Despite growing concern over the last two decades about the low-oxygen “dead zone” that emerges each summer in the fisheries-rich Gulf of Mexico, the nitrate pollution at the root of the problem continues to rise.
That’s the upshot of a study just released by the U.S. Geological Survey (USGS), which monitored nitrate trends at eight key locations in the Mississippi River Basin over the 30-year period from 1980-2010.
At the outlet where the Mississippi empties into the Gulf of Mexico, nitrate levels rose 12 percent just between 2000 and 2010.
High levels of nitrate –sometimes referred to as a “nutrient,” because it encourages plant growth – can cause an abundance of algae in water bodies, which in turn robs water of oxygen as bacteria break down the excess organic matter.
Fish and other aquatic animals get increasingly stressed as the oxygen level declines. The point at which they begin to suffocate varies, but stress is usually evident when oxygen drops below 3 milligrams per liter of water. (For comparison, oxygen levels in air are about 280 milligrams per liter.)
Last summer, the Gulf of Mexico’s dead zone spanned 5,840 square miles, an area about the size of Connecticut.
Commercial and recreational fisheries in the Gulf produced a dockside value of $818 million in 2011 and generated 23 million recreational fishing trips, according to the National Oceanic and Atmospheric Administration.
Unabated, the influx of nitrate pollution continues to threaten the Gulf’s ecology and economy.
The only bright spots in the USGS study were trends in the Illinois River, where nitrate concentrations fell by 21 percent over that last decade, and in the Iowa River, where they dropped by 10 percent.
But the rise in concentration at the Mississippi’s mouth suggests little progress in containing the overall influx of nitrates from fertilizers, livestock waste, farm operations, sewage discharges and other sources.
Although not part of the USGS assessment, the conversion of natural wetlands to farms and other land uses throughout the Mississippi watershed also contributes to the problem. Wetlands absorb nutrients, preventing them from entering streams and rivers, where they become harmful pollutants.
In the upper Mississippi watershed alone, some 35 million acres of wetlands have been drained and converted to other land uses. That’s an area the size of Illinois that’s no longer actively absorbing and processing nitrates and other would-be pollutants.
Along with more optimal use of fertilizer and the creation of buffer zones between farms and streams, strategically reconstructing wetlands and bringing floodplains back into action could help reduce the nitrate levels in Mississippi basin waters.
Studies along the Illinois River, for instance, find that floodplain wetlands can remove about 0.24 tons of nitrate per acre.
One strategy being pursued in parts of Iowa and Illinois, where many farms have underground tile drains that carry runoff directly into nearby rivers and streams, is to strategically construct wetlands where they will intercept this drainage and remove the nitrates before they empty into waterways.
Iowa’s Conservation Reserve Enhancement Program, a joint effort of state and federal agencies and local conservation districts, has found that such strategically placed wetlands remove 40-70 percent of nitrates (and over 90 percent of herbicides) from farm drainage. This pollution trapping not only improves the quality of local waters and the Gulf of Mexico downstream, but the wetlands provide prime habitat for birds and wildlife, opening up recreational opportunities.
Similarly, the Nature Conservancy and partners are working with farmers in Illinois to build wetlands in places that effectively trap nitrates before they pollute the Mackinaw River, a major tributary of the Illinois River and a source of drinking water for more than 80,000 people.
The Conservancy’s research suggests that a wetland area equal to about 6% of the tiled farm field removes about 50% of the nitrates draining off the field.
Clearly, these and other conservation measures to reduce the influx of nitrates into the rivers and streams of the Mississippi Basin show promise for improving the quality of both local water sources and the Gulf of Mexico downstream.
But until such measures are incentivized and more widely implemented, the aquatic ecosystem and the lucrative fisheries of the Gulf of Mexico will continue to be threatened by the appearance of a dead zone in the peak of summer.
Sandra Postel is director of the Global Water Policy Project and Freshwater Fellow of the National Geographic Society. She is co-creator of Change the Course, the national freshwater conservation and restoration campaign being piloted in the Colorado River Basin.