Scientists Seek Ways to Bring Marine Life Back to World’s ‘Dead Zones’

She swims down through cloudy Gulf of Mexico water, trailing silver bubbles, surrounded by black and silver curves of fish.

Dark, almost plate-shaped triggerfish; barred sheepshead; tiger-striped spade fish; silvery, dark-backed bluefish--a dozen or more are within arm’s reach.

Then they’re gone. Nancy Rabalais has hit the “dead zone.”

She is about 30 feet down, near a rig 15 miles offshore. From here to the bottom, 65 feet below the surface, there are no fish. The floor is littered with dead starfish and the shells of anything else too slow to move.

It’s like this for miles and miles. And has been for years and years--Rabalais has been studying the dead zone since 1985 for the Louisiana Universities Marine Consortium, and just won a $250,000 award for her work.

Every summer, a long, narrow stretch of the Gulf of Mexico that may be as big as Massachusetts has too little oxygen for anything to live. The biggest in U.S. waters, it’s the third largest among 61 areas of little or no oxygen in oceans and bays around the world.

For half the year, it is rich with fish, shrimp, starfish and all sorts of other creatures. But the depletion of oxygen begins in the spring as snowmelt and spring rains pour from the Mississippi River into the gulf.

Fresh water is lighter than salt water, so two distinct layers develop. As the water warms, excess nitrogen in the fresh water feeds a growth spurt of algae and microorganisms. They die and fall to the bottom, where their decay consumes oxygen. From the bottom up, oxygen disappears. The water becomes hypoxic, with almost no oxygen, then anoxic: no oxygen.

Summer’s calm keeps the dead water in place.

Shrimp and fish can skedaddle to safer water, but starfish are too slow. The flattest die first. Leggy brittle stars stand on tiptoe, stretching for oxygen. Then they, too, die.

By summer, the dead zone covers 8,000 square miles. The boundary is sharp as you dive, but not as you crawl along the bottom--Rabalais doesn’t know just why.

This year it began earlier than usual.

River discharge peaked in February, with what Rabalais called “a real obvious plankton bloom” in February and March, and low oxygen levels in March. Levels usually do not start dropping until April.

In June, rough weather stirred oxygen into the water for six days. The rest of the time, the water was still, dead. The annual trip to measure the zone’s full extent ended last month.

Last summer, the dead zone covered less seabed than in the last few years--about 4,800 square miles--but went far deeper than ever before.

It also came close, within a couple miles of shore. When winds roiled the water, as they did much of the summer, they mixed in oxygen so the leading edge was farther out. But during August’s calm, it crowded closer.

The Gulf of Mexico’s dead zone is a bit of an oddity. Worldwide, the worst cases are both landlocked, in the Baltic and Black seas.

The Black Sea is up to 6,600 feet deep, and geologists can only speculate how long its deepest waters have been without oxygen.

People polluted the upper layers with sewage, industrial discharge, fertilizer runoff. Now, below about 660 feet, oxygen is replaced by hydrogen sulfide that bubbles to the top, smelling like rotten eggs. Numbers of mackerel, turbot and bonito plummeted. Schools large enough to send boats after are less valuable species such as anchovy and sprat.

Fisheries also have collapsed in the Baltic and in the Kattegat, which runs between Norway and Sweden, said Robert Diaz of the Virginia Institute of Marine Science, who has studied such zones worldwide.

The ecological effects of the Gulf of Mexico’s dead zone are much more subtle, since it is only there for part of the year.

“In others, it’s so clear. You have dead bodies, loss of fisheries, species shift,” Diaz said. “If you analyze fisheries statistics from the gulf, you don’t see those things.”

There is at least one exception: The bigger the dead zone, the fewer brown shrimp are brought to port, said Roger Zimmerman, director of the National Marine Fisheries Service laboratory in Galveston, Texas. Figures since 1985 show a significant correlation.

On the other hand, 80% of Louisiana’s shrimp landings are from inshore waters, so they’re not affected by the dead zone, said Mark Schexnayder of the Louisiana Department of Wildlife and Fisheries.

The department is offering $1,000 a year to shrimpers and charter fishing boat captains willing to log their catch for three years--what, when, where. They’ve found about 30 so far, and are looking for 70 more.

What causes these dead zones? Some of the seas, like the Baltic, have all kinds of pollution. But investigators say there’s a common thread worldwide, traceable because of the Victorian era’s insatiable scientific curiosity.

In 1888, Lauos Winkler published a paper explaining how to measure the amount of oxygen dissolved in seawater. Scientists around the world started using his method and keeping records of what they found.

“There’s a huge upturn in anoxia in the ‘50s. It corresponds with a huge increase in the use of nitrogen fertilizer,” Diaz said. “And that corresponds with the need to do something with the technology used during the war for bombs.”

Farmers put nitrogen on their fields because it works. Maybe one year in four or five the weather is just right, and 20 cents worth of nitrogen can produce $2 or more worth of corn. That price spread makes it worthwhile to use every year.

The U.S. Geological Service estimates that 6.3 million tons of nitrogen and another 2.7 million tons of animal manure are used to fertilize fields in the Mississippi River Basin. Other major nitrogen sources include legumes (1 million tons) and municipal and domestic waste systems, which add 630,000 tons each year, it says.

In 1997, environmentalists and religious leaders in six nations bordering the Black Sea joined in a crusade to clean up its waters. Last year brought a sign of success. For the first time in a decade, winter storms sent thousands of crustaceans scrambling to shore.

But what more can be done?

Wetlands and buffer zones can be created in the United States or restored along waterways to capture some of the nitrogen before it gets into the river.

Fertilizing in spring rather than fall can help, said Otto Doering, a professor of agricultural economics at Purdue University.

Purdue agronomists Ron Turco and Sylvie Brouder suggest some ways to hold nitrogen in the soil include rotating crops and modifying networks of perforated pipes that carry water out of the soil.

More than half of the Midwest’s farms have such tile drains to keep the soil from reverting to wetland prairie. Turco wants to develop systems that would trap and hold water until needed. Then, he pointed out, “if you could store water in the tile system when you wanted to, you could effectively irrigate your field from below ground.”

Planting cover crops in fall and plowing them under in spring can prevent runoff. Bare ground lets the nitrogen escape into the air, while plants absorb and hold it. And, since cover crops are tilled into the soil, their nitrogen goes back into the soil and farmers don’t have to add as much.

Rabalais, who received the San Diego Foundation’s new Blasker Award for Environmental Science and Engineering in July, urges understanding.

“It took a long time for this to develop, and it’s not going to go away overnight,” she said.

Some changes began in the mid-’70s, before she first dove into the dead zone.

In one part of south-central Nebraska, farmers cut their nitrogen use about 10 pounds from a 1980s peak, while yields went up, said Richard Ferguson, an agronomy professor at the University of Nebraska-Clay Center. Because about half of the area’s 3.7 million acres are planted in corn, that’s a lot of nitrogen--8,500 tons or so.

He sees some link between Midwestern agriculture and the Gulf of Mexico’s problem, though he thinks the connection may have been overplayed. Other sources of nitrogen need attention too.

Finding ways for farmers to use less fertilizer makes economic sense, Ferguson said. “It’s something we’ve been working on a long time.”