The number of marine phytoplankton, the microscopic organisms that gobble greenhouse gases and directly or indirectly feed every animal in the ocean, has been declining by about 1% of the global average per year, according to a new study. If the trend continues, it could decimate ocean food chains and accelerate global warming.
Researchers know that phytoplankton numbers have been dropping for the past 30 years. Satellite images show a decline in the concentration of chlorophyll—a green pigment that helps phytoplankton photosynthesize. But because satellites have been collecting data only since the late 1970s, scientists couldn't determine whether this drop was a long-term trend or just a fluke.
To get a more comprehensive record of phytoplankton numbers, Boris Worm of Dalhousie University in Halifax, Canada, and colleagues dug into old shipboard records from sailors who had studied the ocean as far back as 1900. In those days, sailors used a tool called a Secchi disk to gauge how clear the ocean was. They weren't trying to measure phytoplankton, but they inadvertently did because chlorophyll clouds the water.
When Worm and colleagues combined the satellite data, the early shipboard records, and direct measurements of chlorophyll made from the 1950s onward, they found that the recent dip in phytoplankton wasn't a passing phase. It had been happening in most parts of the ocean for more than a century. On average, the planet has lost 1% of its phytoplankton every year since 1900, the team reports in the 29 July issue of Nature.
"You compound that over a century, this becomes a huge, huge decline," says Paul Falkowski, an oceanographer at Rutgers University in New Brunswick, New Jersey, who was not part of the study. Indeed, Worm's team estimates that phytoplankton numbers have plummeted 40% since 1950.
What's more, the team found that phytoplankton numbers were more likely to dwindle in areas of the ocean that were warming, suggesting that climate change is responsible for the drop.
The loss of phytoplankton is a huge problem for marine food chains, says Worm, because every creature in the ocean either eats phytoplankton or eats other organisms that depend on it. If their numbers start to decrease, the populations of these species would drop as well. "The rest of the food web would basically contract," he says.
Even more chilling to marine biologist Anthony Richardson of the University of Queensland in Australia is the potential impact on our atmosphere. The ocean absorbs 40% of the CO2 humans emit. Phytoplankton, in turn, convert that CO2 into oxygen or die and bury it at the bottom of the ocean. If the phytoplankton are disappearing, Richardson says, "the ocean as a carbon sink is declining, and what that means is ultimately more CO2 will stay in the atmosphere instead of being dissolved in the ocean." That will translate into a warmer world, which will wipe out even more phytoplankton.
The study has its drawbacks. The older shipboard data weren't collected with nearly as much regularity as the satellite data, notes marine biologist Mike Behrenfeld of Oregon State University, Corvallis. Still, marine biologist David Siegel of the University of California, Santa Barbara, says that given the sporadic records, Worm and colleagues have constructed a solid report. "They've squeezed as much as possibly can be squeezed out of this data set."