Dead zones increasing in world's coastal waters

SUMMER BRINGS to the Gulf of Mexico each year a giant "dead zone" devoid of fish and other aquatic life. Expanding over the past several decades, this area now can span up to 21,000 square kilometers, which is larger than the state of New Jersey. A similar situation is found on a smaller scale in the Chesapeake Bay, where, since the 1970s, a large lifeless zone has become a yearly phenomenon, sometimes shrouding 40% the bay.

Worldwide, there are some 146 dead zones--areas of water that am too low in dissolved oxygen to sustain life. Since the 1960s, the number has doubled each decade. Many are seasonal, but some of the low-oxygen areas persist year-round. What is killing fish and other riving systems in these coastal areas? A complex chain of events is to blame, but often it starts with farmers trying to grow more food for the world's expanding population. Fertilizers provide nutrients for crops to flourish, but when they are flushed into rivers and seas, they fertilize microscopic plant life as well. 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 seafloor and are digested by microorganisms. This process removes oxygen from the bottom water and creates low-oxygen, or hypoxic, zones.

Most sea life cannot survive in low-oxygen conditions. Fish and other creatures that can swim away abandon dead zones. Yet, they still are not entirely safe by relocating, they may become vulnerable to predators and face other stresses. Other aquatic life, like shellfish, that cannot migrate, in time suffocate.

Dead zones range in size from small sections of coastal bays and estuaries to large seabeds spanning some 70,000 square kilometers. Most occur in temperate waters, concentrated off the U.S.'s East Coast and in the seas of Europe. Others have appeared off the coasts of China, Japan, Brazil, Australia, and New Zealand. 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 overfertilized the water. Similar problems have created hypoxic areas in the northern Adriatic Sea, Yellow Sea, and Gulf of Thailand. Offshore fish farming is another burgeoning source of nutrient buildup in some coastal waters.

Forty-three of the world's known dead zones occur in U.S. 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 U.S. annual catch. Gulf shrimpers and fishers have had to move outside of the hypoxic area to keep providing catches. Landings of brown shrimp, the most economically important seafood product from the Gulf, have fallen from a record high in 1990, with the annual lows corresponding to the highly hypoxic years.

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 1,600,000 tons of nitrogen enter the Gulf from the Mississippi basin, more than triple the average flux measured between 1955-70. The Mississippi drains 41% of the U.S. land mass, yet most of the nitrogen originates in fertilizer used in the productive Corn Belt. Worldwide, annual fertilizer use has climbed to 145,000,000 runs, a tenfold rise over the last half-century. This coincides with the increase in the number of dead zones around the globe.

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 are drained and areas along riverbanks become more and more developed. In the U.S., key fanning 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.

There is no one way to cure hypoxia, as the mix of contributing factors varies among locations. The keys are to reduce nutrient pollution and to restore ecosystem functions. There even have been a few successes. One is the Kattegat Strait between Denmark and Sweden that 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.

For the Gulf of Mexico, curbing nitrogen runoff from farms can shrink the dead zone. Applying fertilizer to match crop needs more precisely would allow more nutrients to be taken up by plants instead of being washed out to sea. Preventing erosion through conservation tillage and changing crop rotations, along with wetland restoration and preservation, also can play a part.

Janet Larsen is a research associate at the Earth Policy Institute, Washington, D.C.

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