Gulf “Dead Zone" Projected to be One of the Largest on Record

08-08-2011 // Craig Guillot
Dead Zone map

As of mid-July, researchers from Texas A&M University had measured the “dead zone” in the Gulf of Mexico to be more than 3,300 square miles. Fertilizer runoff from farms combined with the historically higher waters in the Mississippi River could make it grow to more than 9,400 square miles, making it the largest on record.

Nancy Rabalais, Ph. D., executive director and professor at the Louisiana Universities Marine Consortium, said the “dead zone” in the Gulf is caused by nutrients from agricultural runoff.

According to the National Oceanic and Atmospheric Administration (NOAA), 41 percent of the land in the U.S. eventually drains into the Mississippi River. Farming fertilizers, urban runoff, treated sewage and other pollutants from as far north as South Dakota work their way into the river every spring and empty into the Gulf of Mexico in Louisiana.

There, an accumulation of nitrogen and nutrients deplete oxygen levels in the seawater. When they drop to dangerously low levels, it triggers hypoxia, a condition that can kill all sea life in the area. Dead zones are often described as “ocean deserts” as they support little life and fish can’t be found for miles.

“The animals that live on the bottom will swim away if they can but things that can not move or are attached to the bottom will eventually die off,” said Rabalais.

Every summer when the Gulf dead zone swells, it can cause problems for the seafood industry as fishermen must venture further offshore for their catch. That costs more time and money in fuel for fishermen who are still reeling from the impacts of the Gulf oil disaster.

The dead zone started to appear in the Gulf in the 1950s when nitrogen levels from runoff started to increase. By many estimates, those levels have further tripled in the past 50 years.

NWF senior wildlife biologist Doug Inkley said the root causes of the dead zone can be found not in Louisiana but in places like Illinois, Iowa, South Dakota and Nebraska.

“It’s a direct result of the fertilization in the Midwest. Fertilizers run off farms into streams and tributaries, then into the Mississippi and they all end up down [at the mouth of the Mississippi River],” he said.

Flooded With More than Water

Bonnet Carre opening the spillway

David Muth, Louisiana state director for NWF, said this year’s dead zone is exacerbated by the extraordinary flooding in the Mississippi River. Record rainfalls in April combined with springtime snowmelt sent record amounts of water flowing down the river. Levels rose so high that spillways were opened in Louisiana to relieve the pressure on the levees to avoid flooding Baton Rouge and New Orleans. While the water has since subsided, the excess nutrients will impact the Gulf for years to come.

Muth said the encasement of the river with levees prevents the nitrogen-rich water from being naturally filtered by wetlands on their way to the Gulf. Instead, the waters are all funneled to the mouth of the river where they inundate the waters off the coast of Louisiana.

We have too much fertilizer in the river and we’ve turned it into a big sewer pipe. We don’t let it spread out in its floodplain and trickle through the wetlands where it would use up a lot of the nitrogen,” said Muth.

Start at the Top

Most environmentalists and scientists agree that solutions need to start on farms where fertilizers are used. Using less toxic fertilizers, smaller quantities and avoiding some of the runoff into the river could go a long way in minimizing the dead zone. Small runoff from a farm may not seem significant but Muth said with millions of acres of crops, it can quickly add up to massive quantities. All of that eventually ends up in the river.

Delta NWR

“The solution has to come from farther up the system but we also have to make changes here. If the waters were allowed to pass through estuaries, they would use up a lot of the nitrogen," said Muth.

He also advocates more buffers and wetlands between farms and streams to absorb some of the nutrients that are found in the runoff.

On the lower end of the river, NWF's coastal restoration campaign advocates for reconnecting the river with its floodplain in a controlled way through sediment diversions. During the annual high water in the spring, when nutrient runoff is at its highest, the waters of the river would flow into wetlands and bayous adjacent to the river. In addition to depositing sediment, the river water would be "scrubbed" of nitrogen taken up by the vegetation of the estuary. The water that entered the Gulf, then, would contribute substantially fewer nutrients to the dead zone.

Dead zones are not unique to the Gulf of Mexico. They happen all over the world and are well documented off the coast of Texas near the Brazos River, off the coast of Oregon, in the Chesapeake Bay, the Baltic Sea and the Black Sea.

The Gulf dead zone typically recedes in the fall but the nitrates tend to build annually, growing the zone larger with each passing year. A 2008 study by Danish researchers showed a direct link between climate change and the growth of dead zones. Their model showed that global warming would reduce the ocean’s ability to store oxygen while trying to push oxygen down to ocean depths. That would result in exacerbated dead zone growth. In a worse-case scenario, dead zones could encompass almost a fifth of the world’s oceans by 2100.

“Global warming is a contributing factor. The warmer the water gets, the less oxygen it is capable of holding. It's a growing problem,” said Inkley.

Get Our E-Newsletter 
Connecting...
Join today and get a 1 year subscription to National Wildlife magazine