Hypoxia in the Pacific Northwest

How does the pacific northwest dead zone form


The figure shows oxygen levels as they change with the depth of water, from the ocean's surface (top of graph) down to depth (where low-oxygen is normal).  Oxygen levels below 1.4 ml/l are hypoxic, too low to support most life.  Data: Chan, et al, 2008. Emergence of Anoxia in the CA current large marine ecosystems. Science, vol 319.

Every summer off Oregon and Washington, northerly winds and the earth’s rotation combine to drive surface waters offshore and bring naturally nutrient-rich but oxygen-poor waters to the coast (see Figure at right). When this water wells up to the ocean surface, microscopic plants called ‘phytoplankton’ bloom in the sunlight creating a rich broth that feeds a variety of fish and marine invertebrates. It is this upwelling of nutrient-rich waters that makes the west coast of North America one of the most productive marine ecosystems on earth. 

But upwelling is a double-edged sword. The rich broth of phytoplankton that blooms at the surface ultimately sinks toward the sea floor where it decomposes. The process of decomposition consumes oxygen from the water column, and far from the surface, this oxygen is not easily replaced. The upwelling process can thus lead to oxygen depletion near the sea floor and extending up through 2/3rds of the water column. Hypoxia does not usually affect the sea surface or areas close to shore, where breaking waves efficiently mix oxygen from the atmosphere into the water. 

If coastal winds become southerly (downwelling-favorable), surface waters to move back toward the shore, driving bottom waters away from the coast -- a process called downwelling. When upwelling periods alternate with periods of sufficiently strong downwelling, low-oxygen waters do not accumulate near the seafloor.

During seasons with strong upwelling not punctuated by downwelling, low-oxygen waters can accumulate, causing a dead zone. Each successive pulse of upwelling brings the low-oxygen waters closer to shore and more nutrients to the lighted zone. The resultant phytoplankton blooms eventually sink and decay, further depleting oxygen near the sea floor. Repetitions of these events cause the mass of low-oxygen water near the sea floor to gradually become thicker as well as lower in oxygen. Changes in the strength and pattern of upwelling winds and the oxygen and nutrient content of the deep offshore waters thus impacts the likelihood and severity of hypoxia events. 

The normal upwelling period runs from about April to September. October to March is generally dominated by downwelling-favorable coastal winds. Even during years in which hypoxia develops in the summertime, high-oxygen conditions have reappeared in October, with the beginning of downwelling that pushes low-oxygen waters away from the coast.

The New Normal

It is normal to find naturally low-oxygen conditions in deep, offshore waters, e.g., at the edge of the continental shelf and slope. However, the occurrence of low-oxygen water close to shore (the inner shelf, less than 50 m (165’) of water) is highly unusual and had not been reported prior to 2002 despite over 50 years of scientific observations along the Oregon coast. Moreover, the appearance of anoxia, or zero-oxygen conditions, in 2006 was unprecedented and resulted in mass die-offs of long-lived marine animals such as seastars and sea cucumbers. The presence of long lived species on the seafloor, with some individuals likely decades old, suggests that such low-oxygen conditions are not normal in this system.

 

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