The drought plaguing more than half the United States is having a positive effect on the Gulf of Mexico's "dead zone."

This year, the oxygen-free zone is the fourth-smallest since record-keeping began in 1985, according to a survey by scientists supported by the National Oceanic and Atmospheric Administration.

"The smaller area was expected because of drought conditions and the fact that nutrient output into the Gulf this spring approached near the 80-year record low," said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium, which led the survey.

The dead zone is fed by nutrient runoff from the Mississippi River watershed, leading to explosive algae growth, which sucks up oxygen in the water as it decomposes.

Drought conditions in the Midwest have reduced the amount of water -- and nutrients -- flowing through the Mississippi River system, and as a result, the dead zone measures about 2,889 square miles, about the size of Delaware, the scientists found. This year's dead zone is much smaller than last year, when flood conditions brought large amounts of nutrients to the Gulf of Mexico, resulting in a dead zone that was 6,770 square miles, or about the size of New Jersey.

This year's smaller dead zone confirms the "strong relationship between the size of the hypoxic zone and the amount of fresh water and nutrients carried to the Gulf by the Mississippi River," Rabalais said in a statement.

Still, the zone is larger than the 1,900-square-mile goal identified by the Gulf of Mexico/Mississippi River Watershed Nutrient Task Force and threatens commercial and recreational fisheries. Gulf commercial fisheries were valued at $639 million in 2010.

The smallest dead zone recorded to date measured 15 square miles in 1988, which was a record drought year (ClimateWire, July 30). The largest dead zone was measured in 2002 at more than 8,400 square miles.

Earlier this summer, models issued conflicting dead-zone forecasts. One suggested it would be larger, like last year, if organic matter from last year's large algal blooms stored in undersea soils stimulated algae growth again this year. The actual results suggest a limited role for this "carryover effect" under low-flow conditions.

While the smaller dead zone was expected this year, the survey found an unusually patchy distribution of hypoxia. A follow-up survey will be conducted in mid-August.