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Scientists Measure Ocean Health by Its Color

Scientists have developed a technique to determine how healthy the ocean is by assessing its color. They say how green the water is can tell them the condition of microscopic plants crucial to ocean life and to the oxygen cycle we depend on.

You wouldn't order phytoplankton from a menu, but these microscopic green algae are an important food. They are the first link of the food chain for all marine life and are most abundant where fish are in great supply.

Phytoplankton is also the undersea lungs of the planet. Ocean biologist Michael Behrenfeld at the U.S. space agency, NASA, says that, like plant life on land, these single-celled floating organisms inhale carbon dioxide and exhale oxygen. "Most people that I talk to are actually very surprised to hear that about half of the oxygen produced by plants on this planet actually comes from the oceans, not trees or shrubs or grasses. That's what is fueling our global ocean ecosystems," he said.

Phytoplankton also takes in about half of the carbon dioxide absorbed from the atmosphere by plants. Carbon dioxide is considered a major contributor to global warming. Therefore, the vitality of phytoplankton is crucial to Earth's health.

Scientists have been trying to develop an accurate way to measure its abundance and growth rate since the plants were discovered more than a century ago. A traditional technique measured just the variations in the green color reflected up to satellite cameras by the chemical chlorophyl in phytoplankton. Mr. Behrenfeld says the problem is that no one knows how well the color of the chlorophyl, which can vary based on temperature and the amount of nutrients and light, relates to phytoplankton's volume, or biomass.

The new method Mr. Behrenfeld and his associates developed uses a complex mathematical formula to compare the chlorophyl color to the amount of carbon in the phytoplankton. Carbon is a better predictor for biomass. They are also getting a truer measure of the greenness of the chlorophyl by assessing not only its hue but also brightness, and by correcting for the brighter light bouncing back from land and the atmosphere.

"We can now determine actually how green the individual phytoplankton are from space. From years and years of laboratory studies, we know that the greenness of the cells provides a fingerprint [clue] to growth rate. So that's what we're doing. We're getting the greenness of the cells as well as the biomass, and that gives us growth rate and biomass," he said.

The NASA scientist says the two measures together provide a more accurate assessment of ocean quality. The increased clarity will help determine how well the oceans' organisms are holding up under stresses such as pollution and global warming. Mr. Behrenfeld says it will also contribute to improved computer models that better predict how climate change will alter the environment.

"One of the hopes NASA has with all the investment it is putting into Earth-observing satellites is that with this information, we will get a better understanding of how our biosphere functions today. If we can understand how it is working today, we hope that we can predict better how it will behave in the future," he said.

Earlier analyses of satellite imagery showed a decline of phytoplankton over the past two decades. But co-researcher David Siegel, a geologist at the University of California at Santa Barbara, says the good news from the improved analytical method is that it shows much more of the algae in the tropics than previously thought.

"The differences are 200 percent for the tropics. We are predicting much more production in the tropics by taking into account that the phytoplankton can change the amount of pigment per cell. So it's very exciting from the point of view of a satellite oceanographer," he said.

However, Mr. Siegel says the previous measurement technique overestimated how much phytoplankton production occurs in cooler latitudes.

The researchers warn that they have just begun using their new methods. They say they must collect much more satellite data before they can determine its production rate more precisely to give a more accurate picture of the health of the oceans.