Dead Zones in the World’s Oceans

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. There, a large lifeless zone has become a yearly phenomenon since the 1970s, sometimes shrouding 40% of the bay.

Worldwide, there are some 146 "dead zones" — that is, 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.

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 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 find fish and shrimp.

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. That is more than triple the average flux measured between 1955 and 1970.

The Mississippi River drains 41% of the U.S. landmass, yet most of the nitrogen originates in fertilizer used in the productive Corn Belt.

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.

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.

For the Gulf of Mexico, curbing nitrogen runoff from farms can shrink the dead zone. 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.

This points to the need for early action. After all, while dead zones shrink or grow depending on nutrient input and climatic conditions, the resulting fish die-offs are not so easily reversed.