What Is Nutrient Pollution? Causes, Impacts, and Mitigation

Nutrient pollution refers to any excess nitrogen and phosphorus in bodies of water. This kind of pollution has many causes. In some cases, nutrient pollution comes from natural processes, such as the weathering of rocks and the mixing of ocean currents. However, it is most commonly caused by human activities, such as soil erosion from agriculture, stormwater runoff in cities, and everyday operations in industrial facilities.

Causes of Nutrient Pollution

Nitrogen and phosphorus occur naturally in the atmosphere and waterways. Living organisms need these chemical elements to grow—but too much can be damaging. Here are some situations that cause an overabundance of these nutrients.

Agriculture

Chemical fertilizer that contains nitrogen and phosphorus is applied to crops, usually in excess, to help them grow. However, these nutrients often make their way into water bodies through surface runoff and leaching into groundwater. Through the process of ammonia volatilization, they also vaporize into the atmosphere.

In addition, the rise of animal production has led to an increase in manure. While manure can be used as a natural form of fertilizer for crops, it also makes its way into water through leaching and runoff.

Aquaculture—the practice of cultivating aquatic organisms through controlled methods—can also cause nutrient pollution. Fish farming often occurs in pens or cages located in enclosed bays. These farms produce excess amounts of nitrogen and phosphorus from uneaten food, feces, and other forms of organic waste.

Urban and Industrial Sources

The most common urban source of nutrient pollution is human sewage. Sewage is estimated to contribute 12% of riverine nitrogen input in the United States, 25% in Western Europe, and 33% in China.

In developing countries, when sewage is treated, the main objective is to remove solids, not nutrients; therefore, nutrient pollution remains after treatment. And in developed countries, septic systems purify sewage by leaching it through soil, which reaches groundwater and nearby surface waters.

Stormwater runoff is another cause of pollution; during rainfall events, the stormwater in cities is flushed into nearby rivers and streams. Other sources of industrial nutrient pollution are pulp and paper mills, food and meat processing plants, and discharge from maritime vessels.

Fossil Fuel Sources

The burning of fossil fuels releases nitrogen oxides into the air, which results in smog and acid rain. The nitrogen oxides are then redeposited to the land and water through rain and snow.

The most common sources of nitrogen oxides are coal-fired power plants and exhaust from cars, buses, and trucks. Fossil fuel combustion contributes to 22 teragrams of nitrogen pollution globally every year.

Environmental Impacts

Nutrient pollution is detrimental to the environment as it harms water quality, destroys ecosystems, and disrupts plant and animal species. An excess of nitrogen and phosphorus causes algae to grow faster than ecosystems can handle, resulting in the growth of algal blooms. These algal blooms produce toxins that are harmful to fish and other aquatic life.

Algal blooms are also detrimental for ecosystems as they block sunlight from reaching plants, which prevents them from growing. Additionally, these blooms cause dead zones in the water, resulting in a decrease in oxygen for aquatic life.

Nutrient pollution in the atmosphere causes acid rain which damages waterways, forests, and grasslands. It causes increased acidity in water bodies which is deadly to aquatic life, and it dissolves vital nutrients that trees and plants need to survive, such as magnesium and calcium. Nutrient pollution in the air also contributes to the formation of other air pollutants.

Where Is Nutrient Pollution Happening?

Nutrient pollution from agriculture is a major problem in the United States. In 2018, Florida had record-breaking amounts of algal blooms, stretching more than 100 miles along the Gulf Coast. This was detrimental to fish, turtles, and dolphins and it hospitalized more than a dozen people.

There are also dead zones in the Gulf of Mexico and the Chesapeake Bay. In 2020, the dead zone in the Gulf of Mexico covered approximately 4,880 square miles. On average, the Chesapeake Bay dead zone covers between 0.7 and 1.6 cubic miles during the summer months, when the water is at its warmest and oxygen levels are at their lowest.

Algal blooms are also a major problem in Lake Erie, which spans the U.S. and Canada. The main source of nutrient pollution in the lake is agricultural runoff. Both countries' governments and various environmental organizations have worked for decades to reduce pollution in the lake as it threatens the health of the environment and humans.

Mitigation

Reducing nutrient pollution is crucial. In the United States, the EPA is working to combat nutrient pollution by promoting the collaboration of stakeholders and overseeing regulatory programs. In a regulatory program, the EPA reviews and approves state water quality standards.

The EPA also conducts outreach by developing community materials to increase public awareness of the issue, communicating the latest scientific information to stakeholders, and organizing outreach programs.

The EPA also develops partnerships and provides states with the technical guidance and resources to help them develop water quality criteria for nitrogen and phosphorus.