Curled up in the elbow-crook of nine countries in Northern Europe, the Baltic Sea holds a small portion of the world's water but a sizable portion of its history. It saw the Roman Empire rise and fall. The Vikings built a trade empire around it. Slavic pirates, German settlers and Dutch merchants alternately laid claim to its shores. It swallowed thousands of sunken planes and warships during both world wars.
Today the Baltic Sea holds a less illustrious claim to fame: it contains the largest man-made "dead zone" of oxygen-starved water in the world. In the last century, the sea has experienced a ten-fold increase in hypoxia, or low oxygen, primarily due to overloading of fertilizers, sewage, and other nutrients. The chronic shortage of oxygen lowers the Baltic's water quality, suffocates fish, and leads to large blooms of toxic bacteria.
For almost a decade now, Anders Stigebrandt, a 72-year-old retired oceanographer, has been working on a scheme to breathe oxygen back into the Baltic. He wants to use pumps to mix oxygen-rich water into the sea's hypoxic bottom depths. Now, he is one step closer to realizing his vision. In March's issue of the International Society for Microbial Ecology Journal, he and a team of other investigators reported that they had successfully used the pumping method to oxygenate a fjord called Byfjord off the west coast of Sweden that is roughly 2.5 miles long and 1 mile wide.
"We wanted to do a pilot experiment that could tell us something about oxygenating the Baltic," said Stigebrandt, who was a professor of oceanography at the University of Gothenberg for nearly two decades before retiring a couple years ago. "The Byfjord was the best example we could come up with, because it's very similar to the Baltic in many ways."
"This is a way to throw money into the sea and deviate the focus from solving the problem to treating symptoms."
However, the two bodies of water are different in at least one very important regard—the Baltic is vastly larger, which is why Stigebrandt estimates just getting his project off the ground would cost around $4 billion. "The funding would have to come from countries around the Baltic Sea," he said. After the initial set-up the pumps would have to run continuously, powered by electricity from the grid, supplemented by wind and possibly some small-scale wave energy.
Executing such a massive project requires scientific consensus and political will—but many experts are skeptical.
Oxygenating a fjord is one thing, but making a difference in the Baltic would require an extravagant number of pumps and power, said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium in the United States. She pointed out that while the total volume of Stigebrandt's study area was around 0.14 cubic kilometers, the Baltic Sea is nearly 21,000 cubic kilometers, five magnitudes of order larger.
Others are concerned that relying on geoengineering—the deliberate, large-scale manipulation of a natural system—would be a relatively easy way out of a problem that humans have created. "It's a pity because money is really needed in this area to do things like reduce nutrients from land and control overfishing," said Lars-Anders Hansson, a lake ecologist at Lunds University in Sweden. "This is a way to throw money into the sea and deviate the focus from solving the problem to treating symptoms."
Hansson also has feasibility concerns. In the 1970s, he said, many researchers experimented with pumping to oxygenate lakes, a process called hypolimnetic aeration. "The method proved not to work in small, closed systems," said Hansson. "So it's very surprising that someone wants to take this old technology and apply it to a large, open system."
Even researchers who are not reflexively opposed to the idea think it needs much more careful study first, including the creation of more oceanographic models to understand how the pumping would play out. "I don't have a strong opinion about geoengineering. We've been modifying our coastlines and polluting the Baltic Sea for centuries," said Jacob Carstensen, a marine ecologist at Aarhus University in Denmark. "But I think you have to be very cautious about going out and doing some large-scale manipulations with the Baltic Sea. What happens if they're not right? They could actually end up making things worse than they were before."
Some environmentalists, for instance, are worried that pumping could threaten already dwindling populations of cod, the most economically lucrative fish in the Baltic. Cod reproduction requires highly specific levels of salinity and oxygen, which a large-scale pumping project could disturb, according to Carstensen. "I'm quite certain if they do manipulations, it would reduce the cod reproductive volume of the Baltic Sea," he said.
Stigebrandt is not dismayed by all the criticism. He believes that concerns about cod—as with other worries that people have raised—are more of a kneejerk reaction against geoengineering than an evidence-based critique. "There are many problems that people raise without having thought about them very much," he said. "We have to do more studies to understand if these actually are problems."
This January, Stigebrandt published a study that addressed the concerns about cod. He and his collaborators developed and used a circulation model to forecast how deep-water ventilation in the Bornholm Basin, a region of the southwestern Baltic, would affect salinity and oxygen levels. "Our model showed that pumping would actually improve conditions for cod recruitment in the Baltic basin," he said.
In both the Baltic and the Swedish Byfjord, stratification is what keeps bottom waters hypoxic. In the Baltic, saltwater flows into the sea's depths from the North Sea, making the waters deeper than 100 meters especially dense and resistant to mixing with the lighter, brackish surface waters. In the Byfjord, oxygenated waters from the open ocean sporadically flow in through a shallow entrance but only mix with the fjord's surface waters instead of the dense, salty water that stagnates at the base of its 50-meter-deep basin.
Pumping helps jumpstart mixing in the water column, explained Alexander Treusch, a microbiologist at the University of Southern Denmark and one of the scientists who participated in the Byfjord project. That conveyor belt of mixing then increases inflows of oxygenated water from the open ocean. Those inflows occur without pumping, but relatively infrequently—maybe once every decade or so.
For their project, Stigebrandt, Treusch, and their collaborators rigged a pump that transported the fjord's oxygen-rich surface waters down to 35 meters depth. From about 12,000 total hours of on-and-off pumping between 2010 and 2013, the scientists observed four inflows of oxygen-rich water from the ocean. After two months of pumping, they could already detect higher oxygen concentrations in the fjord's bottom waters. By the end of the project, oxygenated water had replaced approximately half of the fjord's water below 15 meters.
Excessive nutrient loading is a problem because it boosts the growth of phytoplankton in waters. The phytoplankton, and the other creatures that eat them, sink to the water's depths when they die. There, bacteria respire the decaying organic matter, a process that uses up oxygen. The more nutrients there are, the more oxygen gets depleted.
Since the late 1980s, the countries surrounding the Baltic have been working together through an intergovernmental organization called the Baltic Marine Environment Protection Commission to reduce nutrient inputs to the Baltic Sea. The program has successfully slashed nitrogen and phosphorus inputs by around 30 to 50 percent, mostly by cutting runoff from agricultural and industrial activity. But despite the massive strides, hypoxia remains.
One big reason, according to Stigebrandt, is that phosphorus keeps cycling through the system. When there is oxygen in the deep Baltic, phosphorus that falls to the bottom of the sea gets absorbed by bacteria and bound to the sediments. However, when the deep waters have no oxygen, the phosphorus leaks from the bottom sediment into the water column, where it fuels more phytoplankton growth and further increases oxygen consumption in the deep waters.
"It's a vicious circle," said Stigebrandt. He estimates, based on a model he published last September, that it would take ten years of pumping water at 10,000 cubic meters per second to oxygenate the Baltic for a "long, indefinite amount of time." An alternative strategy of just reducing fertilizers and other nutrient supplies alone would require much longer—maybe a century or so, he said.
Hypoxia is a growing worldwide problem that extends beyond the Baltic Sea. A lot of the scientific focus has been on studying huge oxygen minimum zones (OMZs) in the Arabian Sea and the Eastern Pacific that are caused primarily by natural circulation patterns, said Frank Stewart, an assistant professor at the Georgia Institute of Technology who studies the microbes that live in OMZs.
"We have to wait maybe 100 years or oxygenate with pumps. That's the choice."
The Baltic Sea, he said, is an example of how human activities can also drive rampant hypoxia in coastal waters. Other similar systems include the Gulf of Mexico and Chesapeake Bay.
These hypoxic zones are not only driving away marine life and damaging water quality, they also potentially play a role in climate change by influencing global nitrogen and carbon cycles, said Bo Thamdrup, a microbial ecologist at the University of Southern Denmark.
For Stigebrandt, the Baltic pumping project has been almost a decade's worth of work so far. In his retirement he continues to collaborate with other researchers to clarify the environmental consequences of pumping. He thinks it could take another decade to convince policymakers and critics that his project is worth considering.
But Stigebrandt is ready to go today, and is certain that pumping is the cure the Baltic needs. Without it, he believes, people living on the Baltic most likely won't see the sea improve significantly in their lifetime.
"We have to wait maybe 100 years or oxygenate with pumps," he said. "That's the choice." To him, the way to proceed is clear.