Skip booklet index and go to page content

Threats to Sources of Drinking Water and Aquatic Ecosystem Health in Canada

6. Nutrients—Nitrogen and Phosphorus

Available nitrogen and phosphorus have increased dramatically in the past few decades, with widespread environmental consequences.P.A. Chambers,1 M. Guy,2 E. Roberts,3 M.N. Charlton,1 R. Kent,3 C. Gagnon,4 G. Grove,2 N. Foster,5 C. DeKimpe6 and M. Giddings7

1Environment Canada, National Water Research Institute, Burlington, ON
2Environment Canada, National Water Research Institute, Saskatoon, SK
3Environment Canada, Guidelines & Standards Division, Ottawa, ON
4Environment Canada, Centre Saint-Laurent, Montreal, QC
5Natural Resources Canada, Great Lakes Forestry Research Centre, Sault Ste. Marie, ON
6Agriculture and Agri-Food Canada, Research Branch, Ottawa, ON
7Health Canada, Water Quality Program, Ottawa, ON


Current Status

Nitrogen (N) and phosphorus (P) are natural resources for which there is intense competition in terrestrial and aquatic ecosystems not greatly affected by human activity. Until recently, the supply of N and P for most plants, and ultimately to animals, was limited. The most abundant N source, N gas, could only be used by plants once it was fixed by certain bacteria or algae into ammonium or nitrate compounds. Similarly, the most abundant P source, P-bearing minerals, only became available by weathering. Consequently, N or P was a limiting nutrient in most ecosystems prior to human settlement and agricultural development. Moreover, because N and P were in high demand, these nutrients were stored and recycled in close proximity to the locale from which they were scavenged. This pattern was true for plants and animals, including humans because, prior to urbanization, feces from livestock and humans along with other composted waste was returned to the soil, thereby closing nutrient recycling loops and maintaining the fertility of the soil.

The amount of N and P available for plant uptake has increased dramatically in the past several decades. The causes are a massive increase in the use of fertilizer, burning of fossil fuels, development of large urban populations, and an upsurge in land clearing and deforestation. The amount of available N has more than doubled since the 1940s, with human activities contributing 210 million tonnes per year to the global supply of N, compared to only 140 million tonnes generated per year by natural processes (Vitousek et al. 1997). Similarly, natural weathering of phosphate- bearing rocks is now overshadowed by mining activities as a source of P, with approximately 140 million tonnes of phosphate- bearing rock now mined each year (Steen 1998).

This influx of nutrients has disrupted the natural cycles of both N and P. Where animal manure and human wastes were historically spread on farmland to recycle nutrients, a "once-through" system now predominates. Thus, phosphates extracted from mined phosphate rock and inorganic N fixed from N gas by industrial processes are applied to agricultural land or fed to livestock.

Nutrients in the form of foodstuffs flow from the farm to the cities, where most ultimately end up in landfill (sewage sludge, incinerator ash), or in surface or ground waters (Caraco 1993; Nixon et al. 1996). Reactive N and P released to the atmosphere as a result of agricultural practices, as industrial emissions and, in the case of N, as by-products of home heating and automobile engine combustion may be transported and deposited hundreds or thousands of kilometres from their origin.

The environmental consequences resulting from addition of bioavailable N and P to the Earth's ecosystems could be profound. Based upon our review of available scientific evidence, we are certain that N and P loading from human activity has:

  • Accelerated eutrophication of certain rivers, lakes and wetlands in Canada, resulting in loss of habitat, changes in biodiversity and, in some cases, loss of recreational potential.
  • Increased the frequency and spatial extent to which the drinking water guideline for nitrate has been exceeded in groundwaters across Canada and caused economic burden for those Canadians who must transport household water from off-site sources.
  • Caused and continues to cause fish kills in southwestern Ontario due to ammonia toxicity.
  • Contributed to the decline in amphibians in southern Ontario due to long-term exposure to elevated nitrate concentrations.
  • Led to elevated risks to human health through increased frequency and spatial extent of toxic algal blooms in Canadian lakes and coastal waters.
  • Contributed to quality of life concerns for Canadians through water use impairments (e.g., excessive algal and aquatic weed growth); aesthetic (taste and odour) concerns related to water supplies; and contamination of water supplies (e.g., by nitrate and by trihalomethanes [THMs] produced as by-products of disinfection of water containing organic material).
  • Increased the economic burden to Canadians as a result of the need for treatment, monitoring and remediation of contaminated water.


At present, environmental problems caused by excessive nutrients are less severe in Canada than in countries with a longer history of settlement and agricultural production. This trend is due to our relatively small population compared to our land base and the protective measures implemented by both the federal and provincial/ territorial governments in the last 30 years. However, while successes have been realized, environmental and human health problems related to nutrients are evident across Canada.

Household sewage is the largest point source of N and P to the Canadian environment and will likely continue to be so. In 1996, an estimated 5.6 thousand tonnes of total P and 80 thousand tonnes of total N were released to lakes, rivers and coastal waters from municipal wastewater treatment plants in Canada (Table 1). This load occurred despite the fact that, in 1996, 73% of Canadians were served by municipal sewer systems and at least 94% of the wastewater collected by sewers received primary or higher treatment. Most of the N and P in household sewage are from human waste (urine and feces). In addition to household sewage collected in sewers, septic sewage systems, urban runoff and combined sewer overflows are also major sources of nutrients to ground and surface waters. There are no national figures for losses due to leaching from municipal landfills.

Table 1. Comparison of P and N loading to Canadian surface and ground waters from various sources, 1996
Nutrient SourceNitrogen
(103 t/yr)
(103 t/yr)
Municipal wastewater treatment plants80.35.6
Municipal Sewers11.82.3
Septic systems15.41.9
(residual in the field after crop harvest)
Atmospheric deposition to water182
(NO3- and NH4+ only)

1Industrial N loads are based on NO3 + NH3 and are thus DIN not TN; industrial loads do not include New Brunswick, Prince Edward Island or Nova Scotia. Quebec data are only for industries discharging to the St. Lawrence River.

2Agricultural residual is the difference between the amount of N or P available to the growing crop and the amount removed in the harvested crop; data are not available as to the portion of this residual that moves to surface or ground waters.

Discharge of industrial wastewater is another major source of N and P to the environment. Improvements to process technologies during recent years have resulted in reductions in nutrient loading to the environment from certain industrial sectors. However, not all industries are monitored in all provinces or territories, thereby making it impossible to obtain accurate estimates of industrial N and P loading to the environment. There are also no national figures for losses due to leaching from industrial landfills.

Agricultural activities are the largest non-point source of nutrient loading to the environment. In 1996, approximately 55 thousand tonnes of P and 293 thousand tonnes of N remained in the field after crop harvest (Table 1). Although there is no national information on how much of this residual P and N moves to surface or ground waters, a recent assessment of N losses from agricultural land where the soils have a water surplus predicted that 17% of Ontario, 6% of Quebec and 3% of Atlantic farmland would produce runoff or seepage water with >14 mg N/L (Macdonald 2000). In British Columbia, 5% of the agricultural land has a water surplus and 69% of this area was predicted to generate runoff or seepage water with N concentrations >14 mg/L. Given projected increases in intensive livestock operations and crops with high nutrient demands, nutrient losses to surface and ground water as a result of agricultural activities are likely to increase in future.

Aquaculture is a small but growing source of nutrients to Canadian waters (Table 1). Nutrient release from fish production systems results from the excretion of dissolved or solid waste and from unconsumed feed. Although aquacultural losses represent a comparatively small quantity on a national scale, they can be a substantial input to the small bays where aquaculture is typically practiced.

Forest management practices that disrupt the cycle of nutrients between the soil and trees (e.g., timber harvest, site preparation and slashburning, and fertilization) may increase stream water concentrations of N and, to a lesser extent, P. However, because the effects have been studied at relatively few sites in Canada, changes in nutrient loading caused by forest management practices cannot be described for most of the country.

Knowledge and Program Needs

This review has demonstrated the national scope of nutrient-related impacts on aquatic ecosystems in Canada. There is clear evidence that nutrients released to the environment from human activity are impairing the health of certain ecosystems, contributing to quality of life concerns for Canadians and, on occasion, endangering human health.

Although we have documented deleterious changes in Canadian ecosystems as a result of nutrient loading and the impacts of these changes on the quality of life of Canadians, our ability to assess ecosystem change was constrained by data limitations. These limitations could largely be divided into two categories: insufficient knowledge as to the effects of nutrient additions to ecosystem and human health, and insufficient monitoring data of emissions and discharges and ambient conditions.

Insufficient knowledge of the effects of nutrient addition on ecosystem and human health

Nutrient management is a persisting environmental issue unlike others, such as toxic chemicals that can be eliminated by reformulation or discontinuance. Additional research is required to understand the effects of added nutrients on Canadian ecosystems. Areas requiring particular attention are:

  • The role of nutrients in inducing blue-green algal blooms and toxin production.
  • The role of nutrients in causing taste and odour problems in drinking water supplies.
  • Interactions of nutrients with organic contaminants and their effects on aquatic food webs.
  • Effects of sewage and industrial wastewater plumes on aquatic life during periods of ice cover (i.e., limited mixing of the plume and cold water temperatures).
  • Fate and transport of nutrients within different ecosystems (wetlands, coastal waters, rivers, and lakes) and effects on biota.
  • Effects of long term (decades) of nutrient loading on aquatic and terrestrial ecosystems, including water and sediment/soil quality and food webs.
  • Effects of forest management practices and agricultural activities on nutrient loss and transport to aquatic ecosystems and groundwater.
  • Cumulative effects on the aquatic environment from the combination of several nutrient sources all operating within a region.
  • The relationship between nutrient concentrations and aquatic plant biomass, particularly for streams and coastal waters, and the level of aquatic plant biomass that begins to impair beneficial uses of streams.

Insufficient monitoring data

Although every attempt was made to define the status of Canadian ecosystems with respect to nutrients, data on sources and impacts became progressively less available as one moved from lakes to rivers/streams to wetlands to groundwater to coastal waters to forests. Topics requiring particular attention are:

  • Industrial loading to surface waters. At present, the availability of N and P data for industries not connected to municipal wastewater treatment plants is erratic: monitoring and reporting requirements vary among provinces and territories, and among industrial sectors. Of the 2130 industries in Canada with discharge permits, we obtained data on nutrient loading for only 91 for nitrate, 142 for ammonium, and 191 for total P. Moreover, the data are not stored in any single database.
  • Municipal wastewater treatment plant loading to surface waters. At present, data on N and P loading are available for certain municipal wastewater treatment plants in Canada but the data are not consistent in parameters measured or frequency of sampling. In addition, the data are not stored in any single database. Our analysis of nutrient loading from municipal wastewater treatment plants was achieved by applying per capita nutrient loading coefficients to the population served by the various levels of sewage treatment.
  • Agricultural loading to surface and ground waters. Although studies have been conducted at the scale of plots, fields or small watersheds, regional or national estimates of nutrient loading to surface and ground water could not be calculated.
  • Atmospheric deposition of P and total N. Although estimates of atmospheric deposition of dissolved inorganic N are available through a network of provincial and federal monitoring sites, data are not available for total N or total P nor are estimates available for release from various sectors.
  • Groundwater quality. Water well survey programs are patchy across the country. Some wells are already above or close to guidelines for nitrate. Little information is available on ammonia and P in groundwater.
  • Fish kills from accidental spills/discharges of nutrient-related compounds. Currently, reporting is on a voluntary basis.

This review has clearly documented symptoms of environmental degradation from anthropogenic nutrient loading in Canada. However, science-based solutions are available that can assist in further reducing nutrient losses and, in turn, improving environmental quality. New technologies are emerging that can minimize nutrient loading to the environment. Options for reducing nutrient loading are available, particularly from countries with a long history of nutrient problems. A multi-pronged approach is needed to ensure that protection of water quality from the discharge of nutrients and should include:

  • Development of watershed management plans for specific watersheds where lakes, rivers or estuaries are already eutrophic due to human activity or are, as yet, undeveloped and sensitive to nutrient enrichment.
  • Development of nutrient guidelines for the protection of aquatic life for different types of water bodies (streams, lakes, coastal waters and wetlands) and for different ecoregions in Canada.
  • Improved monitoring of industrial, municipal wastewater and agricultural nutrient loading to surface and ground waters.
  • The development of nutrient management plans or codes of practices to reduce nutrient loading from specific sectors which have a broad geographic coverage and which in general need better nutrient management (e.g., municipal wastewater treatment plants, industries, agricultural activities, aquaculture operations).
  • Research on environmental indicators, technologies to recover and recycle nutrients, management practices (technology- based and environmental) that minimize nutrient losses.
  • Expanded public education on the prevention of water contamination by nutrients.

This range of different instruments is needed if we are to counter nutrient impacts on surface and ground waters and give effect to the principles of economic and environmental sustainability.


  • Caraco, N.F. 1993. Disturbance of the phosphorus cycle: a case of indirect effects of human activity. Trends Ecol. Evolut. 7: 51-54.
  • MacDonald, K.B. 2000. Risk of water contamination by nitrogen.In T. MacRae, C.A.S. Smith and L.J. Gregorich (ed.), Environmental sustainability of Canadian agriculture: report of the agri-environmental indicator project. Research Branch, Policy Branch, Prairie Farm Rehabilitation Administration, Agriculture and Agri-Food Canada.
  • Nixon S.W., J.W. Ammerman, L.P. Atkinson, V.M. Berounsky, G.B. Billen, W.C. Boichourt, W.R. Boynton, T.M. Church, D.M. Di'Toro, R. Elmgren, J.H. Garber, A.E. Giblin, R.A. Jahnke, N.J.P. Owens, M.E.Q. Pilson and S.P. Seitzinger. 1996. The fate of nitrogen and phosphorus and the land-sea margin of the North Atlantic Ocean. Biogeochemistry 35: 141-180.
  • Steen, I. 1998. Phosphorus availability in the 21st century: management of a non-renewable resource. Phosphorus Potassium 217: 25-31.
  • Vitousek, P.M., J.D. Aber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.H. Schlesinger and D.G. Tilman. 1997. Human alteration of the global nitrogen cycle: sources and consequences. Ecol. Appl. 7: 737-750.