Marine ‘dead zone’ predicted to be biggest yet

The USA’s National Oceanic & Atmospheric Administration (NOAA) and partners have been measuring the expanse of the dead zone every summer since 1985 and their modelling suggests that this year it could be as large as 8,500 square miles.

The problem is caused by a seasonal change where algal growth, stimulated by nutrients such as nitrogen and phosphorus from the Mississippi and Atchafalaya rivers, settles and decays on the ocean floor, consuming oxygen faster than it can be replaced on the surface.

Scientists use data on the levels of agricultural fertilisers found in the rivers which feed into the gulf and predictions of tropical storms and hurricanes, which can churn up the waters and aerate the bottom layer, to draw up a best guess of how large the oxygen-starved region will be.

As well as being of ecological interest, the information is of vital economic importance as the profitability of the regional fishing industry can depend on the size of the zone.

“I am anticipating a historically large hypoxic zone this summer because the nitrate loading this May, a critical month influencing the size of the area, was very high,” said lead researcher R Eugene Turner of the Louisiana State University.

“The difference between 2007 and 2002 cannot be explained by increased river flow. The riverine flow in May 2007 was 77% of the May 2002 discharge, but it contained 35% more nitrogen. The relatively high nitrate loading may be due to more intensive farming of more land, including crops used for biofuels, unique weather patterns, or changing farming practices.”

Research indicates that nearly tripling the nitrogen load into the Gulf over the past 50 years has led to the heightened hypoxia problem. The scientists say their research will improve assessments of hypoxic effects under various Gulf Coast oceanographic conditions.

These research, observational, and modelling studies are part of a larger NOAA sponsored effort to support effective management of the waters by developing a fundamental understanding of the northern Gulf of Mexico ecosystem with a focus on the causes and effects of the hypoxic zone.

David Gibbs