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In late summer 1988 the dead zone disappeared as the great drought caused the flow of Mississippi to fall to its lowest level since 1933. During times of heavy flooding in the Mississippi River Basin, as in 1993, "the "dead zone" dramatically increased in size, approximately 5,000 km (3,107 mi) larger than the previous year". [72]
The Mississippi River drains about 41% of the nation's water systems into the Gulf of Mexico. The river itself begins at Lake Itasca in Minnesota and runs through 10 states before it reaches the Gulf.
The political and engineering focus in the 20th century was to separate the Lower Mississippi River from its floodplain.Levees and channelization—along with substantial loss of bottomland forests to agriculture in the alluvial valley—have resulted in a loss of wildlife and fish habitat, decreased water quality, and an expansion of the hypoxic zone in the Gulf of Mexico.
The likely starting cause is waste nutrients being carried into the estuaries by the Mississippi River, although the oil made it worse. The dead zone appeared to be created by low amounts of oxygen in the region, known as hypoxic zones, as a result of phosphorus and nitrogen blocking out sunlight. It grows the most during the summer, when the ...
An example of this is the dead zone located off the coast of the Mississippi River. According to NOAA , the 2016 predicted size of this dead zone is going to be approximately 5,898 square miles with a nitrate concentration of 146,000 metric tons of nitrate flowing down the Mississippi and Atchafalaya River into the Gulf of Mexico. [ 5 ]
Each summer, hypoxic conditions form in bottom waters where the Mississippi River enters the Gulf of Mexico. During recent summers, the aerial extent of this "dead zone" is comparable to the area of New Jersey and has major detrimental consequences for fisheries in the region. [12] Nitrogen is most often transported by water as nitrate (NO 3).
Tripling of NO 3 − loads in the Mississippi River in the last half of the 20th century have been correlated with increased fishery yields in waters surrounding the Mississippi delta; [31] however, these nutrient inputs have produced seasonal hypoxia (oxygen concentrations less than 2–3 mg L −1, "dead zones") in the Gulf of Mexico.
Scientists prepare to collect near-bottom water aboard the R/V Pelican to verify oxygen measurements used to determine the size of the Gulf of Mexico hypoxic zone. (NOAA/LUMCON/LSU) A "dead zone ...