For half a century, researchers have struggled to estimate changes in the size and condition of phytoplankton stocks.

But the University of Maine School of Marine Sciences may be about to change all that.

Scientists there said on Tuesday they are now using satellite technology to determine phytoplankton growth rates and physiology.

Using colours

Chlorophyll - the green pigment that powers photosynthesis - has long been the benchmark for estimates of the size of phytoplankton populations and how productive they are.

But estimates have always been made on Earth.

The new approach also comes down to the colour of water and intensity of that light but as seen from space. Satellite data may lead to significant revisions in estimates of how much carbon the oceans absorb from the atmosphere.

It may also lead to new understanding of how shifts in phytoplankton populations echo through marine ecosystems, from the smallest bacteria to whales.
 
Phytoplankton provide the foundation for marine fisheries and, like all plants, help regulate the climate by using carbon to grow.

Old method problems

Until now, scientists have used measurements of chlorophyll to estimate the size or biomass of phytoplankton stocks, they have been well aware of weaknesses in that method.

Chlorophyll in the water varies with sunlight intensity and phytoplankton physiology.

Plants growing in low light conditions produce as much or more chlorophyll per cell as plants growing in full sunlight, says Boss.
 
However, plants need nutrients such as nitrogen and phosphorus, and when nutrients are in short supply, phytoplankton cells respond by not producing as much chlorophyll.

"There's no point absorbing all that light if you can't use it to fix more carbon. So 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 Maine University.

New technology

In place of chlorophyll, the new method substitutes an analysis of the amount of carbon in phytoplankton.

Key to the approach is ocean colour as measured by satellite. Satellites collect colour over a wide spectrum, but a component of that colour known as backscattering, the reflection of light off particles in the water back to space, is particularly useful.

Two Maine scientists, Collin Roesler of the Bigelow Laboratory of Ocean Sciences and Mary Jane Perry of University of Maine, developed a method to obtain backscattering from ocean colour in the mid-1990s.

"There is a big leap of faith in our work. And that's how you assess carbon," says Boss. "The leap of faith is based on several reports in the literature that the proportion of the backscattered light [attributed to] phytoplankton is relatively constant. If that's correct, then we can use backscattering to give us 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."