The study found that the location of a nitrogen source in relation to streams of different sizes in the watershed has an important influence on the amount of nitrogen reaching the Gulf from that source.
The rates of nitrogen reaching the Gulf from upstream areas near large rivers, in such states as Ohio and Minnesota, are much higher than those from neighbouring areas near small streams, the study found.
Moreover, areas near large rivers in these same states, located more than 1,500 miles from the Gulf, deliver more nitrogen to the Gulf than areas near small streams in the states of Mississippi and Arkansas, located only a few hundred miles from the Gulf.
It was not clear until now whether a unit of nitrogen released in
different areas of the Mississippi River drainage basin has an equal chance of reaching the Gulf. It had been assumed that the percentage of nitrogen travelling downstream to the Gulf decreased as the distance increased. However, the study found that nitrogen pollution is removed from water more rapidly in small streams than in large rivers. As a result, nitrogen delivery from point and non-point sources is determined by the amount of time the nitrogen travels through small streams, rather than the source’s distance from the Gulf.
The study found that nitrogen is released from the water by bacteria in the bottom sediments of streams. The amount of nitrogen removed from water depends on the amount of water in contact with bottom sediments. Because water in small, shallow streams has more contact with the bottom sediments than water in deep, large rivers, more nitrogen is expected to be removed in smaller streams than larger rivers. The results of this study strongly support this theory.
Nitrogen increases in the Mississippi River have been cited as the leading cause of eutrophication (excessive algal growth) and chronic hypoxia (low dissolved oxygen) in Louisiana coastal waters and the Gulf of Mexico during the latter half of the 20th century. This area of oxygen-depleted waters is the largest in the western Atlantic Ocean.