Dead zone is much smaller than expected

By Nikki Buskey, Staff Writer Houma Courier
Published: Saturday, July 25, 2009 at 6:01 a.m.

HOUMA — This year’s Gulf of Mexico ‘dead zone,’ the area of low-to-no oxygen that appears annually off the coast of Louisiana, is less than half the size scientists expected.

But some areas of the dead zone are also severely low in oxygen and pressed close against the coast. That caused what scientists call a “jubilee” — rampant massing of fish and shellfish — in the surf at Grand Isle.

A week long mapping trip departing from the LUMCON Cocodrie marine research station found this year’s dead zone was about 3,000 square miles, nearly 5,000 square miles less than scientists predicted.

“This was surprisingly small given the forecast to be among the largest ever and the expanse of the dead zone earlier this summer,” said Nancy Rabalais, chief scientist for the mapping expedition and director of LUMCON.

Researchers predicted in June that the 2009 dead zone could be the largest on record, somewhere between 7,450 and 8,456 square miles, an area roughly the size of New Jersey.

The dead zone is an area of oxygen-starved water, a phenomenon known as hypoxia, and is fueled mostly by nitrogen and phosphorus runoff from agriculture that flows down the Mississippi River.

The extra nutrients trigger an explosion of algae growth that soon sinks, decomposes, and consumes most of the life-giving oxygen supply in the water, causing marine life in the area to either flee or die.

The forecast for a huge dead zone was based on the high nutrient load in the Mississippi River and high freshwater flows from the Mississippi and Atchafalaya rivers in May and June.

The dead zone was quite large in June, Rabalais said, but as river flows slowed in July to below average, fewer nutrients came down the river to stimulate algae growth. Less dense fresh water from the river helps to create the dead zone when it flows out on top of the denser, saltier Gulf of Mexico waters, trapping bottom waters and allowing them to stagnate.

However, areas of ‘dead’ water are unusually thick and severely low in oxygen this year, Rabalais noted, likely due to persistent winds out of the west that pushed low oxygen waters up against the Louisiana coastal shelf. The footprint of the dead zone on the Gulf of Mexico looks smaller, Rabalais added, but the volume of ‘dead’ water is still great.

At some of the research stations where severely low levels of oxygen were found, scientists saw blue crabs, eels, and shrimp swimming at the water’s surface, a sign of stress caused by the dead zone because they can’t survive below.

Other studies found low oxygen levels in early July that caused jubilees off Grand Isle.

Named for the emotion experienced by fishermen who find themselves in the middle of one, jubilees are caused when the dead zone, changing winds and currents push great volumes of fish, crabs and shrimp into the shallows along barrier islands.

The 2008 dead zone was also projected to be a record setter, but the active hurricane season churned Gulf waters and curbed its formation. Last year’s dead zone covered about 8,000 square miles.

The largest dead zone was recorded in 2002, measuring 8,484 square miles.

Models currently used to forecast the dead zone are good for long term planning, but can’t predict short term weather and water patterns that can cause the dead zone to be unpredictable, Rabalais said.

Crews may need to take more-frequent measurement trips throughout the summer to capture a true picture of the dead zone, she added.

Nikki Buskey can be reached at 857-2205 or nicole.buskey@houmatoday.com.