ICES CM 2015/R:15

Understanding and predicting hypoxia over the continental margin in the northern California Current

John A. Barth1, Francis Chan2, Stephen D. Pierce1, Katherine A. Adams3, R. Kipp Shearman1 and Anatoli Y. Erofeev1

1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin Bldg, Corvallis, Oregon, 97331-5503, U.S.A. E-mail: [email protected] 2Department of Zoology, Oregon State University, 3029 Cordley Hall, Corvallis, Oregon, 97331, U.S.A. 3School of Marine Science and Engineering, Plymouth University, Plymouth, Devon, PL4 8AA, United Kingdom

Near-bottom waters over the inner shelf off central Oregon in the northern California Current have been increasingly hypoxic over the last decade, including the appearance of anoxia in summer 2006. Near-bottom, inner-shelf hypoxia is driven by of low-, nutrient-rich source water onto the , followed by the decay of organic matter from surface blooms. We are using data from moorings, ship surveys, and from over 60,000 kilometers of autonomous underwater glider tracks to understand the temporal and spatial distribution of dissolved oxygen over the continental margin off Oregon. The inshore side of , 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 offshore upwelling source water dissolved oxygen concentrations and local wind forcing. 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. Ship surveys show that near-bottom hypoxia can cover a substantial region of Pacific Northwest waters, nearly 18,000 square kilometers inshore of the 200-m isobath. Low- oxygen events can have a substantial impact on the inner-shelf coastal ocean ecosystem including observed absence of and die-offs of invertebrates. The predicted decrease of subsurface oxygen concentrations and possible changes in alongshore, upwelling-favorable winds may exacerbate low-oxygen conditions in the future.