Dead in the Water

“Dead zones” might be something you’d find in a zombie apocalypse, but unfortunately, they’re also a part of our oceans. Thanks to untreated human waste and agricultural runoff, huge chunks of the world’s coastal waterways are nearly void of oxygen and are unable to sustain aquatic life. In the summer of 2002, a dead zone in the Gulf of Mexico swelled to the size of Massachusetts; others have popped up everywhere from the Long Island Sound and Chesapeake Bay, to the Baltic Sea in Europe.

But not all dead zones are man made. And not all of them are sticking to shallow waters either. In a paper published Thursday in Biogeosciences, a team of German and Canadian researchers disclosed that the first known dead zones in the open ocean are located in the tropical North Atlantic, several hundred miles off the West African coast. These dead zones are enveloped in swirling masses of water known as eddies.

“We are pretty sure that this is a natural phenomena,” says lead author Johannes Karstensen, a researcher at GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany.

He explains that winds along the African coast drive strong currents that become unstable as they pass by headlands. In the process, huge eddies occasionally spin off westward into the open ocean. The same effect, Karstensen says, can be observed by twirling a spoon in coffee or running your hand through water.

In 2006, Karstensen and his team installed an oxygen sensor at a mooring located about 500 miles from the coast of Mauritania. The sensor began detecting oxygen levels as low as 2 micromoles per liter of seawater, catching the researchers completely by surprise. “At first, we didn’t believe the measurements,” Karstensen says. “We thought there was something wrong with the calibration.” Prior to that, the lowest oxygen levels in the open waters of the North Atlantic were thought to be around 40 micromoles per liter of seawater.

With the help of additional oxygen sensors, satellite observations, drifting research floats, and ship expeditions to the area, Karstensen’s crew confirmed three dead zones that were up to 90 miles wide and 330 feet deep. The drop off in oxygen starts at about 60 feet below the water’s surface. The scientists believe that eddies are driving nutrients up to the surface of the ocean. These nutrient-rich patches are then taken over by algae, which as they die are decomposed by bacteria that sap the water of all its oxygen. It’s similar to man-made algae blooms caused by fertilizer and sewage runoff.

Dead zones along the coast can lead to massive fish and shellfish die-offs, which in turn, are harmful to aquatic predators like seabirds. There are now hundreds of these algae-plagued areas around the world. “The overall area occupied by these dead zones is increasing,” Karstensen says, “and it is really a serious threat to the ecosystems.” He worries that these open-ocean dead zones in eddies could likewise provoke animal die-offs if they hit one of the Cape Verde Islands, the closest land mass. Currently, he’s using an alert system to track them.

On the flip side, small animals called zooplankton normally hide in the dark depths of the ocean during the day to avoid their numerous predators. But that’s not an option when there’s a dead zone lurking underneath. With no other place to breathe, the zooplankton have to remain near the water’s surface. “I could imagine that seabirds would benefit from this,” Karstensen says. They'd better bring an oxygen tank, if they plan on sticking around.