Shrinking The Dead Zone

TO SHRINK THE dead zone in the northern Gulf of Mexico, the U.S. may need to reform agricultural subsidies and financial incentives for corn-based ethanol, according to a draft report from EPA science advisers.

The gulf's dead zone, characterized by extremely low levels of oxygen, or hypoxia, grew to a near-record 20,500 km² this summer. When final, the report is expected to influence a federal-state task force that is updating an action plan to minimize the dead zone.

To shrink the dead zone to its historic size of 5,000 km², the draft report recommends cutting the amounts of nitrogen and phosphorus carried by the Mississippi River into the gulf by at least 45%. These plant nutrients encourage excessive growth of phytoplankton, which eventually die and are eaten by bacteria that consume much of the oxygen in the water, leading to hypoxia.

The draft report warns that U.S. agricultural subsidies and the economic incentives favoring corn-based ethanol "are at odds with the goals of hypoxia reduction." It identifies five "significant opportunities" for reducing the amount of nitrogen and phosphorus entering the Mississippi. One is for farmers in the Midwest to convert to crops for producing cellulosic ethanol, which require less fertilizer than corn, or rotate corn with other crops. Another is helping farmers better manage fertilizer use to curb runoff. A third involves use of conservation buffers to capture nutrient-laden runoff before it enters surface water.

In addition, wastewater treatment plants need to lower the amount of nitrogen and phosphorus they discharge, the draft report says. A fifth recommendation is to construct or restore wetlands, which can absorb nitrogen and phosphorus before they get to the river.

EPA's Science Advisory Board was scheduled to vote on approval of the draft report last week. But the advisers delayed their decision until December to allow for technical revisions to the draft.

Virginia H. Dale, corporate fellow at Oak Ridge National Laboratory and chair of the board's Hypoxia Advisory Panel, tells C&EN she expects no substantive changes to the draft report's recommendations before it is finalized.