US "dead zones" have increased thrityfold over past 50 years

The number of "dead zones" in the oceans surrounding the US has been increasing at a much faster rate than the global average, according to a report released by the US government last week.

The report, which highlights growing risks to both the US fishing industry and global biodiversity targets, is the first of five mandated by the Harmful Algal Bloom and Hypoxia Amendments Act of 2004.

It was produced by the Interagency Working Group on Harmful Algal Blooms, Hypoxia and Human Health, and charts historic and current trends associated with hypoxia – the phenomenon whereby oxygen in water reaches such low levels that it drives sea life out of an area or kills organisms that cannot travel, creating so-called "dead zones".

Hypoxia is caused by higher than required inputs of organic matter into the water which in turn cause a rise in the phytoplankton population. The phytoplankton then consume the oxygen in the water, leaving a deficit of oxygen for other marine life.

The report concluded that incidences of hypoxia have increased tenfold in the past 50 years on a global basis, while in the US they have soared thirtyfold over the same period. Prior to 1960, there were just 12 hypoxic regions in US coastal waters; now there are more than 300.

The cause of hypoxic areas has changed in the past 50 years, said the report. In the 1960s, it was largely due to "point source" organic input from municipal waste. The Clean Water Act introduced environmental regulations to tackle dumping in the ocean, but it failed to regulate non-point source inputs to the ocean, such as agricultural run-off.

"Overfishing, harmful algal blooms, toxic contaminants and physical alteration of coastal habitats associated with coastal development are several problems that co-occur with hypoxia and interact to decrease the ecological health of coastal waters and reduce the ecological services that they can provide," the report said.

Attempts to reduce hypoxia have failed to make significant headway due to the increased population in coastal wetlands areas, the report added. Increased human population is a key contributor to the non-point source run-off that exacerbates the problem, the document warned.

The Gulf of Mexico has been particularly hard hit by hypoxic conditions. According to a separate report by NOAA published in August, the average size of the dead zone there is 6,653 square miles – 3.5 times the target goal set by the Gulf of Mexico/Mississippi River Watershed Nutrient Task Force.

The report did not assess the effects of the BP Deepwater Horizon oil spill, although the NOAA report suggested the hypoxic effects of the spill would be minimal.