It is a well-known axiom of human endeavor that "little things can make a big difference." This truism, it turns out, also applies to the interplay between Earth's atmosphere and its oceans. Tiny ocean plants known as phytoplankton help regulate the Earth's climate by absorbing carbon dioxide, a major greenhouse gas, from the atmosphere. In fact, phytoplankton account for about half of all the carbon dioxide absorbed by plants each year. These free-floating aquatic plants also form the base of the marine food pyramid. Any change in phytoplankton numbers alters the ocean food chain.

Now, new research at the University of Maryland shows these little marine plants may have an even bigger impact on the health and interaction of the oceans and climate than previously thought. Wendy Wang and colleagues at the University of Maryland, Earth System Science Interdisciplinary Center (ESSIC), have found that phytoplankton population and individual plant size can both change dramatically due to the physical processes associated with the climate phenomena known as El Niño and La Niña. In turn, these changes affect, not only ocean ecology, but also climate. Changes in the size and population of these plants change the amount of carbon dioxide that they absorb from the atmosphere and how much carbon is stored in the ocean as living plants or decayed plant matter.

This ongoing research builds on earlier work and was conducted with colleagues James Christian, Ragu Murtugudde, and Antonio Busalacchi, also from Earth System Science Interdisciplinary Center, University of Maryland.

Image and Video courtesy of NASA

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."

Image and Video courtesy of NASA

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 and stored on the ocean floor. It helps to reduce the "greenhouse effect" and the intensity of global warming by stabilizing concentrations of carbon dioxide in the atmosphere.

Related stories:

Fall 2004 feature article Pieces in a Global Picture: Maryland Research Paints Key Portions of an Earth Science Portrait of Climate Change

September 17, 2003 Ancient Algae Yield New Insights Into CO2 in Early Atmosphere

October 14, 2002 Maryland-Led Research Revises Estimates of Tropical Deforestation

July 19, 2001 Researchers Find El Nino Affects the Global Carbon Cycle

April 24, 2001 Bush's Environmental Policy: The First Hundred Days

December 2, 1999 Science Team Finds That Humans Probably Are Contributing to Retreat of Arctic Sea Ice

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