Fisheries in the tropics and mid-latitudes could be badly hit by the loss of these micro-organisms as a result of warmer waters, the paper implies.

Phytoplankton grow in the upper layers of the ocean, needing light as well as nitrogen, phosphate and iron to grow. These nutrients come from the cold deep ocean, and are brought to the surface by currents.

Oregon State University botanist Michael Behrenfeld and colleagues pored over nearly a decade's-worth of satellite data to see how these tiny, unsung plants of the ocean surface respond to shifts in temperature.

The NASA satellite SeaWiFS uses sensors to record light that is reflected back by the ocean. Banks of phytoplankton can be spotted because they contain chlorophyll, which absorbs red and blue parts of the light spectrum.

Behrenfeld's "map" of phytoplankton found that the mass underwent two big changes over the study period.

In 1997-98, phytoplankton increased, matching a period when the El Nino effect was in reverse and the seas were relatively colder. Production of phytoplankton then declined from 1999 to 2004 as El Nino went back into an extended warming cycle. There was then a rise from 2005 to 2006.

The scientists say the results clearly link the sea's surface temperature with the abundance of phytoplankton, and thus provide an excellent indicator of what could happen in a warming climate.

Their paper appears in Thursday's issue of Nature, the weekly British science journal.

If -- as some simulations suggest -- global warming will lead to more frequent or permanent El Ninos, then the distribution of ocean biomass could be radically reshaped, the authors say.

The biggest victims would be tropical and mid-latitude waters.

In these regions, the light is good for phytoplankton photosynthesis but warmer water also tends to "sit" on denser, colder waters, which means there will be less upwelling of the vital nutrients.

Conversely, colder, higher-latitude seas could benefit.

These regions get less light, which is bad for photosynthesis, but they offer a good nutrient flow because of strong upwelling from the ocean floor.

Very often, though, these currents are so intense that the phytoplankton are drawn hundreds of metres (feet) into the ocean depths, where sunlight does not penetrate.

In the future, higher temperatures and an influx of fresh water from precipitation and melting ice may help dampen the currents, which would thus spur phytoplankton growth.

In a commentary on the research, Scott Doney, a geochemist of Woods Hole Oceanographic Institution in Massachusetts said there remained many unknowns about the future of phytoplankton.

"Ecosystems are complex and nonlinear... and unexpected phenomena may arise as we push the planet into this unknown climate state," said Doney.

Phytoplankton are not just an essential first link in the food chain on which other ocean lifeforms depend.

They also absorb carbon dioxide (CO2) from the atmosphere as part of photosynthesis -- so any disruption to this process could accelerate the climate-change mechanism.

Roughly 100 million tonnes of carbon are gobbled up each day by phytoplankton, according to the Behrenfeld study.

A similar amount of carbon sinks to the seafloor, through dead phytoplankton, or is gobbled up by other marine life which feed on phytoplankton.

Other factors that influence phytoplankton growth include dust blown from the land, and variations in solar radiation.

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