NASA Study Identifies Factors That Limit Growth of Ocean Plants

Washington, 01 Sep 2006

A NASA-sponsored study shows that by using a new technique, scientists can determine what factors limit the growth of ocean algae, or phytoplankton, and how that growth affects Earth's climate.

Phytoplankton is a microscopic ocean plant and an important part of the ocean food chain. By knowing what factors limit its growth, scientists can better understand how ecosystems respond to climate change, according to an August 31 NASA press release.

The study focused on phytoplankton in the tropical Pacific Ocean, an area that plays an important role in regulating the quantity of carbon dioxide released into the atmosphere and thereby influencing the world’s climate. This area of the ocean is the largest natural source of carbon dioxide to the atmosphere.

It is important that scientists understand how ocean plants behave because plants play a critical role in maintaining a healthy planet. Plants annually take up billions of tons of carbon dioxide from the atmosphere through photosynthesis and use it to create the food on which nearly all Earth’s other organisms depend for life.

"We concluded that nitrogen is the primary element missing for algae growth and photosynthesis in the northern portion of the tropical Pacific,†said Michael Behrenfeld, an ocean plant ecologist from Oregon State University, “while it was iron that was most lacking everywhere else [in the tropical Pacific]."

Scientists determined that when phytoplankton is stressed from lack of iron, it appears greener, or healthier, than it really is. Normally, greener plants are growing faster than less green plants. When iron is lacking, enhanced greenness does not mean phytoplankton are growing better. They are actually under stress and unhealthy. These findings explain why healthy-looking phytoplankton are actually not so healthy.

"Because we didn't know about this effect of iron stress on the greenness of algae or phytoplankton before,†Behrenfeld said, “we have always assumed that equally green waters were equally productive. We now know this is not the case, and that we have to treat areas lacking iron differently."

In the tropical Pacific, correcting for this "iron effect" in areas other than the northern portion decreases scientists' estimates of how much carbon dioxideocean plants photosynthesize for the region by roughly 2 billion tons. This figure represents a tremendous amount of carbon dioxidethat remains in the atmosphere that scientists until now had thought was being removed.

The information about the false health of phytoplankton allows scientists using computer models to re-create the movement of carbon dioxidearound the world much more accurately.

Satellite data from NASA's Sea-Viewing Wide Field-of-View Sensor made an important contribution to the study, but the real cornerstone of the discovery was ship-based measurements of fluorescence.

Fluorescence occurs when plants absorb sunlight and some of that energy is emitted as red light. Scientists looked at about 140,000 measurements of fluorescence made from 1994 to 2006 along 58,000 kilometers of ship tracks.

They found that phytoplankton emit much more fluorescence when they do not have enough iron. The researchers used this signal to determine which parts of the ocean are iron-stressed and which are nitrogen-stressed.

Nutrients that make ocean plants thrive, such as nitrogen and phosphate, come mainly from the deep ocean when water is mixed by the wind. Iron also can come from dust blowing in the air.

About half the photosynthesis on Earth occurs in the oceans, and the rest on land. Ocean and land plants share the same basic requirements for photosynthesis and growth: water, light and nutrients. When these three are abundant, plants are abundant. When any one of them is missing, plants suffer.

US Department of State
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