In spiring and summer, oxygen levels in the Gulf of Mexico have become so low in a large area off the
coast of Louisiana—sometimes spreading as far as the coasts of Texas and Mississippi—that most fish and shellfish cannot survive, creating what is known as a “dead zone.” Fish, shrimp, and crabs often flee, while less mobile bottom-dwellers such as snails, clams, and starfish may die. The phenomenon is attributed to excess nutrients—mostly from fertilizers—that flow down the Mississippi River and empty into the gulf. Why are excess nutrients bad for marine life? All living things require nutrients containing nitrogen, phosphorus, and trace elements to sustain life. But if too much nitrogen and phosphorus find their way into the ocean, these nutrients fertilize the explosive growth of algae.When these algae sink and die, their decomposition consumes most of the oxygen in the bottom water. Algal blooms not only affect fish, but can contribute to the loss of seagrass bed and coral habitats and to the deterioration
of water quality.Over the past 30 years, scientists, coastal managers, and public policy makers have come to recognize that nutrient pollution is a significant problem for the coastal regions of the United States. There are problem areas on all the coasts and also in freshwater lakes, but they are particularly prominent along the mid-Atlantic coast and the Gulf of Mexico. In addition, the algae in some blooms (e.g., red tides) are harmful, containing toxins that can contaminate shellfish and kill marine life. A single harmful algal bloom, if it takes place during the wrong season,can cost a region millions of dollars in lost tourism or lost seafood revenues.