Tale of two dead zones: Gulf’s larger, Bay’s smaller

By Tim Wheeler, The Baltimore Sun
June 28, 2010

As if the Gulf of Mexico doesn’t have enough problems right now, scientists are predicting that a larger-than-average "dead zone" will form there this summer.  The Chesapeake Bay, meanwhile, appears to be in line for one of its smallest areas of oxygen-starved water – though that doesn’t necessarily mean it’s well on the road to restoration.

In the northern Gulf, University of Michigan aquatic ecologists Donald Scavia and Mary Anne Evans forecast that there’ll be from 6,500 to 7,800 square miles of hypoxic or oxygen-poor water – an area roughly the size of Lake Ontario (or New Jersey, if a closer example helps you picture its scale).  Shown above is the 2009 dead zone.  Scavia’s best estimate is the zone this year will be around 6,564 miles – on the low side of the range of possibilities, but still the 10th largest on record.

Researchers say it’s too soon to tell what impact the Deepwater Horizon blowout will have on the dead zone.

"We’re not certain how this will play out," Scavia said in a release. If enough oil gets in the waters normally subject to low oxygen, or hypoxia, the dead zone could be larger, as microbes in the water break down the oil, consuming that much more oxygen in the water in the process.  But the oil might also limit the size of the dead zone, Scavia suggested, by stunting the growth of algae blooms that starve the water of oxygen when they die and decay.

Either way, the combination of a larger-than-normal dead zone and toxic oil are likely a "one-two punch" for the Gulf’s fish and shellfish, which sustain a $659 million fishery, said Scavia, whose work is underwritten by the National Oceanic and Atmospheric Administration.

The Chesapeake, meanwhile, is likely to have the sixth smallest low-oxygen zone in 25 years, Scavia predicts.  He forecasts the volume of water with less than 2 milligrams oxygen per liter will be 5.7 cubic kilometers, which would be below average for recent years and a little smaller than last year’s.

Bay scientists parse the Chesapeake‘s dead zone into low-oxygen, or hypoxic, water and anoxic, or oxygen-deprived, water. They’re predicting that really dead anoxic zone will be the fifth smallest in the past quarter-century.

Though heavy rains and snow melt last winter and early spring sent big pulses of nutrient-laden water into the Susquehanna River and the bay, flows in other months have been below-normal, leading to the milder dead zone prediction.  The zone this summer could actually look a lot like the one mapped in 1988, as seen below, scientists say.

A smaller dead zone would definitely be an encouraging development for the bay.  But senior scientist Denise Breitburg of the Smithsonian Environmental Research Center cautions that doesn’t necessarily mean the Chesapeake‘s beleagured fisheries will automatically rebound.  Habitat loss, fishery pressure and competition from other species, among other things, also play a role.

GRAPHIC:  (Gulf dead zone map: NOAA, Louisiana University Marine Constortium.  Chesapeake map: University of Maryland Center for Environmental Science)

Posted by Tim Wheeler at 11:03 AM