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Microscopic plants in the ocean called phytoplankton are among the most important creatures on Earth. They produce half of the planet's oxygen and form the basis of the ocean's food web. A new study also finds that they are in trouble. Since 1950, the amount of phytoplankton in the ocean's surface waters has declined by 40 percent.

And as NPR's Richard Harris reports, the cause appears to be global warming.

RICHARD HARRIS: Biologist Boris Worm is noted for his studies showing that the world's fisheries are in sharp decline. Most of that trend is due to overfishing, but it turns out that may not be the whole story. Worm, at Dalhousie University in Nova Scotia, has now turned his attention to the marine food chain. In particular, the ocean plant life that ultimately feeds almost everything else.

Dr. BORIS WORM (Biologist): The very fundamental question would be: Is the ocean getting more or less green? Is it increasing in plant life or decreasing?

HARRIS: This is a hard question to answer. Satellite studies show that the greenness of the ocean varies widely year to year, decade to decade. It would take many decades of data to see if there's a long-term trend. And, lo and behold, Worm has now unearthed more than a century of data. In particular, data using an instrument called a Secchi disk, which measures the transparency of ocean water.

Dr. WORM: The Secchi disk is a beauty because it's the simplest oceanographic instrument. Maybe it's also been in continuous use since it was invented in the late 1800s, and it hasn't changed since then.

HARRIS: Basically, oceanographers lower this white disk on a rope and they note how deep it is when it disappears from view. Oceanographers have taken half a million measurements like this throughout the world's oceans, so Worm and his colleagues collected piles of that data and looked for trends.

Dr. WORM: What we found was that phytoplankton was declining in eight out of 10 large ocean regions.

HARRIS: And the trend was pretty dramatic, averaging one percent per year, year after year, according to their study in this week's�Nature magazine. That means, for example, since 1950, they've documented a 40 percent decline in the Northern Hemisphere.

And the cause is pretty clear: The declines are biggest where the ocean is warmest. As the surface of the ocean warms up, that hot water just sits there and prevents colder nutrient-rich waters from coming up from below, so the phytoplankton don't get fertilized. And this turns out to be critical not just for plankton, but for the fish that feed on them.

Dr. WORM: Areas that have a very high phytoplankton production, such as the Peruvian upwelling area, for example, have extremely high fisheries catches, and others with little phytoplankton production have low fisheries catches.

HARRIS: And we know what happens when that Peruvian coastal area gets warmed up and its phytoplankton decline - that condition is called an El Nino. And Michael Behrenfeld at Oregon State University says El Ninos cause sharp declines in fish and in everything that depends on fish.

Professor MICHAEL BEHRENFELD (Marine Algae Specialist, Oregon State University): You see die-offs of marine mammals, die-offs of marine birds. So to me, that's just possibly the most tangible picture that we can have of how important the productivity of these microscopic plants is to the welfare of the higher organisms that live and depend upon the ocean.

HARRIS: Behrenfeld says El Nino is an analogy, not a prediction of what's going to happen. But there's no question the world's oceans have been getting warmer, and the widely accepted forecasts say global warming will continue to make the oceans heat up.

Now, Behrenfeld wasn't part of the study on phytoplankton, but he says the findings are quite consistent with similar studies, like his own, which have used satellites to measure declines in phytoplankton.

Prof. BEHRENFELD: None of them are perfect, but when you have multiple, independent ways of trying to answer the same question and they're all coming up with a similar result, that gives you power in terms of having confidence in the conclusions that you're drawing.

HARRIS: As for Canadian biologist Boris Worm, when he's asked about what all this portends for the future of life in the ocean, he hesitates.

Dr. WORM: What I can say with certainty is it will be different.

HARRIS: Richard Harris, NPR News.

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