stratified water column. Conversely, high bottom-water oxygen concentrations occur in winter due to enhanced water-column mixing associated with North Pacific storm patterns and downwelling. During this study, benthic oxygen fluxes were measured on the Newport Hydrographic Line at 30 m and 80 m water depth during 10 research cruises, that spanned a wide range of upwelling and wave conditions from December 2017 to July 2019. Data is presented from repeated autonomous eddy covariance (EC) lander deployments and core incubations, yielding the first seasonal study of benthic oxygen utilization on the Oregon Shelf. Benthic oxygen exchange rates were found to vary within 24-48 h EC deployments, seasonally and spatially. Average rates of benthic oxygen utilization were higher on the inner shelf than on the mid shelf, with strong relationships between turbulent kinetic energy, significant wave height, and/or current speed and benthic oxygen utilization. At both sites, the highest rates of benthic oxygen utilization were observed in fall and winter and the lowest benthic oxygen utilization rates were observed in the summer under hypoxic conditions. Oxygen fluxes for non-winter and non-fall deployments agree well with previous field studies and model results. The high oxygen utilization rates observed during the fall/winter months indicate that there may be a more complete coupling between annual carbon production and remineralization on the shelf than previously thought.