Nature: Phytoplankton, the tiny photosynthesizing organisms that inhabit Earth's oceans, provide half the planet's oxygen and sequester 100 megatons a day of atmospheric carbon dioxide. Phytoplankton also make up the oceanic food chain's first link. Monitoring the health of this vital population is essential but tricky because it fluctuates strongly on multiple scales of time and space. Now, as Nature's Quirin Schiermeier reports, a team led by Boris Worm of Dalhousie University in Halifax, Nova Scotia, has met that challenge by combining satellite imagery with data gathered over the past century from ocean-going ships. The team's conclusion is alarming: The total biomass is steadily falling at a rate of 1% per year, possibly because seawater is becoming warmer and more acidic. The accompanying image, taken by NASA's SeaWIFS orbiter, shows vast "rivers" of plankton (green) between two masses of seawater off the coasts of Argentina and the Falkland Islands.
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