Fertilizers As Pollutants

Fertilizers, whether they are artificial or organic, can cause serious problems if they contaminate freshwater and marine ecosystems. The same nutrients in fertilizers that nourish crops, lawns, and golf courses can cause rapid growth of algae, also known as an algal bloom. Dying algae in turn feed bacteria, which consume so much dissolved oxygen that fish and plants cannot survive. Some species of algae also produce toxins that can be harmful to humans. (See References 1)


Fertilizer pollution is harder to regulate and reduce than many other kinds of water pollution because it is a type of nonpoint source pollution. A point source -- which comes from one definable location, such as a factory -- is easier to track. But fertilizers are used on private lawns, in agricultural fields and on golf courses. Manure contains high levels of nitrogen and phosphorus and causes the same ecological problems as artificial fertilizers. Farm animals in the U.S. produce huge quantities of manure -- about 130 times more feces than the country's human population (see References 1). Even if fertilizers are not used near a body of water, rainfall eventually washes them into streams and rivers.

Chemistry of Fertilizers

Fertilizers contain three primary plant nutrients: mostly nitrogen and phosphorous with smaller amounts of potassium. The first two are the most important as pollutants. Nitrogen fertilizers containing nitrates can contaminate groundwater because nitrates are highly soluble in water. At high levels, nitrates can poison humans, particularly children. Lower levels of nitrates can also be deadly to amphibians. Certain forms of nitrogen are released as gases, contributing to acid rain and climate change. Rainwater runoff brings fertilizers into streams, rivers, lakes and oceans.

Effects of Eutrophication

Not all organisms in a lake, pond or coastal delta are equally prepared to take advantage of the new nutrients. Fertilizer pollution does not make fish grow bigger, for example. The biggest change occurs in algae populations, which flourish. Algal blooms cover the surface of the water so sunlight does not penetrate as far down as it typically would, reducing the ability of underwater plants to perform photosynthesis and produce oxygen. Dying algae feed microorganisms, which deplete more oxygen. Fish species such as trout and salmon die when dissolved oxygen is too low, while other species like carp often take over.

Dead Zones

In coastal ecosystems, huge dead zones can occur where algal blooms deplete oxygen. More than 400 of these areas exist, totaling more than 94,000 square miles of ocean. One such dead zone in the Gulf of Mexico is roughly the size of New Jersey, and it continues to grow. Dead zones also can form at the mouths of polluted rivers. (See References 1)

Harmful Algal Blooms

A few algae species produce toxins that can kill fish, birds, and mammals and cause health problems for humans. Other algae species are not exactly toxic, but they cause bad smells or ruin the taste of fish caught in the same waters (see References 2). Humans are most at risk when they eat contaminated seafood.