To date, scientists trace the cause of the problem to high levels of nutrients, particularly nitrogen. About two-thirds of the nation's harvested cropland and the treated sewage of 27% of the U.S. population empty into the Mississippi River, and eventually into the Gulf. The nutrients discharged feed algal blooms in the spring and summer, which periodically die and sink to the bottom. Large amounts of oxygen are then consumed by the bacteria that decompose the dead algae. Sometimes, so much oxygen is consumed that there is not enough left in the water to sustain life. This dreadful condition persists until an event such as a tropical storm or a cold front mixes the oxygen-depleted lower waters with the upper waters and brings relief.
Scientists first noted the annual formation of the Gulf of Mexico dead zone in the 1980s. In 2001, a multilevel government task force led by the U.S. Environmental Protection Agency set a goal of reducing the Gulf of Mexico dead zone to a five-year running average of less than 2,000 square miles. The figure represents conditions that were likely to be typical in the early 1970s, and a significant reduction from the current conditions. To achieve this goal, a 30 percent reduction in nitrogen load from the Mississippi and Atchafalaya Rivers is necessary. The effort focuses on refining agricultural practices in the watershed. Tactics include better timing of fertilizer applications, plowing more efficiently, restoring natural etlands, and improving manure management.