So-called dead zones are areas of low oxygen in the ocean, typically caused by algal blooms that are themselves driven by nutrient pollution. Oxygen is depleted by the algae themselves as they grow, and also by the bacteria that feed on dead algal cells. The dead zone in the Gulf of Mexico forms during summer each year and has a significant impact on commercial and recreational fisheries.

Flow rates along the Mississippi this spring were nearly double the long-term average, which significantly increased the amount of nitrogen and phosphorus transported into the gulf. Based on estimates of nutrient inputs compiled by the US Geological Survey, the scientists suggest that the area of the hypoxic zone in the Gulf of Mexico could reach 24,400 square kilometres this year.

The previous record was about 22,000 square kilometres, set in 2002. ‘The growth of these dead zones is an ecological time bomb,’ said Donald Scavia of the University of Michigan. ‘Without determined local, regional and national efforts to control them, we are putting major fisheries at risk.’

August 2011