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Vital ocean phytoplankton a casualty of global warming?

A new study suggests that a global rise in ocean temperatures has cut the number of phytoplankton, which are the bedrock of the food chain, by 40 percent since 1950. Other scientists link the rise in ocean temperatures to global warming.

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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.

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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."