Less oxygen dissolved in the water is often referred to as a “dead zone” (in red above) because most marine life either dies, or, if they are mobile such as fish, leave the area. Habitats that would normally be teeming with life become, essentially, biological deserts. Photo: NOAA
Ah, high summer: Trips to the beach, with dips in the ocean to cool off after volleyball or a nap in the sun, perhaps a bonfire on the shore under the stars. But it’s not only the season for tan lines and late nights. In the Gulf of Mexico, it’s also the season for dead zones.
Dead zones have plagued the Gulf for years. These biological deserts are created when an excessive amount of nutrients, such as sewage or fertilizer, enter the water. Algae consume the nutrients, causing them to grow, thrive, and die. Their decaying bodies slowly sink to the ocean floor, using up oxygen as they decompose. The result is a vast expanse of oxygen-depleted waters where practically no living organism can survive.
While the dead zone appears every year, its size and severity isn’t constant. Last week, two teams of NOAA-sponsored scientists released differing reports predicting the extent of the Gulf of Mexico dead zone this year.Their findings were dramatically different, with one group forecasting a dead zone of 1,197 square miles, while the other team predicted it would be up to 6,213 square miles. The difference comes from two vastly different ways of looking at the ocean environment.
The smaller prediction was announced by researchers from the University of Michigan. They focused on the current amount of nutrients being dumped into the Gulf of Mexico through rivers such as the Mississippi. Due to the drought that has plagued the Midwest this year, the nutrient offload is rather small, causing the researchers to predict a much smaller dead zone. During May 2012, the streamflow in the Mississippi and Atchfalaya Rivers, two of the main rivers which end in the Gulf, were only half that of normal conditions.
The researchers from the Louisiana Universities Marine Consortium and Louisiana State University, on the other hand, took a different approach. Their model took into account the nutrient load from previous years. Nutrients can remain in bottom sediment and be recycled the next year, which is why the researchers predicted a dead zone about the size of Connecticut.
While the size of the dead zone is in dispute, its occurrence is a sure thing. Dead zones have been recorded in the Gulf of Mexico since the 1950s, though researchers who have examined cores taken from the area note that the phenomenon has been around for at least the past 150 years.
Changes in area of the hypoxic zone on the Louisiana continental shelf from 1985 through 2003, plotted on the basis of measurements made by N.N. Rabalais (Louisiana Universities Marine Consortium, written commun., 2005). Photo: U.S. Geological Survey
Yet it seems dead zones, in general, are increasing in both size and duration. The smallest recorded dead zone was in 1988 and stretched a total of 15 square miles. The largest one was in 2002 and encompassed a staggering 8,400 square miles. Although there are fluctuations between years, it seems the general trend is up, the U.S. Geologic Survey reports.
Large dead zones can greatly impact life around the coast. Due to the low oxygen content of the water, fish flee the area before they suffocate. Fishermen follow, travelling farther to find viable fish stock, a tax on their time, fuel use, and their upfront costs.
There are measures that could help reduce the size of the dead zone, such as reducing nitrogen and phosphorus runoff from agriculture and restoring wetlands. There could soon be an influx of funds for the latter if the RESTORE Act, a piece of legislation which has been folded into the Transportation Bill that Congress is currently wrangling over, is signed into law. Under the RESTORE Act, 80 percent of the Clean Water Act fines that BP paid after the Deepwater Horizon oil spill would be given to the Gulf Coast states for environmental and economic restoration. Rebuilding Gulf ecosystems is critical to putting a dent in the dead zone.
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