Scientists Zero In on True Color of the Sea

How blue is the ocean? How green is the sea?

The color of seawater, a key measure of ocean health, is coming into sharper focus due to a breakthrough in analyzing satellite images.

A group of NASA and university scientists on Thursday announced it had figured how to measure the hue and brightness of ocean coloration that, in turn, reflects changes in the tiny plants that provide the base of the ocean food chain and supply half of the world’s oxygen.

The new techniques, scientists say, hold great promise in helping measure changes in ocean plankton, which besides being the undersea lungs of the planet also help determine how many fish are produced.

Phytoplankton exhale life-giving oxygen and are consumed as food by zooplankton and small fish, which, in turn, are eaten by larger fish.

Using earlier versions of the satellite imagery, other scientists from NASA and the National Oceanic and Atmospheric Administration have noted a worrisome decline of phytoplankton over the last 20 years, possibly a result of global warming.

Michael Behrenfeld, a biological oceanographer at Oregon State University, said the new techniques should eventually result in a much more precise picture of phytoplankton, the ocean’s basic biological building block.

Increased clarity will help determine how well ocean health is holding up under stresses such as pollution and global warming.

“We haven’t made it to Oz yet,” Behrenfeld said, who recently left NASA. “But today we are announcing we have found the Yellow Brick Road.”

Scientists say the importance of phytoplankton cannot be overestimated.

These single-cell plants, too tiny to be seen without magnification, are so numerous that their collective weight would be more than all of the trees and shrubs and other terrestrial plants.

They absorb nearly half of the world’s human-produced carbon dioxide, lessening the effect of this primary greenhouse gas linked to global warming. For two decades, scientists have used satellites to study plankton on a global scale by measuring the color of the oceans.

Seawater changes from blue to green as the abundance of phytoplankton increases, and researchers have used this color to determine the overall quantity of plankton.

But this was only half of the picture. The other half was how fast these plankton are growing, an important factor given how quickly phytoplankton bloom and die or are consumed -- usually within six days.

Using a new analytical formula, scientists can measure how green the hue, and this “greenness” provides an indication of how fast plankton are growing.

Phytoplankton, like other plants, shed pigment, or chlorophyll, from their cells, and quit growing when stressed by changes in temperature, light or nutrients.

One of the trickiest parts of measuring this, said David Siegel, a UC Santa Barbara geology professor, was correcting for brighter light bouncing back from land and the atmosphere. “The ocean isn’t the brightest target,” he said.

Siegel likened the project, which is being published by the journal Global Biogeochemical Cycles, to fiddling with the settings for color and brightness on a television set.

In this case, the new mathematical formula to achieve those settings took a decade to perfect.