A sensor on the Aqua satellite measures the health of phytoplankton in the oceans by checking the amount of florescent red light they emit, NASA reported.

The Moderate Resolution Imaging Spectroradiometer (MODIS) is able to determine how well and efficiently the plankton are functioning, while measuring chlorophyll determines how much plankton are present in the oceans.

Using MODIS on Aqua permitted researchers to conduct the first global analysis of the health and productivity of ocean plants, according to the space agency.

Researchers also can study how changes in the global environment alter these processes at the center of the ocean food web.

Single-celled phytoplankton fuel nearly all ocean ecosystems, serving as the most basic food source for marine animals. Phytoplankton account for half of all photosynthetic activity on Earth and play a key role in the balance of carbon dioxide in the atmosphere. The health of these marine plants affects the amount of carbon dioxide the ocean can absorb from the atmosphere and how the ocean responds to a changing climate.

"This is the first direct measurement of the health of the phytoplankton in the ocean," said Michael Behrenfeld, a biologist who specializes in marine plants at Oregon State University. "We have an important new tool for observing changes in phytoplankton every week, all over the planet."

All plants absorb energy from the sun, typically more than they can consume through photosynthesis. A small fraction of this extra energy is re-emitted as fluorescent light in red wavelengths.

MODIS is the first instrument to observe this signal on a global scale.

"The amount of fluorescent light emitted is not constant,

Behrenfeld said. "I changes with the health of the plant life in the ocean."

Scientists previously used satellite sensors to track the amount of plant life in the ocean by measuring the amount and distribution of chlorophyll.

"Chlorophyll gives us a picture of how much phytoplankton is present,"

said co-author Scott Doney, a marine chemist from the Woods Hole Oceanographic Institution in Woods Hole, Mass. "Fluorescence provides insight into how well they are functioning in the ecosystem."

With this new measurement, the scientists discovered large areas of the Indian Ocean where phytoplankton were under stress from iron deficiency. They were surprised to see large portions of the ocean "light up" seasonally as phytoplankton responded to a lack of iron in their diet. The amount of fluorescence increases when phytoplankton have too little iron, a nutrient in seawater. Iron reaches the sea surface on winds blowing dust from deserts and other arid areas, and from upwelling currents.

The research team detected new regions of the ocean affected by iron deposition and depletion. In the fall and winter and especially the summer, significant southwesterly winds over the Indian Ocean stir up ocean currents and bring more nutrients up from the depths for the phytoplankton to feed on. At the same time, the amount of iron-rich dust delivered by winds is reduced.

Climate change could mean stronger winds pick up more dust and blow it to the sea, or less intense winds leave waters dust-free. Some regions will become drier and others wetter, changing the regions where dusty soils accumulate and get swept up into the air.

Phytoplankton will reflect and react to these global changes.

"On time-scales of weeks to months, we can use this data to track plankton responses to iron inputs from dust storms and the transport of iron-rich water from islands and continents," Doney said. "Over years to decades, we also can detect long-term trends in climate change and other human perturbations to the ocean."

These findings appeared in the May edition of the journal Biogeosciences.