Phytoplankton and Climate: A Decline of Marine Building-Block in Some Oceans

The base of the food chain in the world's oceans, microscopic phytoplankton, have seen some changes in the last decade or so. Researchers' findings on this decline, compiled partly from NASA satellite data, were recently accepted for publication in the journal Global Biogeochemical Cycles.

It turns out that between 1998 and 2012 around the world, diatoms, which are the largest phytoplankton algae, declined in numbers greater than 1 percent a year. The most substantial losses were in the North Pacific, North Indian and Equatorial Indian oceans. Having less of the tiny algae in the oceans may decrease their ability to work as carbon sinks -- to draw carbon dioxide out of the atmosphere and store it, according to a release.

When phytoplankton bloom and extend in a long, colorful span in the ocean that can be seen from space, they take in carbon dioxide from water and change it to organic carbon. They use this converted carbon as food, and they are using it for their photosynthesis, just as trees do. When each alga cell dies, it sinks and takes the carbon stored in its body, said the release.

Diatoms are a bit different, because they are larger and can sink more quickly when they die.

The team was led by Cecile Rousseaux, of Universities Space Research Association and NASA's Goddard Space Flight Center. Their study involved taking measurements of the green of chlorophyll on the ocean from NASA's SeaWiFS (Sea-viewing Wide Field of View Sensor), a high-tech instrument that flew on one of the agency's satellites from 1997 to 2010. After 2002, they also used a certain spectroradiometer, or light reader, that rode aboard the agency's Aqua satellite. The chlorophyll data that they gathered indicated all the phytoplankton types in the oceans, as the release noted.

Using a biogeochemical modeling device that gives an idea of the conditions (sun, currents, etc.) in the ocean, the researchers were able to tell which algae were large diatoms and which were tinier phytoplankton types.

With these learnings that went beyond satellite images, the scientists determined that because the ocean's highest layer, called the mixed layer, is growing more shallow, the diatoms are declining.

This is an area that is most exposed to wind and current churn and to sunlight--conditions that are good for phytoplankton. If the water is more shallow, though, the layer cannot hold as many nutrients. "The phytoplankton can run out of nutrients," said Rousseaux, in the release.

Scientists say that the diatom decline is not yet severe, but should be observed. It could be happening because of changes in wind, although the researchers will need to explore further, as the release said.