Balmy ocean waters are putting the squeeze on phytoplankton, tiny plants that collectively fix as much carbon dioxide as all terrestrial greenery combined. Their decline could threaten ocean ecosystems and contribute to global warming.
Daniel Boyce of Dalhousie University in Halifax, Nova Scotia, and his colleagues estimate that the global phytoplankton stock has plummeted 40 percent since 1950. They reported this finding in July after analyzing 50-plus years of data on light penetration of the ocean surface and plankton abundance in water samples. The die-off is due to a combination of rising sea surface temperatures and decreased ocean circulation between the higher and lower layers, Boyce says. Most phytoplankton dwell within 25 meters of the surface. The warmer this layer is, the more difficult it is for nutrients from the cold depths to mix in. As nutrients dwindle, so do the phytoplankton.
A continued decline would reverberate up the food chain and reduce atmospheric CO2 absorption, potentially accelerating climate change. “I think that the 40 percent global decrease that they report is provocative but not yet fully demonstrated,” says Michael Behrenfeld, an oceanographer at Oregon State University who studies phytoplankton. Analysis of satellite data and historical records could verify the numbers.