Massive 'dead zone' in the Gulf -- but not because of the oil spill

The annual Gulf of Mexico "dead zone" – a low-oxygen region of seawater that appears each spring and summer and either snuffs marine life or sends it fleeing – is one of the largest on record this year.

That's the assessment of a team of scientists who on Monday wrapped up a cruise to take the dead zone's measure. This year it's roughly the size of Massachusetts and stretches from off of Galveston, Tex., east to the Mississippi's "bird's foot" delta.

The patch off of Texas is particularly noteworthy, says Nancy Rabalais, a marine scientist and director of the Louisiana Universities Marine Consortium. Dr. Rabalais heads the annual survey effort.

IN PICTURES: Dead zones

This year's is the largest oxygen-deprived area seen off of the Texas coast since she and her team began conducting the surveys in 1985, she says. Indeed, the dead zone's "total area probably would have been the largest if we had had enough time to completely map the western part."

The mapping cruise left Cocodrie, La., on July 24 – the departure delayed a day because of high seas kicked up by tropical depression Bonnie.

How 'dead zones' form

The dead zone forms each spring and summer as snowmelt and rainfall in the Mississippi River's vast drainage basin leach nutrients from farm fields and to a lesser extent from urban landscapes along the river and its tributaries.

Once these nutrients hit the sea, they represent an all-you-can-eat buffet for plant-like phytoplankton in the Gulf. Phytoplankton that survive grazing from other marine creatures die and sink. They decompose, as does the waste from the organisms that grazed on them.

Bacteria decompose the organic material, sapping the water's oxygen in the process. Fish in the water column and bottom dwellers that can flee the zone do so. Organisms that can't avoid the zone suffocate.

Larger, patchier dead zone this year

Aside from its size, this year's dead zone is notable for its patchiness, Rabalais explains. Usually, the zone is a continuous expanse of low-oxygen water. But hurricane Alex and tropical depression Bonnie stirred the waters enough to temporarily increase oxygen concentrations in some areas, only to have them fall again once the storms passed.

The unusual westward expanse of the zone is likely due to a higher-than-normal discharge from the Mississippi, combined with westward-flowing coastal currents that so far have failed to perform their summertime U-turn. In a typical summer, this westward flow reverses, Rabalais explains in an email exchange. But this year the conditions triggering that reversal either are late or are weaker than usual.

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