Scientists discover trigger for immense Atlantic plankton bloom
Oceanographer Amala Mahadevan with a model showing the role of eddies in the bloom. Credit: WHOI
On the recent 4th of July holiday, U.S. beachgoers thronged their way to seaside resorts and parks to celebrate with holiday fireworks.
Across the horizon and miles out to sea toward the north, the Atlantic Ocean's own spring and summer ritual was unfolding: the blooming of countless microscopic plant plankton, or phytoplankton.
In what's known as the North Atlantic Bloom, an immense number of phytoplankton burst into color, first "greening" then "whitening" the sea as one species follows another.
In research results published in this week's issue of the journal Science, scientists report evidence of what triggers this huge bloom.
Whirlpools, or eddies, swirl across the surface of the North Atlantic Ocean sustaining phytoplankton in the ocean's shallower waters where they can get plenty of sunlight to fuel their growth, keeping them from being pushed downward by the ocean's rough surface.
The result is a burst of spring and summer color atop the ocean's waters.
How important is the bloom to the North Atlantic Ocean and beyond—to the global carbon cycle?
Much like forests, springtime blooms of microscopic plants in the ocean absorb enormous quantities of carbon dioxide, emitting oxygen via photosynthesis.
Their growth contributes to the oceanic uptake of carbon dioxide, amounting globally to about one-third of the carbon dioxide humans put into the air each year through the burning of fossil fuels.
The North Atlantic is critical to this process; it's responsible for more than 20% of the ocean's uptake of carbon dioxide.
An important scientific question is how this "biological pump" for carbon might change in the future as Earth's climate evolves.
In winter, strong winds generate mixing that pushes phytoplankton into deeper waters, robbing them of sunlight but drawing up nutrients from the depths. As winter turns to spring, days are longer and plankton are exposed to more sunlight, fueling their growth.
"Our results show that the bloom starts through eddies, even before the sun begins to warm the ocean," says Amala Mahadevan, an oceanographer at the Woods Hole Oceanographic Institution in Massachusetts and lead author of the Science paper.
Co-authors of the paper are Eric D'Asaro and Craig Lee of the University of Washington, and Mary Jane Perry of the University of Maine.
The National Science Foundation (NSF) funded the research.
Barely visible against roiling seas: scientist Eric D'Asaro with robotic floats he developed. Credit: Craig Lee
"Every undergraduate who takes an introductory oceanography course learns about the ecological and climate significance of the North Atlantic Bloom—as well as what causes it," says Don Rice, program director in NSF's Division of Ocean Sciences, which funded the research. "This study reminds us that, when it comes to the ocean, the things we think we know hold some big surprises."
The newly discovered mechanism helps explain the timing of the spring and summer bloom, known to mariners and fishers for centuries and clearly visible in satellite images.
It also offers a new look at why the bloom has a patchy appearance: it is shaped by eddies that, in essence, orchestrate its formation.
Making the discovery was no easy feat. "Working in the North Atlantic Ocean is challenging," says Perry, "but we were able to track a patch of seawater off Iceland and follow the progression of the bloom in a way that hadn't been done before."
"Our field work was set up with floats, gliders and research ships that all worked tightly together," adds D'Asaro. "They were in the same area, so we could put together a cohesive picture of the bloom."
The scientists focused on phytoplankton known as diatoms. Diatoms live in glass houses—walls made of silica. "When conditions are right, diatom blooms spread across hundreds of miles of ocean," says Lee, "bringing life-sustaining food to sometimes barren waters."
In April 2008, Lee, Perry and D'Asaro arrived in a storm-lashed North Atlantic aboard the Icelandic research vessel Bjarni Saemundsson.
They launched specially-designed robots in the rough seas. A float that hovered below the water's surface was also deployed. It followed the motion of the ocean, moving around, says D'Asaro, "like a giant phytoplankton."
Lurking alongside the float were six-foot-long, teardrop-shaped gliders that dove to depths of up to 1,000 meters. After each dive, the gliders, working in areas 20 to 50 kilometers around the float, rose to the surface, pointed their antennas skyward and transmitted their stored data back to shore.
The float and gliders measured the temperature, salinity and velocity of the water, and gathered information about the chemistry and biology of the bloom itself—oxygen, nitrate and the optical signatures of the phytoplankton.
Scientists aboard two ships, the Woods Hole-operated research vessel Knorr and Iceland's Bjarni Saemundsson, visited the area four times.
Soon after measurements from the float and gliders started coming in, the scientists saw that the bloom had started, even though conditions still looked winter-like.
"It was apparent that some new mechanism, other than surface warming, was behind the bloom's initiation," says D'Asaro.
To find answers, the researchers needed sophisticated computer modeling.
Enter Mahadevan, who then used three-dimensional computer models to look at information collected at sea by Perry, D'Asaro and Lee.
She generated eddies in a model, using the north-to-south variation of temperature in the ocean. The model showed that without eddies, the bloom happened several weeks later and didn't have the space and time structures actually observed in the North Atlantic.
In future research, the scientists hope to put the North Atlantic Bloom into a broader context. They believe that much could be learned by following the bloom's evolution across an entire year, especially with gliders and floats outfitted with new sensors. The sensors would look at the zooplankton that graze on a phytoplankton smorgasbord.
These data could be integrated, say the oceanographers, into models that would offer a more complete story.
"What we're learning about eddies is that they're a critical part of life in the ocean," says Perry. "They shape ocean ecosystems in countless ways."
Eddies and phytoplankton, the researchers believe, are central to the oceanic cycling of carbon, without which climate on Earth would look very different.
"We envision using gliders and floats to make measurements—and models—of ocean physics, chemistry and biology," says D'Asaro, "that span wide regions of the world ocean."
And that, says Lee, would spark a new understanding of the sea, all from tiny plankton that each spring and summer bloom by the millions and millions.
Source: National Science Foundation