Spring in the North Atlantic is formidable. Storm-lashed, frigid, gray. The subpolar region is so tumultuous that ever-vigilant space satellites often can’t penetrate the almost perpetual cloud cover, unable to provide a clear view of one of the most important life-sustaining events on the planet — the spring phytoplankton bloom.
That means if researchers ever hope to understand the phenomenon, they have to take to the high seas.
In 2008, an international expedition called the North Atlantic Bloom Experiment, funded by the National Science Foundation, did just that. It was the first to put marine scientists in the North Atlantic to observe the entire progression of the spring bloom over a three-month period, from development to demise.
They did that by using underwater robotic gliders and a float developed by University of Washington researchers that reported conditions between the surface and 1,000 meters several times per day, from early April through late June. High volumes of data were literally “phoned home” by the robots via Iridium satellite.
Now, the discoveries made possible by unprecedented, in situ data collection are being made public. The most recent announcement came in July when the National Science Foundation and the journal Science reported the results of one of the experiment’s studies — the discovery that the spring bloom can begin up to 30 days earlier than previously thought as the result of eddies stratifying the near-surface waters, rather than springtime warming of the ocean surface.
These new contributions to our understanding of the North Atlantic spring bloom, one of the largest in the world, will inform modeling by marine and climate scientists, according to University of Maine biological oceanographer Mary Jane Perry, who was among the 26 researchers from five countries on the expedition. The research findings also have implications for the Gulf of Maine, which is fed by the waters of the North Atlantic and supports similar species.
Major changes in the Gulf of Maine — including the influx of freshwater from accelerated melt in the Arctic and Greenland, and shifts in the marine food web — often occur first in the North Atlantic. But unlike the terrestrial ecosystem, scientists understand much less about North Atlantic phenology, Perry says, including annual variability, patterns and mechanisms.
“The North Atlantic is a really special place — a really important part of the ocean — because what happens there is so important to the atmosphere’s carbon dioxide cycle,” says Perry. “This subpolar region is responsible for more than 20 percent of the entire ocean’s uptake of carbon dioxide, and phytoplankton have an important role in that drawdown.”
Phytoplankton, which include diatoms and dinoflagellates, are microscopic plants at the base of the marine food web that fuel the ecosystem. The photosynthetic organisms also help maintain the health of the atmosphere by absorbing and sequestering carbon dioxide caused by the burning of fossil fuel.
For more than a quarter-century, starting at the University of California, San Diego, Perry has studied marine phytoplankton in an effort to understand its biomass variability and production dynamics. In recent years, her focus has been on the interaction of phytoplankton and light in the ocean.
The quest to better understand phytoplankton has taken Perry on two major expeditions to the subpolar North Atlantic. Her first research cruise was as a faculty member at the University of Washington in the early 1990s as part of a Cold War-era initiative. The focus was on light propagation in the open ocean, where phytoplankton play a role in how deep in the ocean light can penetrate. That variability was particularly important when employing laser technology to detect Russian submarines and for friendly undersea laser communications.
While it was the shortcomings of technology — the inability of moorings to provide a comprehensive spatial view and of satellites to see through clouds — that sent Perry to the North Atlantic, it was the latest in autonomous underwater profiling floats and sea gliders equipped with sensors that made the second expedition so successful. A mixed-layer float hovered in near-surface waters, moving with the ocean. The 6-foot gliders surveyed the area to depths of up to 1,000 meters, returning to the surface to transmit data.
The autonomous underwater robots compiled information about the physical environment, including temperature, salinity and velocity of the water, as well as data about the chemistry and biology of the phytoplankton bloom, oxygen and nitrate levels, and unique optical signatures of the tiny plants.
“When we started the expedition in 2008, a big storm greeted us and we were thrown about for three days,” Perry says. “We were tossed about by the waves that were crashing over the wheelhouse. It’s one of the many good reasons for doing data collection autonomously with this kind of technology.”
Just after joining the UMaine School of Marine Sciences in 1999, Perry was instrumental in the development of autonomous glider technology for remote deep-sea data gathering. Her contribution focused on leading a project that designed optical sensors to measure phytoplankton and particulate carbon in the water column. Her efforts to improve the efficiency and effectiveness of the sensor technology, including miniaturizing it from the size of a football to a hockey puck, is reflected in every autonomous marine glider manufactured in the United States today.
Perry’s interest in robotic technology was driven by her life-long interest in trying to measure and observe the patterns and amounts of phytoplankton as the base of the food web, and understand why the patterns change. On the 2008 expedition, which involved four research cruises of up to 21 days south of Iceland, data collected by optical, chemical and physical sensors on four gliders, a float and the ship were coupled with a 3D biophysical model. The result was unprecedented documentation of the spring bloom from beginning to end, including previously unknown aspects of its mechanics.
The discovery announced in July was the result of a study led by Amala Mahadevan of Woods Hole Oceanographic Institution, Eric D’Asaro Craig Lee of the University of Washington, and Perry. Their research revealed that eddies or small whirlpools of swirling seawater can switch on the bloom up to 30 days earlier than the natural confluence of seasonal heat and light.
Until this latest research, scientists using climate models understood that springtime warming of the ocean surface triggered the near-surface vertical density gradation, known as stratification. That stratification, which prevents vertical mixing of the phytoplankton, and the increased seasonal light exposure that occurs every spring were thought to be the primary prompts of the bloom.
Among the other breakthroughs was unprecedented documentation of critical phenomena essential to carbon sequestering. One study focused on the aggregate flux event that feeds the deep ocean and contributes to carbon dioxide sequestering. During these events, phytoplankton growing on the ocean surface form layers of aggregates and sink, providing food for deep-sea ecosystems and a carbon cycling function vital to the atmosphere.
Scientists have struggled to detect or estimate aggregate flux events that could ultimately inform estimates of carbon flux in the ocean. The study, led by Perry and the subject of a UMaine master’s thesis by Nathan Briggs, used optical sensors to collect data on the flux event, including sink rates, distribution, relative abundance and chlorophyll content of aggregates.
Perry has also co-authored other papers with expedition colleagues, including one to be submitted shortly, that reveals the importance of a specialized life cycle stage of a diatom species in carbon export from surface waters. Researchers discovered that as an essential nutrient, silicic acid, is depleted, the diatom enters an encapsulated life stage that makes it highly resistant to degradation and extremely efficient for transporting carbon to the depths of the sea.
“It’s all coming together in terms of our abilities to observe complete cycles in remote places for extended periods of time,” Perry says. “That’s important, because if the ocean changes, how will we know if we don’t look?”
“We have to be able to be there more than once or twice with a ship,” she says. “Such a snapshot is biased by whatever is occurring at that moment. We need a better view than what we get from satellites. We need long-term, sustained measurement. A persistent presence. We need a combination of autonomous sensing and detailed validation sensing — important parts of moving our understanding of the ocean forward.”