Scientists Discover New Trigger for Immense North Atlantic Plankton Bloom

Phenomenon of spring and summer is jump-started by swirling currents of seawater

On this July 4th week, U.S. beachgoers are thronging 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 is 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 percent 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.

"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.

 -NSF-

Global Warming Refuge Discovered Near At-Risk Pacific Island Nation of Kiribati

Ocean currents may mitigate warming near handful of equatorial islands

Scientists predict ocean temperatures will rise in the equatorial Pacific by the end of the century, wreaking havoc on coral reef ecosystems.

But a new study shows that climate change could cause ocean currents to operate in a way that mitigates warming near a handful of islands right on the equator.

Those islands include some of the 33 coral atolls that form the nation of Kiribati. This low-lying country is at risk from sea-level rise caused by global warming.

Surprisingly, these Pacific islands within two degrees north and south of the equator may become isolated climate change refuges for corals and fish.

"The finding that there may be refuges in the tropics where local circulation features buffer the trend of rising sea surface temperature has important implications for the survival of coral reef systems," said David Garrison, program director in the National Science Foundation's (NSF) Division of Ocean Sciences, which funded the research.

Here's how it could happen, according to the study by Woods Hole Oceanographic Institution (WHOI) scientists Kristopher Karnauskas and Anne Cohen, published today in the journal Nature Climate Change.

At the equator, trade winds push a surface current from east to west.

About 100 to 200 meters below, a swift countercurrent develops, flowing in the opposite direction.

This, the Equatorial Undercurrent (EUC), is cooler and rich in nutrients. When it hits an island, like a rock in a river, water is deflected upward on an island's western flank.

This upwelling process brings cooler water and nutrients to the sunlit surface, creating localized areas where tiny marine plants and corals flourish.

On color-enhanced satellite maps showing measurements of global ocean chlorophyll levels, these productive patches of ocean stand out as bright green or red spots--for example, around the Galapagos Islands in the Eastern Pacific.

But as you gaze west, chlorophyll levels fade like a comet tail, giving scientists little reason to look closely at scattered low-lying coral atolls in that direction.

These islands are easy to overlook because they are tiny, remote, and lie at the far left edge of standard global satellite maps that place continents in the center.

Karnauskas, a climate scientist, was working with coral scientist Cohen to explore how climate change would affect central equatorial Pacific reefs.

When he changed the map view on his screen in order to view the entire tropical Pacific at once, he saw that chlorophyll concentrations jumped up again exactly at the Gilbert Islands on the equator.

Satellite maps also showed cooler sea surface temperatures on the west sides of these islands, part of Kiribati.

"I've been studying the tropical Pacific Ocean for most of my career, and I had never noticed that," he said. "It jumped out at me immediately, and I thought, 'there's probably a story there.'"

So Karnauskas and Cohen began to investigate how the EUC would affect the equatorial islands' reef ecosystems, starting with global climate models that simulate effects in a warming world.

Global-scale climate models predict that ocean temperatures will rise nearly 3 degrees Celsius (5.4 degrees Fahrenheit) in the central tropical Pacific.

Warmer waters often cause corals to bleach, a process in which they lose the tiny symbiotic algae that live in them and provide vital nutrition.

Bleaching has been a major cause of coral mortality and loss of coral reef area during the last 30 years.

Even the best global models, with their planet-scale views and lower resolution, cannot predict conditions in areas as small as these small islands, Karnauskas said.

So the scientists combined global models with a fine-scale regional model to focus on much smaller areas around minuscule islands scattered along the equator.

To accommodate the trillions of calculations needed for such small-area resolution, they used the new high-performance computer cluster at WHOI called "Scylla."

"Global models predict significant temperature increases in the central tropical Pacific over the next few decades, but in truth conditions can be highly variable across and around a coral reef island," Cohen said.

"To predict what the coral reef will experience in global climate change, we have to use high-resolution models, not global models."

The model predicts that as air temperatures rise and equatorial trade winds weaken, the Pacific surface current will also weaken by 15 percent by the end of the century.

The then-weaker surface current will impose less friction and drag on the EUC, so this deeper current will strengthen by 14 percent.

"Our model suggests that the amount of upwelling will actually increase by about 50 percent around these islands and reduce the rate of warming waters around them by about 0.7 C (1.25 F) per century," Karnauskas said.

A handful of coral atolls on the equator, some as small as 4 square kilometers (1.54 square miles) in area, may not seem like much.

But Karnauskas' and Cohen's results say that waters on the western sides of the islands will warm more slowly than at islands 2 degrees, or 138 miles, north and south of the equator that are not in the path of the EUC.

That gives the Gilbert Islands a significant advantage over neighboring reef systems.

"While the mitigating effect of a strengthened Equatorial Undercurrent will not spare corals the perhaps-inevitable warming expected for this region, the warming rate will be slower around these equatorial islands," Karnauskas said.

"This may allow corals and their symbiotic algae a better chance to adapt and survive."

If the model holds true, even if neighboring reefs are hard-hit, equatorial island coral reefs may survive to produce larvae of corals and other reef species.

Like a seed bank for the future, they might be a source of new corals and other species that could re-colonize damaged reefs.

"The globe is warming, but there are things going on underfoot that will slow that warming for certain parts of certain coral reef islands," said Cohen.

"These little islands in the middle of the ocean can counteract global trends and have a big effect on their own future," Karnauskas said, "which I think is a beautiful concept."

 -NSF-

Scientists Assess Radioactivity in the Ocean from Japan Nuclear Power Facility

With current news of additional radioactive leaks from the Fukushima nuclear power plants, the impact on the ocean of releases of radioactivity from the plants remains unclear.

But a new study by U.S. and Japanese researchers analyzes the levels of radioactivity discharged in the first four months after the accident.

It draws some basic conclusions about the history of contaminant releases to the ocean.

The study was conducted by Woods Hole Oceanographic Institution chemist Ken Buesseler and two colleagues based in Japan, Michio Aoyama of the Meteorological Research Institute and Masao Fukasawa of the Japan Agency for Marine-Earth Science and Technology.

They report that discharges from the Fukushima Dai-Ichi nuclear power plants peaked one month after the March 11 earthquake and tsunami that precipitated the nuclear accident, and continued through at least July.

Their study finds that the levels of radioactivity, while high, are not a direct threat to humans or marine life, but cautions that the effect of accumulated radionuclides in marine sediments is poorly known.

The release of radioactivity from Fukushima--both as atmospheric fallout and direct discharges to the ocean--represents the largest accidental release of radiation to the ocean in history.

Concentrations of cesium-137, a radioactive isotope with a 30-year half-life, at the plants' discharge points to the ocean peaked at more than 50 million times normal/previous levels.

Concentrations 18 miles offshore were higher than those measured in the ocean after the Chernobyl accident 25 years ago.

This is largely related to the fact, says Buesseler, that the Fukushima nuclear power plants are located along the coast, whereas Chernobyl was several hundred miles from the nearest salt water basins, the Baltic and Black Seas.

However, due to ocean mixing processes, the levels are rapidly diluted off the northwest coast of Japan.

The study used data on the concentrations of cesium-137, cesium-134 and iodine-131 as a basis to compare the levels of radionuclides released into the ocean with known levels in the sea surrounding Japan prior to the accident.

The resulting paper, Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity, is published in the current issue of the journal Environmental Science & Technology.

Buesseler was awarded a rapid-response grant from the National Science Foundation's (NSF) Division of Ocean Sciences to establish baseline concentrations of radionuclides in the Atlantic and Pacific Oceans.

"Understanding and management of the long-term geochemical fate and ecological consequences of radiochemical contamination of the sea is dependent on our knowledge of the initial conditions," says Don Rice, director of NSF's Chemical Oceanography Program. "Acquiring that knowledge depends on our ability to deploy experts to the scene with minimal delay."

The investigators compiled and analyzed data on concentrations of cesium and iodine in ocean water near the plants' discharge points.

The data were made public by TEPCO, the electric utility that owns the plants, and the Japanese Ministry of Culture, Sports, Science and Technology.

The team found that releases to the ocean peaked in April, a fact they attribute to "the complicated pattern of discharge of seawater and freshwater used to cool the reactors and spent fuel rods, interactions with groundwater, and intentional and unintentional releases of mixed radioactive material from the reactor facility."

The scientists also found that the releases decreased in May by a factor of 1,000, "a consequence of ocean mixing and a primary radionuclide source that had dramatically abated," they report.

While concentrations of some radionuclides continued to decrease, by July they were still 10,000 times higher than levels measured in 2010 off the coast of Japan.

This indicates that the plants "remain a significant source of contamination to the coastal waters off Japan," the researchers report.

"There is currently no data that allow us to distinguish between several possible sources of continued releases," says Buesseler.

"These most likely include some combination of direct releases from the reactors, or storage tanks or indirect releases from groundwater beneath the reactors or coastal sediments, both of which are likely contaminated from the period of maximum releases."

Buesseler says that at levels indicated by these data, the releases are not likely to be a direct threat to humans or marine biota in the surrounding ocean waters.

There could be an issue, however, if the source remains high and radiation accumulates in marine sediments.

"We don't know how this might affect benthic marine life, and with a half-life of 30 years, any cesium-137 accumulating in sediments or groundwater could be a concern for decades to come," he says.

While international collaborations for comprehensive field measurements to determine the full range of radioactive isotopes released are underway, says Buesseler, it will take some time before results are available to fully evaluate the impacts of this accident on the ocean.

The Gordon and Betty Moore Foundation also funded the research.

-NSF-