Hypoxia on the Oregon Shelf: a Modeling Study

During the last decade frequent summertime episodes of hypoxic (< 1.43 ml DO/l; DO - Dissolved Oxygen) waters occupying significant areas of the Oregon coastal shelf have been observed. The spatial extent and severity of low oxygen conditions on the shelf vary interannually and are the cumulative results of both physical and biological processes. Among the most important factors are the amount of total primary production, the retention (or residence) time of water before it is flushed from a local region and DO and nitrate concentration of the source waters that are transported onto the shelf. In order to examine the importance of these physical and biological processes as well as their interaction, a 6-component (nitrate, ammonium, phytoplankton, zooplankton, detritus, oxygen) ecosystem model has been coupled to a ROMS (Regional Ocean Modeling System) circulation model. We analyzed model hindcasts for three years: 2002, with strong hypoxia record, 2006, when hypoxia was severe (even approaching anoxia), spatially extensive and lasted for several months, and 2008, when hypoxic events were shorter and not as widespread along the coast. It was found that the supply of nutrients to the shelf is a key factor leading to hypoxia off Oregon during the summer. The differences in wind forcing, mainly in the frequency of succession of upwelling and downnwelling events, in 2002 and 2006 also appear to influence the extent of hypoxia.