‘Dead zone’ ranks third

‘Dead zone’ ranks third

             
COCODRIE — The recurring “dead zone” off the Louisiana and Texas coasts isn’t quite as big as predicted earlier this year, but — at 7,900 square miles — is still the third-biggest ever mapped, according to a scientist who has been studying the phenomenon since 1985.

Crabs, eels and other creatures usually found on the bottom were swimming in crowds on the surface because there was too little oxygen to survive in their usual habitat, said Nancy Rabalais, chief scientist for the northern Gulf of Mexico hypoxia studies.

Boats trawling for shrimp were as conspicuous in their absence, she said.

“We very often see swarms of crabs, mostly blue crabs and their close relatives, swimming at the surface when the oxygen is low,” she wrote in an e-mail from the research ship as it headed toward a Sunday landfall.

Eels, which live in sediments 60 feet to 70 feet below the surface of the water, are a less common sight, she said.

“We saw brown shrimp swimming at the surface at our regular observing station off Terrebonne Bay in late June and early July,” she wrote.

“This doesn’t happen very often … and the oxygen at the bottom that day was close to zero.”

The area with almost no oxygen, a condition called hypoxia, was about the size of Connecticut and Delaware together. It would be big enough to hold New Jersey if only land area was counted.

The Louisiana-Texas dead zone is the world’s second-largest hypoxic area, she said.

This year’s is about 7.5 percent smaller than predicted by Eugene Turner, a professor of oceanography and coastal sciences at LSU and Rabalais’ husband.

Basing the prediction on nitrogen content in the Mississippi River watershed, he estimated the dead zone would be about 8,540 square miles, the largest measured in at least 22 years.

More storms than usual in the first half of July and the tropical low between Cameron, La., and Galveston, Texas, may have reduced hypoxia by keeping the waters roiled, Rabalais said.

Hypoxia occurs when still weather lets fresh water pouring in from the Mississippi River float above the heavier salt water in the Gulf. Currents carry the fresh water west toward Texas.

Algae die and fall to the bottom, where their decay uses oxygen faster than it is brought down from the surface. Eventually, the lower layer holds too little oxygen for fish and other aquatic life.

Nitrogen, from sources including fertilizer, erosion and sewage, speeds up the process by feeding algae.

The dead zone was larger in 2002 and 2001, when it spanned 8,500 and 8,006 square miles respectively, and almost as big (7,728 square miles) in 1999.

This year’s dead zone was big enough to boost the long-term average from about 5,000 square miles to 5,200 square miles. Scientists want to reduce it to about 2,500 square miles.