Coastal dead zones expanding
The number of coastal areas known as dead zones is on the rise. A new tally reports more than 400 of the oxygen starved regions worldwide.
You’re traveling through another ecosystem, once filled with fish and clams, but now, bacteria. It’s a journey into a disastrous land with boundaries that are expanding beyond imagination. That’s a signpost up ahead: Your next stop — The Dead Zone.
The number of dead zones, marine areas with so little oxygen that they barely support life, has increased worldwide by a third since 1995, reports a new study in the Aug.15 Science. These areas seriously stress marine ecosystems and deplete stocks of edibles such as fish and clams.
Warming coastal waters and greater demand for corn will probably exacerbate the problem, says Alan Lewitus, chief of the National Oceanic and Atmospheric Administration’s Ecosystem Stressors Research Branch in Silver Spring, Md., which partially funded the study. Restricting the flow of nutrients into waterways is the first step in beginning to resuscitate these zones, but recovery may take years, he says.
Dead zones begin with an abundance of life. Nutrients from fertilizer runoff, sewage discharge or natural upwellings in the water column feed the growth of phytoplankton, tiny free-floating organisms such as some algae.
“When you add extra nutrients to the sea, you get fertilizer effects that are identical to fertilizer effects on land,” says study coauthor Robert Diaz of the College of William & Mary’s Virginia Institute of Marine Science in Gloucester Point, Va. “You make the land greener, you also make the sea greener.”
When the phytoplankton, which may appear as large, visible blooms or “tides,” die, they sink in the water column and are eaten by bacteria.
“The bacteria think this is great,” says Diaz. But these bacteria consume oxygen and soon there isn’t enough to go around. “Now fish and crabs are getting uncomfortable, so they leave. Worms and clams come up out of the sediment and extend their body parts up to reach oxygen that might be further from the bottom. Some animals will just shut down, lower their metabolism,” he says. But if they can’t physically escape the zone, the animals suffocate.
Oxygen from surface waters usually replenishes the oxygen-starved bottom. But when the water column becomes stratified, a typically seasonal occurrence where water is separated into distinct layers of temperature, that replenishment may not happen. The Gulf of Mexico’s dead zone, the largest in the United States, is slightly smaller this year than scientists had predicted because Hurricane Dolly came in and mixed-up the waters. (Though this year, the Gulf dead zone is the second-largest size it’s been since 1985 — slightly larger than the state of Massachusetts.)
When every last bit of oxygen is used up, a new suite of bacteria bloom with breath that can make these oxygen-starved waters reek of sulfur.
Diaz, who has been studying and tallying dead zones for more than 20 years, and colleague Rutger Rosenberg of the University of Gothenburg in Sweden, pored through the literature for reports of dead zones, which were noted in the Baltic Sea as early as the 1930s. Dead zones have now been reported in more than 400 systems and affect a combined area of more than 245,000 square kilometers, an area roughly the size of the United Kingdom, the scientists report.
The scientists also calculated the crippling effect dead zones have on certain fisheries. In the Gulf of Mexico, for example, the zone has robbed the area’s brown shrimp of about 210,000 metric tons of food.
“This lost biomass could feed 75 percent of the brown shrimp caught in a season — that’s not a trivial amount. We might have more shrimp and they might be bigger,” Diaz says.
The flow of nutrients, be it sewage or fertilizer, must be managed if dead zones are to recover, says Eugene Turner of Louisiana State University in Baton Rouge, who recently published an assessment of the Gulf of Mexico’s dead zone online in Environmental Science & Technology. Developing biofuels with crops other than corn, which has meager roots and “leaks nitrogen,” will be an important part of the recovery process, says Turner. The causes of dead zones, he says, “are co-mingled with how we use and abuse the land and do or do not value water supplies.”
