New Insights about Hypoxia in Oregon’s Coastal Ocean from over a Decade of Autonomous Underwater Glider Observations

Near-bottom waters over the continental shelf off Oregon in the northern California Current have become increasingly hypoxic, including the appearance of anoxia in summer 2006. Observed ecosystem impacts include the absence of fish and invertebrate die-offs. Near-bottom, inner-shelf hypoxia is driven by upwelling of low-oxygen, nutrient-rich source water onto the continental shelf, followed by the decay of organic matter from surface phytoplankton blooms. We are using data from over a decade of autonomous underwater glider missions to understand the temporal and spatial distribution of dissolved oxygen over the continental margin off Oregon. The glider tracks cover in excess of 60,000 kilometers and provide hundreds of thousand of full water-column profiles of water properties including dissolved oxygen. These glider observations, together with moored instrument time series and numerical ocean circulation and biogeochemical modeling of this region, have provided new insights on the distribution and time-dependence of hypoxia in this region. The inshore side of Heceta Bank, a submarine bank that deflects the coastal upwelling jet seaward creating a region of weaker velocities inshore, is particularly vulnerable to hypoxia. Near-bottom dissolved oxygen variability is driven by changes in both the dissolved oxygen concentrations in offshore upwelling source water and local wind forcing. “Source water” is defined as being seaward of the continental shelf break on density surfaces that upwell onto the continental shelf. The strength and depth of the onshore source water flux due to wind-driven upwelling can vary through the upwelling season, influencing near-bottom shelf hypoxia. Late in the upwelling season, upwelled source waters can become lower in oxygen due to off-shelf flux of continental shelf water that has undergone respiration and is, therefore, lower in oxygen than unmodified upwelling source water. For present day source water dissolved oxygen concentrations (~2.3 ml/l), hypoxia over the inner shelf on the inshore side of Heceta Bank during the summer upwelling season is observed about 50% of the time. Given the recent declining trend in source water dissolved oxygen concentration, in 50 years the frequency of the hypoxia over the inner shelf on the inshore side of Heceta Bank is predicted to be about 90%.