Nutrient pollution is a type of water pollution that occurs when the presence of too many nutrients causes environmental damage. The most common effect of this type of pollution is the death of fish and other aquatic life. Nutrient pollution is often caused by the release of nitrogen or phosphorous into a body of water. These are not directly toxic to fish, but act by disrupting ecological balance through a complex process. Nitrogen and phosphorous runoff is associated with agricultural practices, which is one reason that agricultural areas are the most common sites of nutrient pollution.
Having too many nutrients is harmful because it causes the overgrowth of aquatic plant life, especially photosynthetic algae. When stimulated by the presence of excess nutrients, this algae can form mats on the surface of the water. When the surface algae dies, less oxygen is produced in the water because algae below the surface don't receive enough sunlight. If there is a high demand for oxygen in such a body of water, oxygen levels may fall to the point that fish and other aquatic animals begin to die. This process is called eutrophication when it occurs naturally and artificial eutrophication when caused by man-made nutrient sources.
The excess nitrogen and phosphorus that cause nutrient pollution are usually the result of agricultural activity. Nitrogen is used in fertilizers and is present in animal waste from feedlot operations. Phosphorous is also a component of farm fertilizers or treated sewage. Usually these elements, which are necessary for life, only occur naturally in limited amounts. In the same way that these fertilizers increase crop growth, they can cause the growth or overgrowth of algae associated with cases of nutrient pollution.
One prominent example of the process is the so-called dead zone that occurs annually in the Gulf of Mexico. The dead zone is caused by nitrogen carried to the Gulf from the Mississippi River. The Mississippi drains much of the land in the Midwestern United States, an area known for its farming operations and agriculture. In the Gulf of Mexico, the extra nitrogen causes blooms of algae, which die and deplete the oxygen in the water. Fish and shellfish that can't escape the zone are killed.
While it may difficult to end all instances of nutrient pollution, managing such cases includes making sure that too many nutrients don't enter waterways in the first place. Some ways to do this include reducing or better controlling the use of nitrogen fertilizers, using soap that doesn't contain phosphates, and improving water treatment methods.