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Water: Nutrients

Effects of Nitrogen and Phosphorus Pollution

Photo of algae-filled water along a shoreline.

Source: James B. Hyde

Effects of nitrogen and phosphorus pollution are diverse and far-reaching. Not only does nitrogen and phosphorus pollution affect human health and the environment, but its cascading effects (such as the need for additional treatment for drinking water or increases in health care costs for related illnesses) cost millions of dollars per year, directly impacting the economy. Use the tabs below to learn about the various effects of nitrogen and phosphorus pollution on humans, fish, shellfish, wildlife, and the economy.

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Photo of quarantine sign warning against eating clams and mussels.

Shellfish that ingest harmful algae become poisonous to humans, causing authorities to close some shellfish areas at certain times of the year. Source: NOAA

Human health impacts can result from ingestion of elevated concentrations of toxic compounds containing nitrogen, disinfection byproducts formed when treating water, toxins produced by algae, and toxic shellfish. Nitrogen and phosphorus pollution also poses risks to humans that are not related to human health, such as restrictions on recreation (e.g., boating, swimming, kayaking) or increased costs (e.g., increased drinking water treatment, economic losses from tourism). To learn more about the financial impacts of nitrogen and phosphorus pollution, visit the economy tab, which is accessible from this page.

Human Health Impacts from Nitrate and Nitrite

Elevated nitrate and nitrite (chemical compounds that contain nitrogen) levels in drinking waters have been associated with harm to infants, adverse pregnancy outcomes, and possibly cancer.  Methemoglobinemia results when high levels of nitrate or nitrite reduce the blood's ability to deliver oxygen to the skin and organs. The low delivery of oxygen during methemoglobinemia causes the skin to acquire a blue tint, and in severe cases can cause coma and death.1 Methemoglobinemia can also occur in adults, but requires much higher dosages and is typically caused by accidentally ingesting food contaminated with sodium nitrite (mistaken for table salt), rather than from drinking water.2

Although evidence of a direct exposure-response relationship between nitrate in drinking water and adverse reproductive effects is unclear, some reports suggest an association between exposure to nitrates and spontaneous abortions, intrauterine growth restriction, and various birth defects. More research is needed to definitively link the two.3 Numerous studies have investigated linkages between nitrates/nitrites in drinking water and some cancers because nitrates and nitrites are precursors to formation of N-nitroso compounds (carcinogens, which are substances that cause or aggravate cancer).4, 5, 6, 7, 8

Human Health Impacts from Disinfection Byproducts

Human health can also be impacted by disinfection byproducts that are formed when disinfectants (e.g., chlorine compounds or ozone) used to treat drinking water react with organic carbon from the algae in source waters. One study has shown that algal production increases the formation of trihalomethanes, a carcinogenic byproduct.9 Disinfection byproducts that may be associated with high organic carbon levels resulting from nitrogen and phosphorus pollution include trihalomethanes, haloacetic acids, and N-Nitrosodimethylamine (NDMA).

According to many studies, chlorine and disinfection byproducts can potentially lead to bladder, rectal, and colon cancers; reproductive and developmental health risks (e.g., low birth weight, small for gestational age; decreased fetal viability, fetal malformations); and liver, kidney, and central nervous system problems.10, 11 Both the nitrogen and phosphorus pollution level in the untreated water and the type of treatment determine the human exposure to disinfection byproducts. Because public drinking water systems are required to maintain disinfection byproduct concentrations within levels set to protect human health, drinking water plants impacted by nitrogen and phosphorus pollution may be required to undertake renovations to maintain safe drinking water.

Human Health Impacts from Algal Toxins

A group of algae called cyanobacteria (formerly known as blue-green algae) produces compounds that are toxic to humans (as well as pets and livestock).12 Nitrogen and phosphorus can contribute to large blooms of cyanobacteria whose toxins may be ingested by humans during recreation or in treated drinking water. Some drinking water treatment processes may be effective for removing algal toxins from water, though not all drinking water plants employ those processes. In the event of a large algal bloom, treatment processes may not eliminate all of the toxins.13, 14

In a recent study, one type of cyanobacteria was classified as a potential carcinogen (it is currently on the EPA's "Candidate Contaminant List" to be researched further).15

Human Health Impacts from Toxic Shellfish

Humans do not necessarily need to ingest polluted water to by affected by nitrogen and phosphorus pollution. Ingestion of seafood that is tainted with toxins from harmful algae can cause gastrointestinal distress, which is occasionally followed by memory loss, disorientation, confusion, and even coma and death in extreme cases. There is no cure for these toxic poisonings and medical personnel can only treat the symptoms.16 Illness or death might result from even small doses of these toxins. For example, several documented harmful algal bloom (HAB) events have produced enough toxins that consumption of only one or two mussels from impacted waters could kill a healthy adult.17 Domoic acid, which is produced by marine diatoms and bioaccumulates in fish and shellfish, is a well documented toxin that has killed people who ate seafood contaminated with this toxin.16, 18 In response to increased illnesses thought to be caused by HABs, the CDC's National Center for Environmental Health (NCEH) has developed a Harmful Algal Bloom-related Illness Surveillance System (HABISS) to support public health decision making.

Recreational Impacts

The presence of toxins in waterbodies can restrict recreation when water quality is impacted. Algal blooms can prevent or reduce advisability of swimming and other recreational activities, such as boating or kayaking and impair the aesthetic quality of surface waters. In areas where recreation is determined to be unsafe because of algal blooms, signs are often posted as warnings.

Fish, Shellfish, and Wildlife

Photo of dead dungeness crabs in a tide pool.

Piles of dead crabs washed ashore in Oregon after oxygen levels dipped so low that the seawater became lethal to bottom-dwelling sea life. Source: Elizabeth Gates

Changes in the environment resulting from elevated nitrogen and phosphorus levels (e.g., algal blooms, toxins from harmful algal blooms (HABs), and hypoxia/anoxia) can cause a variety of effects to fish, shellfish and wildlife populations. When excessive nitrogen and phosphorus loads change a water body's algae and plant species, this can result in altered habitat and quality of available food resources, and subsequently affect the entire food chain. Algal blooms can also increase the turbidity and impair the ability of fish and other aquatic life to find food.19 Algae can also damage or clog the gills of fish and invertebrates.20

Some HABs form toxins that can cause illness or death for some animals. Some of the more commonly affected marine animals include sea lions, turtles, seabirds, dolphins, and manatees.20 More than 50 percent of unusual marine mortality events are thought to be associated with HABs.21

Although lower level consumers, such as small fish or shellfish, may not be affected by algal toxins, higher level consumers eat a large enough quantity of smaller fish (and therefore toxins) for the toxins to build up (bioaccumulate) in the body, resulting in health impairments and possibly death.18, 21 Domoic acid (produced by diatoms), which accumulates in the tissue of mussels, crabs, and fish, causing consumers of these animals to become ill, is one example of an algal toxin.16

There are many examples of HAB toxins significantly affecting marine animals. For example, 19 humpback whales near Georges Bank in the Gulf of Maine died because of domoic acid poisoning.21 In addition, between March and April 2003, 107 bottlenose dolphins (Tursiops truncatus) died, along with hundreds of fish and marine invertebrates along the Florida Panhandle. High levels of brevetoxin, produced by a species of dinoflagellate, were measured in all of the stranded dolphins examined, as well as in their fish prey.18 Finally, scientists report that California sea lions exposed to low doses of domoic acid as a fetus can result in an increase of epileptic seizures and behavioral abnormalities.22

In freshwater, cyanobacteria can produce toxins that have been implicated as the cause of a large number of fish and bird mortalities. These toxins have also been tied to the death of pets and livestock that may be exposed through drinking contaminated water or by licking themselves after bodily exposure.21 In 2004, two dogs died within hours of drinking water from a lake in Nebraska contaminated with high concentrations of cyanobacteria toxin. By the end of the year, there had been five dog deaths, numerous wildlife and livestock deaths, and more than 50 accounts of human illnesses, including skin rashes, lesions, or gastrointestinal illnesses reported at Nebraska lakes.23 Another recent study showed that at least one type of cyanobacteria has been linked to cancer and tumor growth in exposed animals.15

Algal blooms deplete dissolved oxygen levels by blocking sunlight that underwater grasses need to grow or by causing oxygen depletion (hypoxia) as dead algae are decomposed.20, 24 Mobile species, such as adult fish, can sometimes survive hypoxic events by moving to areas with more oxygen.25 However, migration to avoid hypoxia depends on species mobility and availability of suitable habitat. Unfortunately, less mobile or immobile species, such as oysters or mussels, cannot move to waters with life-sustaining levels of oxygen and are often killed during hypoxic events.25 Crab jubilees can occur when crabs find themselves in oxygen-deprived waters. As they become stressed by the low levels of oxygen, they come out of the water in search of oxygen.26

Although aquatic animals can tolerate a range of dissolved oxygen levels, younger life stages of species like fish and shellfish typically require higher levels of oxygen to survive. As fish and shellfish grow, they generally become more tolerant to a wider range of oxygen levels in their environment or can migrate to water with better conditions.

Ultimately, hypoxia causes a severe decrease in the amount of aquatic life in hypoxic zones and affects the ability of aquatic organisms to find necessary food and habitat. In extreme cases, anoxia or anoxic zones occur. In these dead zones, there is no oxygen and only anaerobic bacteria are able to survive.25


Nitrogen and phosphorus pollution and eutrophication can impact the economy through additional costs for medical treatment for humans who ingested HAB toxins, monitoring water for shellfish and other affected resources, upgrading wastewater treatment plants to remove nitrogen and phosphorus, and treating drinking water supplies to remove algae and organic matter.

Economic losses from algal blooms and HABs can include reduced property values for lakefront areas, commercial fishery losses, and lost revenue from fishing and boating related trips, as well as the tourism industry and tourism-related businesses. Commercial fishery losses occur because of a decrease in the amount of fish available for harvesting due to habitat and oxygen declines, some HAB toxins that can make seafood unsafe for human consumption, and an overall reduction in the amount of fish bought because people might question if eating fish is safe after watching the news.27 To put the issue into perspective, consider the following examples of estimated costs resulting from nitrogen and phosphorus pollution:

Photo of a fish market.

Source: NOAA

  • The Chesapeake Bay has collectively invested more than $10 billion to-date, and must invest more than $28 billion more to achieve current water quality standards.28
  • Estimated loss of lake property values from excessive algal growth from nitrogen and phosphorus as high as $2.8 billion annually.29
  • For freshwater lakes, losses in fishing and boating trip-related revenues are estimated at $1.2 billion dollars.29
  • Overall economic costs from HAB outbreaks occurring from 1987 to 2000 were estimated as high as $82 million dollars annually (in 2000 dollars).30
  • Annual commercial fishery losses from HABs are estimated to be as high as $25 million dollars (in 2000 dollars).31
  • Costs for small communities to remove nitrate from drinking water can be significant. For example, in 1991 Des Moines, Iowa (population of approximately 200,000) constructed a $4 million ion exchange facility to remove nitrate from its drinking water supply.32 On average, it costs the city approximately $130,000 every year to remove excess nitrate levels.33
  • From 1995 to 1997, estimates for beach cleanup for Sarasota County in southwest Florida (17.5 miles of beach) averaged $63,000 annually. These costs covered cleaning up dead fish from HAB events and collecting and disposing of red seaweed that washed up during storms.34
  • At least $813 million is spent annually on bottled water due to taste and odor issues in drinking water supplies. Taste and odor can be linked to nutrient pollution that causes algal blooms.29


  1. U.S. Agency for Toxic Substances and Diseases Registry. 2007. "Case Studies in Environmental Medicine: Nitrate/Nitrite Toxicity." (PDF) (40 pp, 285K, About PDF) Accessed March 2011.
  2. USEPA. 1990. "The Drinking Water Criteria Document on Nitrate/Nitrite." TR-1242-60B. U.S. Environmental Protection Agency.
  3. Manassaram, D.M., L.C. Backer, and D.M. Moll. 2006. A review of nitrates in drinking water: maternal exposure and adverse reproductive and developmental outcomes. "Environmental Health Perspectives" 114(3): 320-327.
  4. Kilfoy, B.A., M.H. Ward, T. Zheng, T.R. Holford, P. Boyle, P Zhao, M. Dai, B. Leaderer, and Y. Zhang. 2010. Risk of non-Hodgkin lymphoma and nitrate and nitrite from the diet in Connecticut women. "Cancer Causes Control" 21: 889-896.
  5. Boffetta, P. and F. Nyberg 2003. Contribution of environmental factors to cancer risk. "British Medical Bulletin" 68(1):71-94.
  6. Coss, A., K. P. Cantor, J. S. Reif, C. F. Lynch, and M. H. Ward. 2004. Pancreatic cancer and drinking water and dietary sources of nitrate and nitrite. "American Journal of Epidemiology" 159(7):693-701.
  7. Joossens, J. V., M. J. Hill, P. Elliott, R. Stamler, J. Stamler, E. LeSaffre, A. Dyer, R. Nichols and H. Kesteloot. 1996. Dietary salt, nitrate and stomach cancer mortality in 24 countries. "International Journal of Epidemiology" 25(3):494-504.
  8. Grosse, Y., R. Baan, K. Straif, B. Secretan, F. El Ghissassi and V. Cogliano 2006. Carcinogenicity of nitrate, nitrite, and cyanobacterial peptide toxins. "The Lancet Oncology" 7(8):628-629.
  9. Jack, J., T. Sellers, and P.A. Bukaveckas. 2002. Algal production and trihalomethane formation potential: an experimental assessment and inter-river comparison. "Canadian Journal of Fisheries and Aquatic Sciences." 59: 1482–1491.
  10. USEPA. 2009. "Drinking Water Contaminants." U.S. Environmental Protection Agency. Accessed December 2010.
  11. CFR. 2006. 40 CFR parts 9, 141, and 142: "National Primary Drinking Water Regulations: Stage 2 Disinfectants and Disinfection Byproducts Rule." Code of Federal Regulations, Washington, DC. Accessed December 2010.
  12. Codd, G. A., L. F. Morrison and J. S. Metcalf 2005. Cyanobacterial toxins: risk management for health protection. "Toxicology and Applied Pharmacology" 203(3):264-272.
  13. Hitzfeld, B., S. Hoger and D. Dietrich 2000. Cyanobacterial toxins: removal during drinking water treatment, and human risk assessment. Environmental Health Perspectives 108 Suppl 1:113-22.
  14. Carmichael, W.W. 2000. "Assessment of Blue-Green Algal Toxins in Raw and Finished Drinking Water." AWWA Research Foundation, Denver, CO.
  15. Falconer, I.R., A.R. Humpage. 2005. Health Risk Assessment of Cyanobacterial (Blue-green Algal) Toxins in Drinking Water. "Int. J. Environ. Res. Public Health" 2(1): 43-50.
  16. Bushaw-Newton, K.L. and K.G. Sellner 1999. Harmful Algal Blooms (PDF) (40 pp, 1.5MB, About PDF). In: NOAA's State of the Coast Report. Silver Spring, MD: National Oceanic and Atmospheric Administration. Accessed April 2011.
  17. NOAA. 2009. "Marine Biotoxins." National Oceanic and Atmospheric Administration. Accessed December 2010.
  18. WHOI. 2008. "Marine Mammals." Woods Hole Oceanographic Institution. Accessed December 2010.
  19. Chesapeake Bay Program. 2009. "Underwater Bay Grasses." Accessed December 2010.
  20. NOAA. 2011. "Overview of Harmful Algal Blooms." National Oceanic and Atmospheric Administration. Accessed March 2011.
  21. WHOI. 2008. "HAB Impacts on Wildlife." Woods Hole Oceanographic Institution. Accessed December 2010.
  22. NOAA. 2008. "California Sea Lions Seizures May Come From Fetal Domoic Acid Poisoning." National Oceanic and Atmospheric Administration. Accessed December 2010.
  23. Walker, S.R., J.C. Lund, D.G. Schumacher, P.A. Brakhage, B.C. McManus, J.D. Miller, M.M. Augustine, J.J. Carney, R.S. Holland, K.D. Hoagland, J.C. Holz, T.M. Barrow, D.C. Rundquist, and A.A. Gitelson. 2008. Nebraska Experience. In: Hudnell, H.K. (ed.) Cyanobacterial Harmful Algal Blooms: State of the Science and Research Needs. Advances in Experimental Medicine & Biology. Vol. 619. Springer. 500 pp.
  24. USGS. 2009. "Hypoxia." U.S. Geological Survey. Accessed December 2010.
  25. ESA. 2009. "Hypoxia." (PDF) (4 pp, 714K, About PDF) Ecological Society of America. Accessed December 2010.
  26. NOAA. 2008. "Dissolved Oxygen." National Oceanic and Atmospheric Administration. Accessed December 2010.
  27. WHOI. 2008. "Hearing on 'Harmful Algal Blooms: The Challenges on the Nation's Coastlines'." Woods Hole Oceanographic Institution. Accessed December 2010.
  28. Chesapeake Bay Program. 2009. "Funding and Financing." Accessed December 2010.
  29. Dodds, W.K., W.W. Bouska, J.L. Eitzmann, T.J. Pilger, K.L. Pitts, A.J. Riley, J.T. Schloesser, and D.J. Thornbrugh. 2009. Eutrophication of U.S. freshwaters: analysis of potential economic damages. "Environmental Science and Technology" 43(1): 12–19.
  30. Hoagland P., and S. Scatasta. 2006. The economic effects of harmful algal blooms. In E Graneli and J Turner, eds., Ecology of Harmful Algae. Ecology Studies Series. Dordrecht, The Netherlands: Springer-Verlag, Chap. 29.
  31. Anderson, D.M., P. Hoagland, Y. Kaoru, and A.W. White. 2000. "Estimated Annual Economic Impacts from Harmful Algal Blooms (HABs) in the United States." Technical Report WHOI-2000-11. Woods Hole Oceanographic Institute, Woods Hole, MA.
  32. U.S. Census Bureau. 2000. "Des Moines (city), Iowa." Accessed December 2010.
  33. Jones, C.S., D. Hill, and G. Brand. 2007. Use a multifaceted approach to manage high source-water nitrate. "Opflow" (June): 20–22.
  34. Hoagland, P., D.M. Anderson, Y. Kaoru, and A.W. White. 2002. The economic effects of harmful algal blooms in the United States: estimates, assessment issues, and information needs. "Estuaries" (25): 819-837.

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