What is Hypoxia?

 Glossary of terms

  • Hypoxia - Meaning 'low oxygen', hypoxia describes a state in which the amount of oxygen in seawater is less than 1.4 milliliters per liter (ml/l).
  • Anoxia - Meaning 'no oxygen', anoxia describes a state in which the amount of oxygen in seawater is less than 0.5 milliliters per liter (ml/l).
  • Phytoplankton - Microscopic plant-like organisms that obtain energy through the process of photosynthesis and serve as the basis for the majority of oceanic food webs.
  • Upwelling - A wind-driven phenomenon bringing deep, cold, nutrient rich, and oxygen-poor water towards the ocean surface.  Off the Oregon and Washington coasts, upwelling occurs during the summer months in response to northerly winds (winds blowing from north to south).
  • Downwelling - A wind-driven phenomenon that pushes nutrient-poor and oxygen saturated surface water to depth.  Downwelling off the Oregon and Washington coasts occurs when winds blow from south or north.

Science of Hypoxia

Unlike the dead zones in estuarine systems that are caused in large parts by excessive nutrient run-offs from land, the Oregon dead zone forms along the open coast where coastal winds drive ocean currents that upwell nutrient-rich but oxygen-poor waters from the deep sea onto the shallow reaches of the continental shelf. This upwelling of nutrients further fuel phytoplankton blooms that eventually sink and decompose to further reduce oxygen levels in the already low oxygen waters along the seafloor. Hypoxic zones along the Oregon coast form seasonally, and can begin in late spring/early summer in response to the onset of upwelling-favorable winds from the North. Hypoxia can persist through the summer months and ultimately recedes for the year during the Fall when winds again shift direction and promote ocean currents that flushes low oxygen water off the continental shelf. 

Upwelling-caused dead zones have their greatest impacts on the shallow waters of the continental shelf where extremely low oxygen conditions are not a typical feature. The 2006 dead zone encompassed over 3000 km of the Oregon shelf, an area that exceeds the total combined area of estuaries in the state. Recent reports have also indicated the possible novel development of an analogous dead zone along the open coast of Washington state. Since upwelling shelves represent one of the largest fraction of coastal marine waters of the US west coast, uncertainties into the potential for further intensification and expansion of upwelling-driven dead zones represent a major scientific and management challenge.

 

 

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