Growing dead zone

by Sierra Club intern Ben Hellerstein


photo from the Carlton College Science Education Resource Center

Good news: this year’s dead zone in the Gulf of Mexico is only 6,765 square miles in area. Scientists had predicted that the dead zone might be as large as 9,400 square miles this summer, owing to the higher-than-normal load of nitrogen and phosphorous fertilizers washed into the Mississippi River by heavy spring floods in the Midwest. Instead of a dead zone the size of the state of Vermont, we ended up with something closer to one and a third Connecticuts.

But don’t start celebrating just yet. The scientists who measured the dead zone believe that their data was compromised by Tropical Storm Don, which swept through the Gulf as they were taking their measurements and temporarily stirred up the oxygen-deprived water. In all likelihood, the dead zone was larger than measured, in line with a decades-long trend of increasing hypoxia in the Gulf.

And according to a recent study from the U.S. Geological Survey (USGS), it’s our fault. We have known for decades that humans cause the Gulf dead zone, but now we know that Minnesota and Wisconsin, in particular, share much of the blame for the growth in the dead zone over the past thirty years.

Dead zones occur where the concentration of dissolved oxygen in ocean water is less than two milligrams per liter. This condition is known as hypoxia, and when it happens, ordinary sea floor organisms like fish and shrimp cannot survive.

Some parts of the ocean are naturally hypoxic, but hypoxia in shallow coastal areas, including the Gulf of Mexico near the Mississippi River Delta, has increased over the past few decades due to human activity. Excess nitrogen and phosphorus from farm fields, suburban lawns, and wastewater treatment plants wash into the Mississippi. When these nutrients reach the Gulf, they cause the phytoplankton near the surface to reproduce rapidly. Soon, the phytoplankton dies and sinks to the bottom, where it decomposes. The decomposition of the phytoplankton consumes oxygen, and due to poor circulation in the Gulf during the summer, oxygen quickly becomes depleted.

Organisms that can swim away fast enough can survive, but those that can’t leave may die. Over the long term, the entire food web in the Gulf of Mexico is disturbed, affecting the survival of all aquatic organisms and the health of the $2.8 billion a year Gulf fishing industry.

The USGS study is entitled, “Nitrate in the Mississippi River and its tributaries, 1980-2008: Are we making progress?” and the answer to that question appears to be “no.” Since 1980, the concentration of nitrates flowing into the Gulf of Mexico at the mouth of the Mississippi River has increased by 10%.

The study evaluated nitrate concentrations at eight sites along the Mississippi River and its major tributaries. Six of the eight sites showed little or no change over the past thirty years. But at the other two sites, the concentration of nitrates has increased by 75%. One of those sites is located at Herman, Mo., on the Missouri River near its confluence with the Mississippi. The other is located on the Mississippi River at Clinton, Iowa, where most of the water in the river comes from Wisconsin and Minnesota.

That means that Minnesota and Wisconsin are responsible for a significant chunk of the increase in nitrate runoff into the Gulf of Mexico over the past thirty years. The dead zone is growing in large part because of what we are doing to our land in the Upper Midwest. Despite extensive efforts to promote good soil and nutrient management practices over the past several decades, the USGS study shows that existing strategies to reduce nitrogen runoff are not working.

We can see the effects locally as well as nationally. The Minnesota Pollution Control Agency and Minnesota Department of Agriculture conduct a “Biennial Assessment of Water Quality Degradation Trends and Prevention Efforts” (appendix A). In 2010, the agencies found that 117 miles of Minnesota’s streams are impaired due to high nitrate levels, and 541,373 acres of our lakes show “nutrient/eutrophication biological indicators” that can be a sign of elevated nitrogen concentrations.

At the so-called “Minnesota Milestone” sites, where up to sixty years of water quality data is available, 75% have shown an increase in nitrate levels over the long term. Nitrates have found their way into our groundwater as well. In Southeast Minnesota, 12% of drinking water wells had nitrate concentrations above 10 milligrams per liter, the official drinking water standard. Elevated nitrate levels in drinking water may cause birth defects and cancer at multiple organ sites, including the stomach, esophagus, bladder, ovaries, and colon. It has also been linked to methemoglobinemia, a disease that leaves the blood unable to bring oxygen to body tissues.

Where does this nitrate come from? 74% of the nitrate that washes into the Mississippi River every year comes from agriculture. Two major sources are fertilizer and manure, especially for nitrogen-hungry corn production. Any nitrogen that is not taken up by the plants can leach into streams and groundwater, eventually draining into the Mississippi River.

The federal Clean Water Act, passed in 1972, regulates some sources of nitrogen pollution, but most agricultural activities are exempt. Unlike a municipal sewage treatment plant, where all potential contaminants are discharged into a water body at a single point, runoff from agricultural fields is a diffuse, “non-point source” of pollution. That makes it much more difficult to address.

In an effort to reduce non-point-source water pollution, the Clean Water Act was amended in 1987 to create the Section 319 program. Under this program, the federal government provides money to state governments, which award grants for projects that reduce water contamination from non-point sources, including agriculture. Local government units, nonprofit organizations, and universities may apply for funding under Section 319.

The State of Minnesota also created a separate Clean Water Partnership (CWP) program to award grants for non-point-source pollution reduction projects. These grants, along with loans and technical assistance, are available to local units of government only.

In 2002, Minnesota passed the Phosphorus Lawn Fertilizer Law, the first law in the nation to regulate the use of fertilizer containing phosphorus on lawns. Phosphorus, like nitrogen, can be harmful to lakes and streams in large quantities, promoting excessive phytoplankton growth. The law stipulates that fertilizers containing phosphorus may not be used on lawns unless a new lawn is being established or a soil test indicates that there is a phosphorus deficiency. Golf courses and sod farms are exempt from this restriction. According to a March 2007 report from the Minnesota Department of Agriculture, phosphorus use for lawn fertilization decreased from 292 tons to 151 tons in the first few years after the law took effect.

Although these state and federal programs are a step in the right direction, the USGS study shows that excess nutrient runoff continues to be a serious, and growing, problem. We can’t ignore the impact of our land use practices on the health of the Gulf of Mexico, and bodies of water closer to home.


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