Ocean ‘desert’ zone extends to isles

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The ocean's dead zones have expanded in recent years to include part of the Hawaiian archipelago, scientists have concluded from space-observation studies.

"What's happening is, large portions of the area are becoming less productive," said Jeffrey Polovina, with the National Marine Fisheries Service in Honolulu.

That is bad news for larger species such as tunas, sharks and marlins, he said.

A sensor aboard the SeaStar spacecraft, launched in 1997, has confirmed that the North Pacific's subtropical "desert" has expanded to the northeast, reaching part of the Hawaiian Islands.

IMAGE COURTESY JEFFREY POLOVINA A figure shows the global ocean surface chlorophyll distribution, with the least productive regions shown in dark blue.

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The least productive ocean waters -- biological deserts -- have reached the Hawaiian archipelago, Hawaii scientists report.

The largest ecosystems in the major ocean basins are subtropical gyres, large-scale regions of winds and currents with low chlorophyll for plant and animal growth. These areas cover 40 percent of Earth's surface and in nine years have expanded 10 to 25 times faster than global-warming models predict, the scientists said.

"What's happening is, large portions of the area are becoming less productive," said Jeffrey Polovina, with the National Marine Fisheries Service in Honolulu.

This will likely decrease the carrying capacity for larger animals such as tunas, sharks and marlins, he said in an interview. "We will just have less productivity at the base of the food web."

He said there might be a change in species composition with the absence of larger, predator animals favoring the smaller fish, such as mahimahi and skipjack.

Polovina and colleagues Evan Howell, research oceanographer, and Melanie Abecassis, research assistant, analyzed data from a nine-year time-series of remotely sensed ocean color data from the Sea-viewing Wide Field-of-view Sensor on the orbiting SeaStar spacecraft.

The craft, launched in 1997, maps the oceans in color, with surface chlorophyll in green reflecting the amount of plant life supporting the base of the food chain.

"SeaWiFS is a remarkable sensor that gives us a picture of ocean biology all around the globe," Polovina said, explaining the data is valuable for many different applications.

From January 1998 to February 2007, all biological deserts in the Pacific and Atlantic subtropical gyres expanded at average annual rates of 0.8 percent to 4.3 percent a year, the scientists said in a paper published in Geophysical Research Letters.

Total expansion was about 6.6 million square kilometers, or about 15 percent, they reported.

In the North Pacific, they said, the gyre "has expanded to the northeast, reaching portions of the Hawaiian archipelago and well into the eastern Pacific." The South Pacific desert area also grew on the southern and northern sides.

The rapid expansion of low surface chlorophyll in four ocean basins suggests global warming as the likely cause, but it is premature to attribute it to global warming, Polovina said.

"If it is due to an increase in greenhouse gases, it really does represent something that is going to be a long, permanent change."

Because of the large size of the ocean deserts, "even modest changes can result in significant impacts on the spatial distribution of surface chlorophyll in the entire ocean," the scientists pointed out.

The only exception to the spreading biological deserts was in the South Indian Ocean. "There is probably a lot more variability in that ocean," Polovina said. However, the researchers noted a trend toward expansion that could show up in a few more years, he said.

SeaWiFS Project scientist Charles McClain at NASA's Goddard Space Flight Center in Greenbelt, Md., was not involved with the study, but told ScienceNOW Daily News, "Everything seems to hold together."

Various oceanographic observations and models point to global warming as the cause of the expanding subtropical gyres and low-productivity ecosystems, he said.

But he said there is a question whether part or all of the expansion might be due to a natural event that could reverse itself in a decade or two -- something Polovina agreed cannot be ruled out.