Nutrient-rich runoff from those farming states ends up in the Mississippi River and eventually makes its way to the Gulf. The amount of nitrogen entering the Gulf of Mexico each spring has increased by about 300 percent since the 1960s, mainly due to increased agricultural runoff.
According to U.S. Geological Survey estimates, 153,000 metric tons of nutrients flowed down the Mississippi and Atchafalaya rivers to the northern Gulf in May 2013, an increase of 94,900 metric tons over last year's drought-reduced 58,100 metric tons. The 2013 input is 16 percent higher than the average nutrient load estimated over the past 34 years.
In the Gulf and the Bay, the nutrient-rich waters fuel explosive algae blooms. When the algae die and sink, bottom-dwelling bacteria decompose the organic matter, consuming oxygen in the process. The result is a low-oxygen (hypoxic) or oxygen-free (anoxic) region in the bottom and near-bottom waters: the dead zone.
Fish and shellfish either leave the oxygen-depleted waters or die, resulting in losses to commercial and sports fisheries. In the Gulf, the dockside value of commercial fisheries was $629 million in 2009, and nearly 3 million recreational anglers contributed more than $1 billion to the region's economy.
Chesapeake Bay dead zones, which have been highly variable in recent years, threaten a multi- year effort to restore the bay's water quality and to enhance its production of crabs, oysters and other fisheries. The Geological Survey estimates that 36,600 metric tons of nitrogen entered the estuary from the Susquehanna and Potomac rivers from January through May, which is 30 percent below the average loads estimated between 1990 and 2013.
The final Ches
|Contact: Jim Erickson|
University of Michigan