As summer comes to the Gulf of Mexico, it brings with it 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 percent of the bay.

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 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 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. But they are still 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 in low-oxygen waters.

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 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.

Coastal Dead Zones Around the World Source: UNEP, GEO Yearbook 2003 (Nairobi: 2004), compiled from Boesch 2002, Caddy 2000, Diaz et al. (in press), Green and Short 2003, Rabalais 2002.

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 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 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 percent of the U.S. 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.

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 percent of the U.S. landmass, yet most of the nitrogen originates in fertilizer used in the productive Corn Belt.

Worldwide, annual fertilizer use has climbed to 145 million tons, a tenfold rise over the last half-century. (See data.) This coincides with the increase in the number of dead zones around the globe. And 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 as areas along riverbanks are 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 percent 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. 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 percent, 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 percent 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.

While phosphorus appears to have been the main culprit in the Black Sea, nitrogen from atmospheric sources—namely, emissions from fossil fuel burning—seems to be the primary cause of the dead zones in the North and Baltic seas. Curbing fuel use through efficiency improvements, conservation, and a move toward renewable energy can diminish this cause of the problem.

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, can also play a part.

Innovative programs such as the American Farmland Trust’s Nutrient Best Management Practices Endorsement can reduce the common practice of using too much fertilizer. Farmers who follow recommendations for fertilizer application and cut their use are guaranteed financial coverage for potential shortfalls in crop yields. They save money on fertilizer purchases and are insured against losses. Under test programs in the United States, fertilizer use has dropped by a quarter.

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. For while dead zones shrink or grow depending on nutrient input and climatic conditions, the resulting fish dieoffs are not so easily reversed.

Copyright © 2004 Earth Policy Institute