NASA satellite data capture a big climate effect on tiny ocean life

Ocean Surface Chlorophyll: This series of images shows chlorophyll levels in the equatorial Pacific Ocean, derived from SeaWiFS data. Chlorophyll indicates phytoplankton. During the La Nina in 1998, the image shows a higher concentration of chlorophyll (yellow and red colors) around the equator (0 degrees). A normal year shows a lighter shading, or much less chlorophyll. During the 1997 El Nino, the indication of chlorophyll is almost not noticeable. Credit: NASA Full size image available here

Turns out that the old cliché, "it's the little things that make a difference," is especially true when it comes to our atmosphere and oceans. Tiny ocean plants, or phytoplankton, help regulate the Earth's climate by accounting for about half of the carbon dioxide, a major greenhouse gas, absorbed annually from the atmosphere by plants. But, these organisms are also the base of the marine food web, responsible for most of the biological activity in the ocean. Microscopic animals called zooplankton, eat the phytoplankton and are in turn eaten by other larger animals. So, any change in phytoplankton numbers alters the ocean food chain.

Now, new research shows these marine plants may have an even greater impact on the health of our oceans and climate than previously thought. In a study published in the January 2005 issue of Geophysical Research Letters, Wendy Wang and colleagues at the University of Maryland, Earth System Science Interdisciplinary Center (ESSIC), College Park, Md., found that phytoplankton population and size can change dramatically due to the physical processes associated with the climate phenomena known as El Niño and La Niña. In turn, these changes not only affect ocean ecology, but also influence our climate by impacting carbon storage in the ocean.

During an El Niño year, warm waters from the Western Pacific Ocean spread out over much of the basin as upwelling subsides in the Eastern Pacific Ocean. Upwelling brings cool, nutrient-rich water from the deep ocean up to the surface. So, when upwelling weakens, phytoplankton do not get enough nutrients to maintain their growth. As a result, surface waters turn into "marine deserts" with unusually low populations of phytoplankton and other tiny organisms. With less food, fish cannot survive in the surface water, which then also deprives seabirds of food.

During La Niña conditions, the opposite effect occurs as the easterly trade winds pick up and upwelling intensifies, bringing nutrients to the surface waters, which fuels phytoplankton growth. Sometimes, the growth can take place quickly, developing into what scientists call phytoplankton "blooms."

Using a computer model and NASA's Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite, Wang examined marine biological changes associated with El Niño and La Niña, and uncovered the mechanisms responsible for such phytoplankton blooms. SeaWiFS measures the amount of light coming out of the ocean at different wavelengths and can determine the intensity of plant pigment, or greenness, and the number of individual phytoplankton cells.

A dramatic recovery from the strong 1997-98 El Niño led to La Niña conditions in the Pacific Ocean, beginning in mid-1998. "During this period, SeaWiFS imagery showed extremely dark greenness along the equator, with chlorophyll concentrations increasing by more than 500 percent, a level not previously observed," said Wang. The computer model showed strong upwelling helped to bring extra iron, an important micro-nutrient for marine organisms, into the surface waters, stimulating phytoplankton growth. The study also found that since most zooplankton died off during the intense El Niño phase, there were fewer of these ocean animals in the surface water to eat phytoplankton, leading to large unhindered phytoplankton blooms.

As phytoplankton flourish, a large amount of carbon is used to build cells during photosynthesis. The plants get carbon from carbon dioxide in surface waters. In the atmosphere, carbon dioxide is an important greenhouse gas. When marine organisms die, they carry carbon in their cells to the deep ocean. Surprisingly, this study found that this "export of carbon increased by a factor of eight due to the large phytoplankton blooms," said Wang. This process, called the oceanic "biological pump." is an important mechanism that enables more carbon dioxide to be transferred from the atmosphere, to be stored in the ocean floor.

Clearly, this process helps to reduce the "greenhouse effect" by stabilizing concentrations of carbon dioxide in the atmosphere. The ocean is truly a key player in the future our global climate. Researchers will continue to study the impact of climate changes on marine ecosystems and NASA will continue to play a unique role by providing satellite observations that offer detailed information to advance scientific knowledge of the Earth system.

Co-authors on this study include James Christian, Ragu Murtugudde, and Antonio Busalacchi from Earth System Science Interdisciplinary Center, University of Maryland.