Marine dead zones on the rise
Dead zones: these ominous sounding areas of the seafloor deprived of oxygen and life, are spreading across the world's ocean floor. According to Rutger Rosenberg from the University of Gothenburg in Sweden and Robert Diaz from the Virginia Institute of Marine Science, College of William and Mary in the US, dead zones have increased to such an extent that they are now considered to be 'the key stressor on marine ecosystems' and 'rank with over-fishing, habitat loss, and harmful algal blooms as global environmental problems'. The scientists published their study results in the journal Science. Dead zones may sound like a horror film. However they are not a work of fiction; if their spread continues their impact could be more horrific than any film. Dead zones are areas of the sea floor which are low in oxygen, and where eutrophication - an increase in chemical nutrients - has occurred. Eutrophication is largely considered to be caused by run-off from the agricultural sector, from fertilisers containing nitrogen and phosphorous, as well as sewage, animal wastes and atmospheric deposition from the burning of fossil fuels, all of which can remove oxygen from the water. Once these chemicals they have entered the water system they boost the growth of algae. When the algae die they provide a rich food source for bacteria, which effectively suck up the oxygen from the surrounding waters, creating dead zones which are incapable of supporting life. According to the scientists, many ecosystems experience periods of low oxygen (hypoxia), usually in the summer after the spring algal blooms. However, if nutrient inputs continue to increase, these zones may persist. Earth's largest dead zone, in the Baltic Sea, experiences hypoxia year-round. This is largely due to the fact that the flow of water in and out of the Baltic Sea is restricted by the islands and narrow channels around Denmark. Chesapeake Bay experiences seasonal, summertime hypoxia through much of its main channel, occupying about 40% of its area and up to 5% of its volume. Professor Diaz began studying dead zones in the mid-1980s after seeing their effect on bottom life in a tributary of Chesapeake Bay near Baltimore in the US. In 1995 he began reviewing dead zones in the world's oceans, when he counted 305 worldwide. The first scientific reports of dead zones from 1910 set their number at four. According to the data he was able to compile, the number of dead zones has approximately doubled each decade since the 1960s. Dead zones now number over 400 worldwide and cover over 245,000 square kilometres. By comparison the United Kingdom is 244,820 square kilometres. Professors Diaz and Rosenberg note that hypoxia tends to be overlooked until it starts to affect the seafood that people eat. A possible indicator of hypoxia's adverse effects on an economically important finfish species in Chesapeake Bay is the purported link between oxygen-poor bottom waters and a chronic outbreak of a bacterial disease among striped bass. The scientists share the belief that the key to reducing dead zones is to keep fertilisers out of the sea. Professor Diaz believes that goal is also shared by farmers who are concerned with the high cost of adding nitrogen supplements to their crops. 'They certainly don't want to see their dollars flowing off their fields into the Bay,' he said. 'Scientists and farmers need to continue working together to develop farming methods that minimise the transfer of nutrients from land to sea.'