Hypoxia and Ocean Acidification
When ocean waters contain so little dissolved oxygen that marine organisms start to suffer, this is called hypoxia. Hypoxia is caused by the microbial decay of organic matter that robs oxygen from the water. This can occur when excess nutrients from land-based sources run off into the coastal ocean to cause plankton blooms that subsequently decay and lead to hypoxia. Off Oregon, nutrients are supplied to the light zone in the coastal ocean by upwelling. Through particular combinations of upwelling, ocean stratification and currents, regions of hypoxia near the sea floor may form during the summer upwelling season, April to September. Ocean acidification is the lowering of the ocean’s pH through the transfer of human-produced CO2 into ocean waters. Ocean acidification is exacerbated in coastal upwelling zones like off Oregon when upwelling brings waters already high in CO2, hence low pH, close to shore.
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