ocean oxygen concentration

In July 2002, scientists with the Oregon Department of Fish and Wild¬life found unusual numbers of bottom¬feeding sculpin lying lifeless on the ocean floor, which would normally be teeming with life. Crabs were also dying, and they washed up onto some beaches in large numbers.
Ocean surface waters normally contain 5-8 millilitres of oxygen per litre of water, which declines rapidly with depth. At a depth of 50 metres the inner coastal waters off Oregon were hypoxic – oxygen levels there were lower than 1.43 millilitres per litre, so low that fish can’t survive,
Many regions of the world have hypoxic coastal waters, usually caused by agricul¬tural fertilizers leaking into the ocean. The excess nutrients fuel plankton blooms, which consume oxygen.
Every summer since then, much of the oxygen has disappeared from a large patch of inner continental-shelf waters. In the most extreme case, in 2006, the waters lost all detectable oxygen for four weeks. Starfish and mussels died, and rockfish and other mobile fish fled the hypoxic zone, which grew to 3,000 square kilometres .
The phenomenon has worried the fishing industry in Oregon, which brings in hun¬dreds of millions of dollars each year and
This is phenomenon is being pound elsewhere in the world. It is not clear if this is secondary to Global Warming. But warmer water temperatures may be important contributory factors.
Usually in the spring , occasional periods of Northerly winds blow surface water off shore, allowing cool water, rich in nutrients but poor in oxygen to rise form deeper offshore layers.
Gewin 2010 Dead in the water Nature vol 104 pp 812-3

Martin Eastwood
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