Sufficient information has been obtained to verify that the shallow inner shelf of the Gulf of Mexico is undergoing accelerated eutrophication as a result of nutrient (nitrogen and possibly phosphorus) input from the Mississippi River. This great river drains over 44% of the area of the contiguous United States, and the watershed encompasses the most productive agricultural area in the World. The net effect of nutrient inputs is the rapid depletion of oxygen in the bottom waters during the summer months. This condition is known technically as hypoxia when the oxygen level declines below 2 mg/liter and as anoxia when oxygen disappears altogether. Hypoxia and anoxia results in death of bottom dwelling nonmotile organisms. Motile organisms such as fish and shrimp migrate to more hospitable areas. Hypoxic conditions also cause a shift in the aquatic population mix and as a result, the general structure of the aquatic community waters. Over 7,000 square miles of the Gulf of Mexico are affected by hypoxia each year.
While overabundance of P is generally regarded as the chief cause of surface water quality problems in fresh waters, marine estuarine systems like the Gulf are usually N-limited. . Silica also is important. Research indicates that the lowering of Si output because of watershed changes over the last century has changed the abundance of various phytoplankton in the Gulf. Introduction of N stimulates marine phytoplankton production. The detritus settles and undergoes decomposition increasing the oxygen demand in the system. Oxygen diffusion downward to replace oxygen used in decomposition is limited by formation of a salinity barrier (halocline) caused by addition of fresh water from the Mississippi River that is not completely mixed with the saline water of the Gulf.
While many of the factors, contributing to hypoxia in the Gulf of Mexico have not been identified, much of the blame has been relegated to agriculture's contribution to non-point source pollution in the Mississippi River basin. Erosion, surface-runoff, and tile-drainage from agricultural fields contribute a major portion of the estimated 1.5 million metric tons of N per year which, on average, have entered the Gulf of Mexico from the Mississippi River since 1960. Analysis of USGS data indicate a doubling of N flowing from the watershed from 1960-1980 and little increase in the past two decades. The primary sources of nitrogen are the Corn Belt states; smaller amounts come from the Missouri and Ohio River watersheds. Lowering the contribution of nutrients from these watersheds will not be an easy task, especially in watersheds where animal manure is the major N source. It will require coordinated research and information transfer programs throughout the north central region, and must involve the farm community as well as the fertilizer input industry.
Site maintained by Carol Sikler for Dr. Jane Frankenberger. Last updated December 18, 2000.