Oregon Sea Grant

In the productive central-Oregon coastal upwelling environment, wind-driven upwelling, tides, and topographic effects vary across the shelf, setting the stage for varied biogeochemical responses to physical drivers. Current, temperature, salinity, and dissolved oxygen (DO) measurements from three moorings deployed during the upwelling seasons of 2009–2011 off the central-Oregon coast are analyzed over three time bands (interannual, subtidal, tidal) to explore the relationship between mid (70 m) and innershelf (15 m) upwelling dynamics and the associated effect on DO. Topographic effects are
observed in each time band due to the Heceta and Stonewall Bank complex. Seasonal cumulative hypoxia (DO<1.4 mL L^-1) calculations identify two regions, a wellventilated inner shelf and a midshelf vulnerable to hypoxia (98+/-5 days annually). On tidal timescales, along-shelf diurnal (K1) velocities are intensified over the Bank, 0.08 m s^-1 compared with 0.03 m s^-1 to the north. Interannual variability in the timing of spring and fall transitions, defined using glider-measured continental slope source water temperature, is observed on the midshelf. Interannual source water DO concentrations vary on the order of 0.1 mL L^-1. Each spring and summer, DO decline rates are modulated by physical and biological processes. The net observed decrease is about 30% of the expected draw down due to water-column respiration. Physical processes initiate low-oxygen conditions on the shelf through coastal upwelling and subsequently prevent the system via advection and mixing from reaching the potential anoxic levels anticipated from respiration rates alone.