Squid invasions signal changes in the Pacific Ocean
In the Pacific, jumbo squid have moved to new waters, signaling changes in the ocean, scientists observe.
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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.
There are two possible explanations for this. An ocean that warms from the top down becomes stratified, like a layer cake. The warmer and more buoyant surface then inhibits oxygenation of cooler waters below. The second possibility: increased productivity. More organic material means more decay and more oxygen removed from the ocean. Some of that oxygen-depleted water then flows north.
Paradoxically, however, productivity in the eastern Pacific is highest when the cycles influencing surface temperatures are in their cool phase. That’s when nutrient-rich water wells up from the deep, fertilizing the algae that ultimately support one of the most productive marine ecosystems on Earth. Recently the ocean has entered a “cool” phase of the 20- to 50-year cycle called the Pacific Decadal Oscillation – warmer waters in the western tropical Pacific, but cooler ones in the eastern tropical Pacific. “When we’re in the cold phase of the PDO, we have lower oxygen,” says Dr. Chavez.