New Diagnosis of Ocean Health

Scientists have developed a new way of determining from satellite images the amount of photosynthesis in the ocean. As compared to previous measurements, the new values are sometimes different by a factor of two or more, depending on the region.

Photosynthesis is the process by which plants convert sunlight, carbon dioxide and water into food. In the ocean, this conversion, also called "primary production," is carried out by phytoplankton, microscopic organisms that form the base of the ocean's food chain.

It's big business for nature.

Although invisible to the naked eye, phytoplankton account for the production of more than 50 billion tons of organic material each year. And because these floating plants absorb as much of the atmosphere's carbon dioxide - a major greenhouse gas - as do terrestrial plants, they are important to any global climate study.

"Scientists have been trying to determine global primary production for a long time," said Michael Behrenfeld of Oregon State University, in a NASA-sponsored teleconference with reporters last week.

Determining the amount of primary production requires knowing how many plants there are, and how fast they are growing. In the ocean, this means measuring phytoplankton levels.

Previously, satellite studies looked at the color of the ocean in a certain region to estimate the amount of chlorophyll - the green pigment in plants that is needed for photosynthesis. The greener the ocean, the more phytoplankton was assumed.

But the method that Behrenfeld and his colleagues have developed includes information about the brightness of the ocean. This extra information gives an indication of the amount of chlorophyll, or "greenness," per plant, which is related to the growth rate.

"Satellite ocean color images are kind of like your television screen, where you have controls for the color setting and controls for brightness," said David Siegel from the University of California, Santa Barbara. "What we've done here is use both the color and brightness signals to determine plant greenness and the number of individual phytoplankton cells."

Siegel and Behrenfeld and their collaborators applied this analysis to data from NASA's Sea-viewing Wide Field-of-view Sensor (SeaWiFS). In a study that appeared in the January 2005 electronic issue of the journal Global Biogeochemical Cycles, the team found that their implied growth rates for phytoplankton matched laboratory studies.

With the new photosynthesis "ruler," the researchers also reassessed the production levels in certain areas. Siegel said that their new measurements in tropical zones are two to three times more than what had been previously estimated. Conversely, in other parts of the ocean, the amount of photosynthesis appears to have been overestimated.

But what these new values mean to the health of the ocean is not yet completely understood. One complication is that more phytoplankton growth is good in some places - like in ocean fisheries - but too much can be a bad thing.

Algal blooms, for example, which are an overabundance of phytoplankton, can lead to a dangerous drop in ocean oxygen levels, due to bacteria eating dead plant material. Moreover, coral reefs appear to do better when phytoplankton are at lower levels.

Jorge Sarmiento of Princeton University, who was not involved in the study, made an analogy to deserts and forests, which also have different levels of photosynthesis.

"We want to conserve that biodiversity - the same is true in the oceans," said Sarmiento.

What the researchers hope is that their new tool will help improve understanding of the effect that climate and nutrient levels have on the vitality of phytoplankton, and correspondingly the ocean as a whole.

"We've found the road, but we have yet to find where it's taking us," Behrenfeld said.