Climate Fact: Discharges and Dead Zones (Wichita, Kansas)

Since 1910, overall precipitation in the lower 48 states has increased by ten percent, and the region with the largest increase in rainfall is the Mississippi River Basin, which includes the Arkansas River. Because of fertilizer use on farms, lawns, and gardens in the Basin, wet years mean that more nitrogen (an ingredient in fertilizer) is carried with rainwater into the Mississippi River, which ultimately leads to the Gulf of Mexico. Excess nitrogen in the Gulf can cause algae populations to boom, and when algae decompose, the process can consume much of the water’s oxygen supply, forming “hypoxic” or “dead” zones in the water. The largest and most severe coastal dead zone in the Western Atlantic Ocean is a 20,000 square kilometer area in the Northern Gulf, offshore from the Mississippi River Delta. The Gulf’s dead zone is less severe during drought years in the Midwest, whereas it is usually at its largest extent during flood years such as 1993. The growth of the dead zone from the mid 1980’s to the late 1990’s resulted in a decline of brown shrimp populations, impacting fisheries.

(Sources: Karlsen, AW et al. “Historic Trends in Chesapeake Bay Dissolved Oxygen Based on Benthic Foraminifera from Sediment Cores.” Estuaries: Volume 23, No. 4, pp. 488-508, August 2000 and National Weather Service: Climate Prediction Center. U.S. Temperature and Precipitation Trends: Annual. Accessed Online 3 July 2007 http://www.cpc.noaa.gov/anltrend.gif)