Mysteries Of The Deep Unlocked From Space

By Stephanie Cartier

In the chilly waters off the coast of the Pacific Northwest, you'll see sea anemones, starfish, and whales. But it's what you can't see that is most astounding. Millions of tiny single-celled plants spend their days floating through the world's oceans, and while you cannot see these plants without a microscope, they are responsible for maintaining life on earth. Known as phytoplankton, the plants consume tons of greenhouse gasses, produce about half the oxygen on Earth, and are the base food supply for marine life. But until recently, scientists had some big questions about a few basics of these tiny lifeforms.

On that quest is Mike Behrenfeld, an Oregon State University (OSU) researcher. OSU has teamed up with NASA and the Universities of Maine and California to come up with a method that can unlock the secrets of the tiny algae's work ethic using satellites that track carbon. And their findings could lead to improved computer models that allow predictions of how climate change will alter ocean and Earth ecosystems.

Productivity has been the main focus of this research, and to determine that, scientists needed to know growth rate and biomass. Productivity is measured by the amount of carbon that gets taken up by phytoplankton and turned into living matter. To find it, scientists must know the growth rate and amount of phytoplankton, but doing so on a large scale has been tricky.

One way to think about productivity is to imagine the differences of a lawn in the spring and winter. Although it may look green during both seasons, the lawn does not need to be mowed as often in the winter because it is not growing as fast. To determine the productivity of the lawn, you need to know the amount of grass and the rate that it grows.

For the past two decades, satellites have tracked the oceans' tiniest residents by observing color. The idea was that when ocean color shifted from blue to green, phytoplankton populations were growing and scientists could estimate the amounnt of the biomass based on production of the green pigment chlorophyll. But researchers knew the flaws of that method. Observations of chlorophyll vary with sunlight intensity and phytoplankton health, and plants growing in low light still produce as much chlorophyll as plants growing in full sunlight, so the green pigment is actually based on the abundance of nutrients in the water. Likewise, when nutrients like nitrogen and phosphorous are in short supply, phytoplankton cells respond by not producing as much chlorophyll. The new method provides information on the amount of carbon in phytoplankton and how green the cells are.

Like all plants, phytoplankton use carbon to grow, so looking at carbon consumption is a more accurate way to gauge productivity. "Being able to assess how much chlorophyll there is per unit of carbon is key to saying how fast these guys are growing, fixing carbon, producing oxygen and all the rest,” says Emmanuel Boss of the University of Maine School of Marine Sciences. While scientists observe ocean color from satellite, part of that color is the reflection of light off particles in the water back to space, known as backscattering. Reports indicate that the proportion of backscattered light due to phytoplankton is relatively constant. Knowing that would help scientists determine biomass. "Chlorophyll divided by this biomass tells us what the physiological status of the algae is. Factor in light, temperature and its correlation to nutrients, and now we can estimate how fast they are growing,” says Boss.

Understanding productivity could lead to better knowledge of marine ecosystems and how they function, including issues related to fisheries, water quality and algal blooms, like red tides, which devastate marine life off the coast of the Pacific Northwest. Having knowledge of the tiny microorganisms could lead to an understanding of global fisheries. "You don't catch fish if there are no phytoplankton,” says Behrenfeld.

For years, oceanographers believed that there was no way to measure growth rate on a global scale. A phytoplankton biomass is highly variable, and turnover is hard to estimate. While there are literally tons of them floating through the world's oceans, their life cycle can be unpredictable. Phytoplankton can double their quantity in as little as 24 hours. On the other hand, they can die or be consumed and sink to the ocean floor in less than a week. It's only been recently that scientists realized the significance of the tiny plants. "It was only in the late 1800s that we even realized these tiny plants formed the base of the marine food web,” says Behrenfeld. "By the 1950s, we had figured out how to accurately measure their production and use observations of chlorophyll to determine their biomass. But until now, we've never been able to measure their rate of production over large areas.”

Now that the measurements are possible, scientists say the applications point out the health of phytoplankton on a global scale, which can act as a litmus test for the health of other ocean life. While the findings represent a big step forward in understanding marine health, there's still a lot of work ahead. "We haven't reached Oz yet, but what we have now found is the Yellow Brick Road,” says Behrenfeld.

"This approach was developed in the lab. Other people working in algal physiology have shown the relationship between chlorophyll and carbon to growth rate and its consistency across different species in the lab. They have shown exactly how this happens. The big leap is to show that we can do it with satellites on a global scale. All the pieces were there,” says Boss.

To be able to fully use the new approach, new satellite systems will be needed that can more accurately determine both the color and brightness of marine waters. Behrenfeld and colleagues are working on a satellite concept that would produce those measurements, called the Ocean Radiometer for Carbon Assessment. Behrenfeld says the team is waiting an opportunity to propose such a mission to NASA. If successful, it would likely take four years between approval time and launch.

Stephanie Cartier is pursuing a master's degree in technical communication at the University of Washington.

Images

Top: Satellites have made detailed measurements of ocean color and help provide new insights to the health of the world's oceans.

Middle: Phytoplankton might not look that powerful individually, but by the ton, they produce about half of the world's oxygen and consume tons of carbon dioxide.

Photos: NASA