There's less floating plastic pollution in the Pacific, sometimes referred to as the Great Pacific Garbage Patch, and in the Earth's other oceans than scientists say they've expected to find -- and that's a worry, they say.

Circular currents in the oceans gather up the floating plastic, creating giant floating areas considered a potential risk to fish as well as other marine wildlife.

An extrapolation from data acquired by a recent global sampling expedition by research ships suggests 7,000 to 35,000 thousand tons of plastic debris is floating in the world's oceans, much less than the estimated 1 million tons data going back as far as the 1970s would suggest should have been found, experts said.

Two Spanish research ships collected more than 3,000 samples from 141 ocean sites during a global cruise of more than 30,000 nautical miles.

While several theories have been put forward for the discrepancy in numbers, the most likely one -- and the most worrisome, researchers say -- is that the debris is slowly sinking into the ocean depths.

While plastic on the surface is bad enough, at least it can be studied, they say; but if it's sinking and out of reach, the harm it may be doing could be harder to estimate.

"The deep ocean is a great unknown," study author Andres Cozar of the University of Cadiz in Spain says. "Sadly, the accumulation of plastic in the deep ocean would be modifying this mysterious ecosystem -- the largest of the world ---before we can know it."

If it isn't sinking, another possibility is that it's being consumed by marine wildlife, which means it could work its way up  the food chain to create a risk to human.

"The plastic pollution in surface waters can more easily interact with the ocean life, because the surface layer of the ocean hosts most of the marine organisms," Cozar says.

The research cruise found concentrations of plastic were greatest in areas where ocean currents were slowly gathering up the floating pollution.

Those areas included west off the United States, between Africa and the U.S., to the west of southern South American and to both the west and east of Africa's southern tip.

"We are putting, certainly by any estimate, a large amount of a synthetic material into a natural environment," says Kara Lavender Law, who studies plastic pollution at the Sea Education Association in Woods Hole, Massachusetts. "We're fundamentally changing the composition of the ocean."

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