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July 30th, 2013
NOAA Scientists Measure and Map the Dead Zone

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Summer can be a tough time for many species in the Gulf of Mexico, when the combination of nutrient-rich river runoff and warm temperatures can rob coastal bottom waters of oxygen. Where that happens, shrimp, fish, and other creatures can be forced to flee to fresher waters, leaving a so-called dead zone behind.

This week NOAA announced the official size of the dead zone. But because low oxygen levels impact valuable species of shrimp and fish, NOAA Fisheries scientists also conduct a separate study that covers a wider area and looks at how oxygen levels affect the distribution of marine animals.

The annual summer groundfish survey runs from the Texas-Mexico border around to the Florida Keys. Scientists stop at nearly 300 randomly selected points along the way to sample the bottom with a trawl net—that’s the only way to know what species are down there—and to measure environmental variables including dissolved oxygen.

Last month, the NOAA ship Oregon II arrived at a sampling station 60 miles off the Louisiana coast and about 150 miles west of where the Mississippi River flows into the Gulf. And when the trawl net came up, it came up empty.

“Water haul,” said Kim Johnson, the NOAA biologist who was leading the expedition. That’s what they call it when the net comes up empty. Or just about empty—the net contained a dozen or so lethargic crabs, a solitary shrimp, and a number of tiny silver fish that looked like small coins.

Johnson didn’t know for sure if the ship had just crossed into the dead zone—maybe the net came up empty by chance—but she would soon find out.

What Causes the Dead Zone?

Dead zones are caused by runoff from land that is rich in nutrients such as nitrogen and phosphorous. These elements aren’t toxic, but they are potent fertilizers. In fact, in the Mississippi River, which drains about forty percent of the continental United States and most of its Midwestern farmland, agricultural fertilizers are the main source of these elements. Air pollution and urban development also increase nutrient runoff.

When these nutrients find their way to the Gulf of Mexico they cause unnaturally large algal blooms. The algae then die and sink to the bottom where they’re decomposed by oxygen-consuming bacteria. During the warm summer months, when there is little mixing in the water column, the bottom water can stagnate and become hypoxic, or low in oxygen. If the hypoxia becomes severe enough, you have a dead zone.

The size of the Gulf of Mexico dead zone varies from year to year. In 2012, the dead zone covered less area than usual, which may have been due to drought conditions in the Midwest—low rainfall means less runoff from land. This year the dead zone is slightly larger than average. At 5,840 square miles, it is about the size of the State of Connecticut.

A larger dead zone means less habitat for shrimp, which in 2011 were worth more than $400 million to Gulf fishermen. Also, many important species of fish, such as red snapper, spend their juvenile years on the muddy bottom and might see their habitat shrink. Many communities along the Gulf Coast depend on fishing and tourism to survive, and for them the dead zone is a serious economic threat.

Although the Gulf of Mexico dead zone is the largest in the United States, dead zones also regularly occur in the Chesapeake Bay, the Long Island Sound, and many other estuaries and coastal zones throughout the nation.

Getting Data From the Deep

Back on Oregon II, scientists on the bow deck prepared to lower a device called a CTD into the water. The CTD is an open cylindrical cage about the size of a small refrigerator, and it contains instruments to measure temperature, depth, salinity, and oxygen.

A hydraulic winch groaned, a cable tightened, and the CTD rose over the gunwale then descended into the sea. 

Water is considered hypoxic if it contains 2 milligrams per liter of dissolved oxygen or less. In the control room, scientists monitored a live readout from the CTD as it made its way through the water. When the data came in there was no longer any doubt: 0.8 milligrams per liter dissolved oxygen. This was the dead zone.

Oregon II continued its journey. Another 30 days at sea with two port calls, another 200 or so randomly selected sampling stations, before arriving at the Florida Keys. NOAA scientists have been conducting the same groundfish survey twice a year for more than thirty years, and they have been measuring the dead zone since 1985. Only by maintaining a continuous, long-term data series can scientists monitor trends in the health of fisheries and the Gulf ecosystem over time.

Johnson is also thinking long term. She grew up in Galveston, Texas, and though she’s not from a shrimping family, most of her friends were, and she spent a lot of time as a kid on shrimping boats. “That why I got involved in fisheries,” Johnson said. “To help my shrimper friends, so that they can teach their children to tow.”

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