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What Causes the North Atlantic Plankton Bloom?

Understanding what causes annual plankton blooms in the North Atlantic could be key to understanding how these microscopic plants will respond to climate change
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Six days from now, every one of the billions of phytoplankton alive today will be dead—eaten by zooplankton or having drifted to the bottom of the sea. In fact, some of these microscopic plants, which collectively perform as much as photosynthesis as all of Earth's land-based plants, live for just two days.

But these microscopic plants have an outsize effect on the levels of carbon dioxide in the atmosphere—both by sucking it up during photosynthesis and by helping to drive the natural circulation of the ocean that lets denser, cooler water that has absorbed CO2 drop to the bottom of the sea in places like the North Atlantic. This natural sink is one of the largest ways that CO2, the most abundant greenhouse gas responsible for climate change, exits the atmosphere. Understanding how and why the tiny plants bloom each spring is therefore critical to understanding how the planet's living systems—and therefore the planet's elemental cycles—might respond to global warming.

For decades scientists have assumed that springtime ocean conditions were responsible for the annual plankton blooms, thanks to pioneering work by oceanographer Harald Sverdrup in the 1950s. But a new analysis of satellite records suggests it may not be as simple as the advent of spring conditions unleashing the photosynthetic potential of phytoplankton, according to a paper in the April issue of Ecology.

Phytoplankton ecologist Michael Behrenfeld of Oregon State University studied nine years of data from the NASA satellite SeaStar and its Sea-viewing Wide Field-of-view Sensor (SeaWiFS). This tool allows researchers to estimate the total surface chlorophyll concentrations in the oceans as well as relative carbon concentration in phytoplankton. Focusing in on the North Atlantic, Behrenfeld found that the increase in numbers of plankton revealed by chlorophyll and carbon concentrations start in the middle of winter—when growth conditions are at their worst—rather than being initiated by the changing spring weather. The reason appears to be the deepwater mixing caused by winter storms churning the ocean, and thereby making it hard for the tiny animals that eat phytoplankton to find their prey.

"The fraction of phytoplankton growth lost to the grazers gets smaller and smaller as you go into winter and deeper mixing," Behrenfeld explains. In essence, because the phytoplankton are spread more thinly throughout more water, or diluted, the would-be grazers have a harder time finding them. That allows the phytoplankton to begin to build up in midwinter, a head start in growth that is a prelude to the massive bloom once the winter's storms cease mixing and conditions for growth improve.

By the end of spring, the grazers catch up, consuming as much plankton as grows and bringing the bloom to a close, as well. But the new hypothesis tweaks the old understanding that zooplankton grazers and other losses essentially eliminate the same amount of phytoplankton at all times, as Sverdrup proposed in 1953.

Biogeochemist Jorge Sarmiento of Princeton University's Atmospheric and Oceanic Sciences Program, who was not involved in the study, calls the new finding that blooms start in midwinter "a provocative idea…the only point in time when growing exceeds grazing is very early on." He adds: "Let's see what the data says in the long run."

Of course, this method of estimating plankton concentrations by interpreting chlorophyll and carbon concentrations remains to be verified by field-testing. "The North Atlantic is a pretty nasty place to go in winter, no one wants to go there. A satellite doesn't care how nasty conditions are," Behrenfeld says, and notes that a new study he is conducting with ocean floats has provided data that supports his new hypothesis. "Now we need to go into the field and make some measurements."

Climate change may also provide a test. If it is indeed the dilution caused by deepwater mixing as a result of winter storms that sets the stage for the annual bloom, then a warmer world with fewer storms in the North Atlantic "should reduce the bloom," Behrenfeld notes. "Winter mixing depths are already shallower in the southern end [of the North Atlantic]. Likewise, we see that the magnitude of the blooms are smaller in the southern end."

Adds Sarmiento: "If [the North Atlantic phytoplankton are] either becoming more efficient or less, there would be feedbacks to the Earth balance of CO2. It's really important to understand the system and understand it well enough that we can predict how it's going to respond to climate change."

After all, if the phytoplankton bloom is diminished Earth might lose two carbon sinks: there would be less dead plankton bodies (having avoided consumption by grazers) that fall to the ocean floor along with potentially less sinking of CO2-rich surface waters. And that, in turn, could further exacerbate the climate change caused by extra atmospheric greenhouse gases.

It is also important because phytoplankton form the base of the marine food web, meaning many of the world's most productive fisheries rely in large part on the activities of these microscopic plants. And other regions of the world's oceans with similar blooms may follow similar patterns, such as the Arabian Sea and its monsoon-related blooms.

The long lead-in and drivers of the phytoplankton bloom are hardly the only mystery of the North Atlantic. After all, these annual blooms show rapid changes in the dominance of one type of phytoplankton quickly followed by another—yet the overall population size grows relatively smoothly. Behrenfeld asks about that mechanism, "How does that actually work?"

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