The Gulf of Mexico’s “dead zone” will be an unwanted summer visitor for decades, persisting even if farmers could immediately end all fertilizer runoff, Canadian scientists say.
The so-called dead zone—an area with too little oxygen to support marine life—forms every summer off Louisiana’s coast. It’s fed largely by nitrogen fertilizer, and a national plan calls for reducing runoff to shrink the area by two-thirds, to 1,950 square miles by 2035.
The zone covered a record 8,776 square miles last year after a swollen Mississippi River carried unusually large amounts of nitrogen to the Gulf.
Runoff from the current year’s fertilizer isn’t all that’s brought down from the 31 states and two Canadian provinces in the Mississippi River Basin. Nitrogen that’s built up underground and in groundwater can take more than 30 years to move into rivers and streams, according to a study appearing in the journal Science.
“My message is that major changes will be needed to improve water quality, and that also we need to be more realistic about these time frames that we’ve set out in the policy goals,” says University of Waterloo researcher Kim Van Meter, the paper’s lead author.
Computer modeling suggests the amount of nitrogen carried down the Mississippi would drop 11% over the next 40 years if current farming and conservation practices continue, as the “legacy sources” decline, she writes. But even getting close to the current national goal would require the impossible: that farmers know ahead of time how much fertilizer their crops will need, and use just that amount.
Donald Scavia, a University of Michigan professor emeritus, and LSU Professor R. Eugene Turner both note that the dead zone changes quickly with annual fluctuations. For instance, they say, its record lows were during droughts when the river was lower than usual and thus carried less nitrogen than usual.
