Processes influencing seasonal hypoxia in the northern California Current System

This paper delineates the role of physical and biological processes contributing to hypoxia, dissolved oxygen (DO) < 1.4 mL/L, over the continental shelf of Washington State in the northern portion of the California Current System. In the historical record (1950–1986), during the summer upwelling season, hypoxia is more prevalent and severe off Washington than further south off northern Oregon. Recent data (2003–2005) show that hypoxia over the Washington shelf occurred at levels previously observed in the historical data. The year 2006 was an exception, with hypoxia covering ∼5000 km 2 of the Washington continental shelf and DO concentrations below 0.5 mL/L at the inner shelf, lower than any known previous observations at that location. In the 4 years studied, upwelling of low DO water and changes in source water contribute to interannual variability, but cannot account for seasonal decreases below hypoxic concentrations. Deficits of DO along salinity surfaces, indicating biochemical consumption of DO, vary significantly between surveys, accounting for additional decreases of 0.5–2.5 mL/L by late summer. DO consumption is associated with denitrification, an indicator of biochemical sediment processes. Mass balances of DO and nitrate show that biochemical processes in the water column and sediments each contribute ∼50% to the total consumption of DO in near‐bottom water. At shorter than seasonal time scales on the inner shelf, along‐shelf advection of hypoxic patches and cross‐shelf advection of seasonal gradients are both shown to be important, changing DO concentrations by 1.5 mL/L or more over 5 days.