More Nitrogen Upstream,
Fewer Filters Downstream

Scientists have found clues to the causes of the Dead Zone in both the Gulf and Mississippi River watershed. By examining sediment cores taken offshore of Louisiana, they report that abundant nutrients and algal deposition, with resulting oxygen depletion, were rare occurrences in the first half of the 20th century. This is consistent with the fact that it was only after 1970 that the hypoxia phenomenon became increasingly common. The researchers also see clues in several dramatic changes across the 1.2 million-square-mile Mississippi drainage basin. These involve the landscape, agricultural and industrial practices, and changes in the Mississippi itself.

First, during the last century, the Mississippi River was constrained with levees to control flooding. Nutrient-rich waters, which were previously spread by floods and natural diversions into filtering buffer zones throughout the floodplains of the vast river basin, now flow directly to the Mississippi delta with the nutrient load intact.

Second, between 1950 and 1996, there was a major increase in the use of nitrogen fertilizer for agriculture. During the same time, the nitrate load in the Mississippi River also increased nearly three-fold. While a small percentage of that increase is attributed to sources such as treated municipal and industrial effluents, livestock operations, atmospheric sources and runoff from urban areas, scientists say an estimated 90 percent of the increased nitrates have come from agricultural runoff—water draining from fertilized fields.

Finally, the upstream landscape was significantly altered, primarily in 1875-1925 and again in 1945-1960, by the destruction of riverside forests and wetlands and by greatly improving drainage efficiency using ditches, buried drainage tiles and/or culverts. In Ohio, Indiana, Illinois and Iowa, for example, 80 percent of their wetlands are gone. These critical buffers once helped to convert fertilizer nitrate into plant matter and atmospheric nitrogen. The loss of the forests and wetlands has effectively eliminated the basin’s capacity to filter out nutrients entering the river system.

“The extensive installment of underground tile drainage systems on farmlands across the upper Midwest allows fields to dry more quickly during the spring thaw, but also to flush out fertilizer and pesticides into creeks and streams that connect with the river,” says Doug Daigle of the Mississippi River Basin Alliance. The improved drainage also causes organic matter in the soil to oxidize more readily, releasing even more nitrogen.

By the time the river reaches the delta, its nutrient load is overwhelming. Unfortunately, the last bastion of major wetlands available on the river system has been dramatically diminished by decades of coastal erosion from both man-made and natural causes. The river has been leveed for flood protection and navigation, ensuring that the delta wetlands have no contact with Mississippi River water. Because the wetlands no longer receive inputs of sediment and nutrients from the river, they are subsiding rapidly and being converted to open water bays and lakes. This subsidence and erosion are exacerbated by the navigation channels and oil and gas canals that have been cut through the wetlands.

Even if the wetlands of Louisiana existed at their greatest natural capacity and were still connected to the river, it is not likely that they could adequately filter the nutrient load the river now carries. Ironically, the Louisiana coastal marshes are starving for the very nutrients that flow by unimpeded—nutrients that move out to sea to cause a surfeit of algal growth and hypoxia in the Gulf.