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coastal dead zones usa, including dead zone in gulf of mexico dead zone in world and causes
Worldwide, there are some 146 dead zones—areas of water that are too low in dissolved oxygen to sustain life. Since the 1960s, the number of dead zones has doubled each decade. Many are seasonal, but some of the low-oxygen areas persist year-round.
What Causes Coastal Dead Zones?
What is killing fish and other living systems in these coastal areas? A complex chain of events is to blame, but it often starts with farmers trying to grow more food for the world's growing population. Fertilizers provide nutrients for crops to grow, but when excess nutrients run off of farm fields with rain water and are flushed into rivers and seas, they fertilize the microscopic aquatic plant life there. In the presence of excessive concentrations of nitrogen and phosphorus, phytoplankton and algae can proliferate into massive blooms. When the phytoplankton die, they fall to the sea floor and are digested by microorganisms. This process removes oxygen from the water and creates low-oxygen, or hypoxic, zones.
Where are the Dead Zones?
Dead zones range in size from small sections of coastal bays and estuaries to large sea beds spanning some 70,000 square kilometers. Most occur in temperate waters, concentrated off the east coast of the United States and in the seas of Europe. Others have appeared off the coasts of China, Japan, Brazil, Australia, and New Zealand. (See map below.)
The world's largest dead zone is found in the Baltic Sea, where a combination of agricultural runoff, deposition of nitrogen from burning fossil fuels, and human waste discharge has over-fertilized the sea. Similar problems have created hypoxic areas in the northern Adriatic Sea, the Yellow Sea, and the Gulf of Thailand. Offshore fish farming is another growing source of nutrient buildup in some coastal waters.
Forty-three of the world's known dead zones occur in US coastal waters. The one in the Gulf of Mexico, now the world's second largest, disrupts a highly productive fishery that provides some 18% of the US annual catch. Gulf shrimpers and fishers have had to move outside of the hypoxic area to find fish and shrimp. Landings of brown shrimp, the most economically important seafood product from the Gulf, have fallen from the record high in 1990, with the annual lows corresponding to the highly hypoxic years.
A Top “Dead Zones” Killer — Excess Fertilizer Nutrients
Excess nutrients from fertilizer runoff transported by the Mississippi River are thought to be the primary cause of the Gulf of Mexico's dead zone. Each year, some 1.6 million tons of nitrogen now enter the Gulf from the Mississippi basin, more than triple the average flux measured between 1955 and 1970. The Mississippi River drains 41% of the US landmass, yet most of the nitrogen originates in fertilizer used in the agriculturally productive Corn Belt.
Worldwide, annual fertilizer use has climbed to 145 million tons, a tenfold rise over the last half-century. (See graph.) This coincides with the increase in the number of dead zones around the globe.
Loss of Wetlands Exacerbates Coastal Dead Zones
Not only has more usable nitrogen been added to the environment each year, but nature's capacity to filter nutrients has been reduced as wetlands have been drained and as areas along riverbanks have been developed. Over the last century, the world has lost half its wetlands.
In the United States, some of the key farming states like Ohio, Indiana, Illinois, and Iowa have drained 80% of their wetlands. Louisiana, Mississippi, Arkansas, and Tennessee have lost over half of theirs. This lets even more of the excess fertilizer farmers apply flow down the Mississippi River to the Gulf.
Solutions to Dead Zone Problems
There is no one way to cure hypoxia, as the mix of contributing factors varies among locations. But the keys are to reduce nutrient pollution and to restore ecosystem functions. Fortunately, there are a few successes to point to. The Kattegat straight between Denmark and Sweden had been plagued with hypoxic conditions, plankton blooms, and fish kills since the 1970s. In 1986, the Norway lobster fishery collapsed, leading the Danish government to draw up an action plan. Since then, phosphorus levels in the water have been reduced by 80%, primarily by cutting emissions from wastewater treatment plants and industry. Combined with the reestablishment of coastal wetlands and reductions of fertilizer use by farmers, this has limited plankton growth and raised dissolved oxygen levels.
The dead zone on the northwestern shelf of the Black Sea peaked at 20,000 square kilometers in the 1980s. Largely because of the collapse of centralized economies in the region, phosphorus applications were cut by 60% and nitrogen use was halved in the Danube River watershed and fell similarly in other Black Sea river basins. As a result, the dead zone shrank. In 1996 it was absent for the first time in 23 years. Although farmers sharply reduced fertilizer use, crop yields did not suffer proportionately, suggesting they had been using too much fertilizer before.
With carefully set goals and management, it is possible for some dead zones to shrink in as little as a year. For other hypoxic areas—especially in the Baltic, a largely enclosed sea with slower nutrient turnover—improvement may take longer, pointing to the need for early action. Although dead zones shrink or grow depending on nutrient input and climatic conditions, the resulting fish die-offs are not so easily reversed.
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