Every year, the north Atlantic ocean turns green with plankton, and for more than fifty years, scientists thought they knew why. Now, a decade’s worth of satellite measurements suggest they were wrong.

The common-sense idea that in the spring, the sun warms up the water column until it hits a key threshold and suddenly comes alive was formalized in 1953 by Norwegian oceanographer Harald Sverdrup. But the true beginning of the plankton blooms probably begins in the dark of winter.

“We found that the north Atlantic bloom was starting much earlier than we thought and it didn’t coincide with an improvement in the growth conditions from the phytoplankton,” Michael Behrenfeld, an phytoplankton ecologist at Oregon State University. “It started in January.”

Plankton blooms are a hot topic in the earth sciences because they are one determinant of how big a carbon reservoir the oceans can be. That’s important for climate science generally, and for would-be geoengineers specifically. A new fundamental understanding of plankton blooms could change the way we model our climate now and long into the future.

Phytoplankton harness the energy of the sun and draw on nutrients in the ocean like nitrogen, iron, and other elements that land plants get from soil. So, Sverdrup and others focused on solar and nutrient availability as the key to their growth. But Behrenfeld found that the phytoplankton’s growth rate started to accelerate in mid-winter, when the conditions for their growth were presumably the worst.

“When we said [the blooms] are beginning in the middle of winter, it meant that the basic understanding of what causes the blooms was wrong,” he said.

He floated an alternative thesis at a NASA event last month, which he says squares better with the new data. During the winter months, cold winds blowing across the water cool the top surface layers. Cold water sinks, pushing up some warmer water, which gets cooled itself and drops. The process creates convection and carries the tiny plankton through a much larger volume of the ocean, diluting them.

When the phytoplankton are spread out, it’s harder for the zooplankton that eat them to find them. Suddenly, the phytoplankton can breed like crazy without as much interference by predators. As spring arrives, the temperature of the surface water and the layers underneath it equalize. The convection stops. The water stops mixing.

Behrenfeld says that it’s at that moment — when the phytoplankton get stuck at the top of the ocean — that we notice the blooms.

“The decoupling between predators and prey occurred before but you start seeing the effect when the mixing stops,” he said.

If he’s right, the new model would have dramatic implications for ocean health in a warming world. If a warmer ocean is all that’s needed to spark plankton blooms, then global warming would lead to larger and larger blooms. With Behrenfeld’s model, a warming ocean would hurt the blooms. Considering that the blooms are the base of the oceanic food chain, that would hurt species ranging from the tiniest fish to the largest whales.

How did the winter plankton blooms not show up in previous data? Well, mostly because very few research vessels want to be bobbing around the north Atlantic in January. Behrenfeld thinks that introduced a bias in the data that seemed to show that the blooms began suddenly in the spring, when more researchers were out looking for them.

Sadly, the excellent satellite that’s provided Behrenfeld’s data — the SeaWIFS — has been on the fritz for the last year. While new fixes keep it working in fits and starts, full datasets don’t seem likely to get beamed down from on high. While the MODIS satellite provides some of the SeaWIFS functionality, it won’t provide the exact same data as older satellite, making long-term studies more difficult. No replacement for SeaWIFS is on the horizon.

See Also:

Animation: NASA/Goddard Space Flight Center Scientific Visualization Studio

WiSci 2.0: Alexis Madrigal’s Twitter, Google Reader feed, and green tech history research site; Wired Science on Twitter and Facebook.