The world’s phytoplankton appear to have been disappearing at a rate of about 1% a year for the last century, researchers announced Wednesday, a disturbing long-term trend for the microscopic algae that form the basis of the marine food chain and produce much of the world’s oxygen.

In reporting their findings in the journal Nature, the Canadian team said that, since 1950, phytoplankton biomass has shrunk about 40%. Scientists had known the population was shrinking, but the long-term nature of that reduction came as a surprise.

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