According to the latest report from the federal Office of Science and Technology Policy the number of dead zones increased dramatically in U.S. waters over the last 50 years. Term "dead zones" refers to condition called hypoxia where in areas of estuaries and coastal waters oxygen levels drop so low that there isn't enough oxygen left to sustain life, meaning that fish and other marine life can't survive in such areas.
There are many different factors responsible for creation of dead zones. Many scientists believe that one of the main causes is water pollution but there are also other factors involved such as fishing, climate change, harmful algal blooms, toxic contaminants, etc. The large number of contaminants leads to high concentrations of nutrients such as nitrogen and phosphorus, and this leads to eutrophication, condition characterized by the excessive algae growth. Once algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, making it incapable to support life.
Such condition can not only cause big environmental but also big economic damage. The last results were very worrying because researchers detected dead zones in almost half of the 647 analyzed US waterways, including the Gulf of Mexico, where scientists discovered one of the largest dead zones in the world.
Lisa Jackson, administrator of the Environmental Protection Agency warned US government by saying that "these growing dead zones endanger fragile ecosystems and potentially jeopardize billions of dollars in economic activity".
Some scientists believe that climate change considerably helped in creation many of these dead zones. Our oceans are becoming warmer due to the climate change, and global climate models predict that the warming of the oceans will cause decline in oxygen levels in deep oceans by 20 to 40 percent in the next century.
Many species that have their habitats in deep oceans are extremely sensitive to changes in oxygen levels, and many of them could go extinct which would create irreversible damage throughout the entire marine food web.
This problem is very serious, and things will become even worse because human population is constantly increasing which means more pollution and bigger climate change impact. The scientists have conclude that "if current practices are continued, the expansion of hypoxia in coastal waters will continue and increase in severity, leading to further impacts on marine habitats, living resources, economies, and coastal communities".