Nitrogen fertilizer dead zone tracked in Gulf of Mexico Agricultural runoff parallels situation of Chesapeake Bay

ABOARD THE ACADIANA — ABOARD THE ACADIANA -- The instant the captain cuts the engines, the crew of the research vessel Acadiana shifts into swift, synchronized action: scooping water samples from the ocean surface and the seabed; lowering an array of instruments overboard to gather data from the depths; whirling and zapping the water in the on-board laboratory until it yields its secrets.

Then, almost before the winch can hoist the last instrument back on deck, the engines roar to life, and the ship heads for the next sampling station - an invisible point along an imaginary line flung out into the Gulf of Mexico.


The key question

The question these scientists are here to answer is simple but profound: Does this swath of the gulf contain enough dissolved oxygen to sustain living creatures, or is the ship in the "dead zone"?


Since 1985, Dr. Nancy Rabalais, 46, has put the dead zone on the map, making dozens of such trips under the auspices of the Louisiana Universities Marine Consortium, collecting thousands of water samples and measurements.

The story they tell her is this: Nitrogen fertilizer, applied as far north as Wisconsin and carried down the Mississippi and Atchafalaya rivers, is making many bottom waters along the Louisiana coastal shelf inhospitable to life.

This summer's dead zone measured almost 7,000 square miles. It stretched from the Mississippi Delta to Calcasieu Pass, near the Texas-Louisiana border. It extended 30 miles out to sea and to depths of 120 feet.

That is not welcome news in certain quarters - among Midwest farmers and operators of charter fishing boats, to name a few.

"Nothing could have a more negative effect on farmers" than curtailing fertilizer use, the president of the American Corn Growers Association recently told the Omaha World Herald.

"It may be sexy to say the gulf is devoid of life," echoed Todd Masson, editor of Louisiana Sportsman magazine, adding that fishermen routinely catch fish in the dead zone. "It may help attract research money. I would question the scientists' motives."

On board the Acadiana, researcher Jim Lee regales the crew with an article from the magazine that goes to elaborate lengths to ridicule the dead zone and those who study it.

"Blasphemy! Blasphemy!" Rabalais cries in mock horror. Then she grows grave. "Yeah, well, I can't fight all the battles."


After a dozen years of scrounging for funds just so she can spend dawn-to-midnight days at sea and carry her message before hostile audiences, she is tired.

"People ask me how long I can keep doing this, and sometimes I wonder," she said. "But I think this is important."

So, from March through June and August through October she takes the Acadiana out one day each month, sampling along the line designated Transect C. Each July she and her crew embark on a six-day marathon, sampling 24 hours a day, back and forth across the coastal shelf. On those cruises, they work 12-hour shifts, except for Rabalais, who works 18 to 24 hours at a stretch. "If I don't do it, who will?" she asked.

The scientific term for what happens in the dead zone is hypoxia, which is defined as less than 2 milligrams of dissolved oxygen per liter of water. Once hypoxia develops, any creature that cannot crawl or swim to oxygen-richer waters will die.

A worldwide problem

Hypoxia is a worldwide problem where rivers carrying large amounts of agricultural runoff empty into coastal waters. It has been documented from the Chesapeake Bay to the Adriatic Sea to Osaka Bay.


Even in the worst hypoxic conditions, water near the surface usually contains sufficient oxygen; it is the lower levels that are starved. In August, at each of the nine stations along Transect C, roughly the lower half of the "water column" was hypoxic. At the seabed, there was virtually no oxygen.

According to earlier studies and anecdotal evidence, hypoxia has occurred on the Louisiana coast for decades. Until Rabalais began her work, it was generally regarded as an isolated phenomenon.

"People knew it was there, but no one had mapped it," she said.

When she did, "lo and behold, it was huge, and it lasted all summer," said Dr. Donald F. Boesch, then the director of the state's marine consortium, known as LUMCON.

The next question -- what caused the oxygen depletion - was "a very difficult nut to crack," said Boesch, now president of the marine studies institute at the University of Maryland.

Rabalais and her colleagues used sediment cores to determine that hypoxia has worsened dramatically since 1950 in almost direct proportion to the use of nitrogen fertilizer in the Mississippi watershed. From 1985 to 1992, the dead zone fluctuated between 3,000 and 4,500 square miles. Then in 1993 it nearly doubled, to 7,000 square miles. That was to be expected: The floodwaters that year carried an unprecedented load of fertilizer downstream.


What disturbs Rabalais is that in subsequent years the zone has not shrunk back to its pre-flood dimensions - and no one, including her, is sure why.

Research on other coastal systems suggests that the total nutrient load can cross a threshold, where hypoxia kicks into a higher gear, Boesch said. But there's no way to know whether that's happening here.

"I would love to know," Rabalais said.

The other question she can't answer is what the dead zone does or may do to the state's half-billion-dollar commercial fisheries.

There's no question that hypoxia can drive fish away temporarily. A trawling cruise this summer encountered "large areas that were devoid of fish," said Dr. Bruce Thompson of the Coastal Fisheries Institute at Louisiana State University. He said, however, that doesn't mean the fish died. "Most fish are very mobile. Fish are going to pack up and leave."

In addition, he said, they are generally "very opportunistic feeders." If bottom-dwellers such as worms die off, the fish will probably find something else to eat.


"The changes [in the fish population] would be very subtle," he said. "It's going to take a lot of study to understand it."

'No dead bodies'

The fact that there are, in Boesch's words, "no dead bodies" makes it harder to generate a sense of urgency about either study or solutions.

"If you go to Iowa and tell a farmer, 'You should be more careful what you put on your fields,' you're not going to get very far," said Mark Davis, executive director of the Coalition to Restore Coastal Louisiana. "You need to be able to say, 'What you do up here affects our ability to live and work.'"

Perversely, hypoxia's observable effects on marine life are sometimes hailed as a godsend. Just days before Rabalais' August trip, winds forced the dead zone very close to shore, driving fish and shrimp onto the beaches near Grand Isle.

"Bad news for fish is great for fishers," crowed a headline in the New Orleans Times-Picayune.


"They [fish and shrimp] were practically jumping into people's nets," a state marine biologist marveled to the reporter. It was left to Rabalais' co-researcher -- and husband -- Dr. Gene Turner of LSU, to note that the area only narrowly escaped a major fish kill.

Starved for oxygen

This is how hypoxia works:

* Once the rivers deposit nitrogen and other nutrients in the gulf, currents distribute them across the shelf, generally flowing westward. Algae, particularly one-celled phytoplankton, thrive.

* When the algae die, they sink to the bottom. As they decompose, the process consumes oxygen from the bottom waters. This process accelerates in the summer months, following the spring's peak river flows.

* At the same time, an invisible barrier prevents the oxygen-rich top waters from mixing with those below. The barrier occurs because the relatively fresh water on top is both lighter and warmer than the bottom waters. Until fall winds churn the layers, the bottom waters remain hypoxic.


"Stuff is alive on the surface, but there's a large area on the bottom with no oxygen," Dr. Nancy Rabalais says, "and that's not good."

Pub Date: 10/06/96