Dead zones increasing in world's coastal waters.

• Author: Larsen, Janet
• Position: Eye On Ecology

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...