Gulf of Mexico 'dead zone' caused by Midwest fertilizer runoff is 4th smallest since 1985 (but it's not all good news)

The 2018 Gulf of Mexico dead zone (Louisiana State University map)

The annual low-oxygen 'dead zone' along the coast of Louisiana is only 2,720 square miles this year, much smaller than in recent years, according to researchers at Louisiana State University, the Louisiana Universities Marine Consortium, and the National Oceanic and Atmospheric Administration, Mark Schleifstein reports for The Times-Picayune in New Orleans. That's only one-third the size of last year's record-breaking 8,776-square-mile dead zone and significantly smaller than the 5,770-square-mile average since 2014.

But it's hard to draw conclusions from the smaller area of this year's dead zone. Marine scientist Nancy Rabalais told Schleifstein that strong winds in the weeks before the July 24-28 monitoring cruise likely pushed the largest concentrations of low-oxygen water further out. This year's zone is sized like others that occurred in years with the same weather patterns, such as 2014.

The dead zone is driven mainly by fertilizer run-off from Midwestern farms that flows down the Mississippi River and feeds algae in the Gulf of Mexico. When the algae die and sink to the bottom, microorganisms break them down, using up the oxygen in the water. "This year's smaller dead zone also comes as a surprise because the amount of nutrients carried by the river in May was at near-record levels,"Schleifstein reports. "LSU oceanographer Gene Turner had predicted then that, based on the river's nutrient levels, the dead zone would be 6,570 square miles."