The foundation of the ocean food chain is eroding, and global warming is partly to blame.
That's the broad conclusion from a newly released study of a century's worth of measurements of the abundance of phytoplankton in the world's oceans.
Between 1899 and 2008, phytoplankton – microscopic, plant-like organisms in ocean surface waters – declined by roughly 1 percent of the global average per year, the study estimates. That works out to a 40 percent drop in amount of phytoplankton between 1950 and 2008, according to the study, which appears in tomorrow's issue of the journal Nature.
Beyond disruptions to the ocean food chain, such a decline would undercut the ocean's ability to take up the carbon dioxide humans have pumped into the atmosphere through increased burning of coal, oil, and gas, as well as through land-use changes, say scientists.
If the findings hold up to additional scrutiny, "that's quite remarkable," says Peter Franks, a phytoplankton ecologist at the Scripps Institution of Oceanography in La Jolla, Calif. "If it's true, there's a lot of bad stuff going on."
Phytoplankton use sunlight to convert carbon dioxide into oxygen and into the sugars that keep the plankton alive long enough to become another creature's meal. By some estimates ocean phytoplankton are responsible for half of all the photosynthetic activity on the planet.
The trend noted in the study becomes most pronounced near the poles and in the tropics since 1950, the researchers say.
Of the factors the team considered to explain the decline, the most influential appeared to be rising sea-surface temperatures – a trend many other scientists have traced to global warming.
How 'staggering' study was done
The size of the change "is staggering," says Daniel Boyce, the lead author of the Nature paper describing the results and how the team arrived at them.
Mr. Boyce, who is working toward a doctorate at Dalhousie, says he began the study three years ago as part of a larger effort to tease out the factors accounting for a significant decline globally in fish at the top of the food chain. Overfishing is one obvious suspect. But changes at the bottom of the food chain also could be playing a role.
Satellite-based estimates of trends in phytoplankton abundance date only to 1979. So Boyce and two Dalhousie colleagues explored the possibility of pushing the record back deeper in time.
The trio spent three years combing historical oceanographic data gathered with a low-tech device known as a Secchi disk – a white and black disk lowered into the water to a point where someone on the surface no longer can detect the pattern on the disk. In the open ocean, the nearly 150-year-old technique has proven remarkably accurate at estimating the concentration of phytoplankton at or near the surface. The Secchi disk results closely match those taken by more modern, high-tech methods, the researchers say.
The team discarded measurements that could have been influenced by factors such as silt from river run-off or by pollution-induced coastal algae blooms. The scientists also tossed out measurements that had been miscalcuated or were biologically impossible.
That process left them with slightly more than 445,000 measurements over the 100-year period that met their quality-control requirements.
As they analyzed the data, the scientists also placed less weight on the oldest data and those from the southern Atlantic Ocean and the southern ocean around Antarctica, because those data were gathered most sparsely and those regions were the least studied over the full period.
After carving up the marine map into 10 regions, the team found a lot of year-to-year variation in phytoplankton populations, as well as regional variations in abundance. But the century-long decline was evident in 8 of 10 ocean regions and was strong enough to offset gains in two others centered in the Indian Ocean.
In effect, warming surface waters have acted as a lid, preventing deeper, nutrient-rich waters from mixing upward to feed phytoplankton at the surface.
Too dramatic too be believed?
While Boyce says the process of gathering and analyzing the information was a challenge, colleague Marlon Lewis says that for him, the most difficult part of the study was was believing the results.
"The toughest hurdle I had was coming to grips with the results," Dr. Lewis says. "We sent Daniel back I can't tell you how many times to redo the calculations or look at it in different ways."
In the end, however, the results held.
For his part, Scripps' Dr. Franks says he remains "slightly skeptical" of the results. The Secchi-disk data allows the team to get pretty good coverage of the oceans over the period. But the less-than-complete data from some oceans gives him pause, he says.
Still, he adds, "given that this is about the only data set that would speak to this issue, I think they've done it as carefully as one can do it. I tend to believe the results."