Gulf dead zone to be biggest ever

This year could see the biggest "dead zone" since records began form in the waters of the Gulf of Mexico. The dead zone is an area of water virtually devoid of oxygen which cannot support marine life. It is caused by nutrients such as fertilisers flowing into the Gulf, stimulating the growth of algae which absorbs the available oxygen. The volume of nutrients flowing down rivers such as the Mississippi into the Gulf has tripled over the last 50 years.