An oxygen-depleted layer of water exists naturally many hundreds of feet below the ocean surface. But for the past 50 years in the Pacific Ocean, this layer has become less saturated with oxygen and moved upward. At depths between 656 and 1,640 feet, areas of the north Pacific have lost between 1 and 2 percent of their oxygen each year during the past 25 years, says Frank Whitney, a scientist emeritus with Fisheries and Oceans Canada in Sidney, British Columbia. And the top edge of this low-oxygen zone has advanced upward at an average rate of almost 10 feet per year.
Most sea life that has gills prefers to avoid these hypoxic waters. For these species, the ocean has effectively become 246 feet shallower in the past quarter century. This may explain why some fish species off the coast of British Columbia have moved to shallower areas, and, in some cases into Alaskan waters, says Mr. Whitney. “I would suggest that would be in response to hypoxia.”
How ocean waters lose oxygen
Some of the deep water along the west coast of North America originates off the coast of equatorial South America, where the water is already “old,” meaning that it hasn’t been in contact with the atmosphere for many years. And it’s further leached of oxygen when organic detritus drops from the highly productive surface waters of the tropical eastern Pacific. As this organic material sinks, it decays, sucking oxygen from the water and creating one of the largest hypoxic zones in the world.
The planet has warmed in the past 30 years and, generally, sediment cores indicate that the warmer Earth becomes, the larger this eastern Pacific hypoxic zone grows, says Francisco Chavez, a researcher with the Monterey Bay Aquarium Research Institute in Moss Landing, Calif.