Summary

During the past 30 years, scientists have observed hypoxic zones in various water bodies around the world.  Excessive nutrient enrichment (eutrophicaton) helps promote overgrowth of opportunistic bacteria, cyanobacteria and algae and surface waters.  After these organisms grow, die and fall to the bottom of a water body, the bacteria decompose, which can lead to depletion of dissolved oxygen in areas of the water body. This oxygen depletion, which is known as hypoxia, occurs when dissolved oxygen concentrations fall below two milligrams per liter in shallow coastal and estuarine systems.

Two examples of hypoxic zones that have garnered significant media attention, the Gulf of Mexico and the Chesapeake Bay, illustrate the limitations of focusing solely on agriculture as the sole source of nutrients causing the hypoxic condition.

Background

Gulf of Mexico

A closer look at the hypoxia issue in the Gulf of Mexico reveals that the cause and the magnitude of the problem are quite complex.  Hypoxic conditions in coastal waters are not the result of nutrient loadings alone, but are caused by complex interactions between climate, weather, basin morphology, water circulation patterns, water retention times, freshwater inflows and stratification or different “layers” of water in a particular aquatic system.

There are many sources of nutrients in the Gulf of Mexico: Fertilizer nutrients have frequently been cited as the principle source of nutrients reaching the Gulf of Mexico and as a result, have been considered by many as the principle source of hypoxia. However, because there are many manmade and natural sources of nutrients, it has not been possible to directly identify the source of nitrogen flowing down the Mississippi River and into the Gulf.

Nutrient Concentrations in the Gulf Have been Decreasing

There is a common misperception that the nitrogen flowing into the Gulf has been steadily increasing, however, scientific data indicate that the average total nitrogen load actually declined 19-21 percent when comparing the 2001-2005 period with 1980-1996.  These reductions are, at least in part, the result of the agricultural community’s implementation of various nutrient management practices.

Chesapeake Bay

The issue in the Chesapeake Bay watershed is slightly different than that in the Gulf of Mexico.  The enclosed nature of the Bay means that water “turnover” is limited. Sewage treatment plant effluent, nutrient runoff from residential and commercial lawn care and agricultural runoff from dairy, poultry and crop farms in the Bay area are often blamed for water quality problems in the Bay, but changes in the volume of freshwater inflows from the area around the northern part of the bay also play a role.

Fertilizer is being used more efficiently than ever before

Any discussions about fertilizer’s contribution to water quality problems must take into account important data regarding U.S. fertilizer use. Between 1980 and 2010, U.S. farmers nearly doubled corn production using slightly fewer fertilizer nutrients than were used in 1980. The announcement is based on fertilizer application rate data released earlier this year by the U.S. Department of Agriculture’s (USDA) National Agricultural Statistics Service (NASS).

Specifically, in 1980, farmers grew 6.64 billion bushels of corn using 3.2 pounds of nutrients (nitrogen, phosphorus and potassium) for each bushel and in 2010 they grew 12.45 billion bushels using 1.6 pounds of nutrients per bushel produced. In total, this represents an 87.5 percent increase in production with 4 percent fewer nutrients during that same timeframe. Corn production accounts for half of U.S. fertilizer use.  This leap in efficiency speaks volumes about the impact that improved agricultural practices are having on U.S. water quality.