hypoxia.JPG

NOAA/National Marine Fisheries Service/National Coastal Data Development Center

This map shows dissolved oxygen levels in the Gulf of Mexico for 2009 in milligrams per liter. Zones with concentrations less than 2 milligrams per liter are considered hypoxic.

Oil and the Dead Zone

By: 
Lauren Schenkman
Robert F. Service
Erik Stokstad
2010-05-19 17:38
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As oil continues to gush into the Gulf of Mexico, a "dead zone" is also having its annual growth spurt. It's not clear how these two complex systems will interact, but scientists are sampling the water for clues.

Fertilizer and urban runoff into the Mississippi River dump nutrients into the Gulf of Mexico. In the spring, higher flows contribute a pulse of nutrients that triggers huge blooms of algae. When the algae dies and sinks to the sea floor, it is consumed by bacteria that guzzle oxygen from the water. If oxygen levels drop too low, shrimp, fish, and other sea life must flee or die, leaving a dead zone that sometimes grows to the size of the state of New Jersey.

According to satellite images, the oil slick overlaps one of the areas where hypoxic zones typically form. The possibilities for what that means are myriad and contradictory.

Some factors might worsen the dead zone. The oil's sheen could prevent oxygen from entering the water, lowering oxygen levels in surface water at a time of year when surface and deeper waters are already stratified. Another negative mechanism: Microdroplets of oil dispersed in the water might set off a feeding frenzy of microbes able to dine on the hydrocarbons, further reducing oxygen levels.

Alternately, the oil could lessen the severity of the dead zone. Oil reflects light, which is needed by the photosynthetic phytoplankton. And toxins in the oil could also diminish the phytoplankton blooms. Both factors could potentially mean fewer dead phytoplankton reaching the bottom, leading eventually to less oxygen depletion.

But how these factors are playing out is far from clear. "We have no idea right now what's going on," says Nancy Rabalais, a biological oceanographer at the Louisiana Universities Marine Consortium in Chauvin who has studied the dead zone for the past 25 years.

Rabalais is currently aboard the research vessel Pelican, conducting the first measurements of the dead zone since the spill. Besides collecting water and sediment samples, her team is using electronic instruments to measure temperature, salinity, dissolved oxygen, chlorophyll biomass, sunlight penetration, and other parameters at depths from 5 to 30 meters. The ship will collect data near and far from shore in Terrebonne Bay, about 125 kilometers west of the Southwest Pass of the Mississippi Delta, and Atchafalaya Bay, another 150 kilometers west. Meanwhile, a instruments on oil rigs are recording parameters in real time.

Next month, a team from Texas A&M University in College Station and other institutions will head into the gulf to try to glean more clues to the impact of the spill. Meanwhile, oceanographer Steven DiMarco at Texas A&M is trying to add the oil plume into a computer model of the biology and currents of the gulf. The goal is to try to get an overall sense of the impact on the dead zone from the spill. "It's become a significant part of the project," DiMarco says.

For more on the gulf oil spill, see our full coverage.

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