updated 6/28/2010 7:26:58 PM ET 2010-06-28T23:26:58

Researchers predict the Gulf of Mexico "dead zone," an underwater area with little or no oxygen, will be unusually large this year. But it's unknown how the oil spill will affect it.

Scientists funded by the federal government expect a zone measuring 6,500 to 7,800 square miles — about the size of New Jersey. The condition is also known as hypoxia.

Over the past five years, it's averaged 6,000 square miles.

The prediction comes from measurements of nutrient flows in the Mississippi River, which pours farm waste into the Gulf. Those nutrients stimulate excessive growth of algae that is broken down by oxygen-sucking bacteria.

University of Michigan ecologist Donald Scavia said the BP spill could make the dead zone larger because microbes gobble oxygen when consuming oil. But the effect could be offset if the oil limits algae growth.

"We're not certain how this will play out," he said in a statement. "But one fact is clear: The combination of summer hypoxia and toxic-oil impacts on mortality, spawning and recruitment is a one-two punch that could seriously diminish valuable Gulf commercial and recreational fisheries."

Farm fertilizers and livestock waste that run off into the Mississippi River are the main sources of the nitrogen and phosphorus that cause the dead zone.

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