Nutrients in the St. Lawrence River

Key results

• Over the 2023 to 2025 period,
  • Phosphorus and nitrogen concentrations were above the guidelines at most monitoring stations
  • For nitrogen, 9 stations had more than 50% of their samples above the guideline, 2 stations had 10% to 50%, and 1 station had less than 10%
  • For phosphorus, 8 stations had more than 50% of their samples above the guideline, 3 stations had 10% to 50%, and 1 station had less than 10% 

Status of total phosphorus and total nitrogen concentration exceedances for the 2023 to 2025 period in the St. Lawrence River and its major tributaries, Canada

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How this indicator was calculated

Note: For the purposes of this indicator, the guidelines are 0.03 milligrams of phosphorus per litre (mg/L of phosphorus) and 0.64 milligrams of nitrogen per litre (mg/L of nitrogen). For more details about the water quality guidelines and categories, please refer to the  Data sources and methods section.
Source: Environment and Climate Change Canada (2026).

Phosphorus and nitrogen levels in the St. Lawrence River are affected by the river’s flow and a variety of human activities along the river. The Île Sainte-Thérèse and Lavaltrie monitoring stations are located downstream from Montréal, Longueuil and Laval (for the Lavaltrie station only) wastewater treatment plants. Elevated levels of phosphorus and nitrogen at these stations may be linked to wastewater treatment processes that do not fully remove these nutrients.

Farther downstream, tributary rivers draining agricultural regions transport higher concentrations of phosphorus and nitrogen because of the chemical fertilizers and manure used to grow crops. These high nutrient levels along the St. Lawrence River can contribute to harmful algal blooms.^{Footnote 1}

Water from tributaries draining the north shore of the St. Lawrence River, such as the Carillon station, generally have lower phosphorus and nitrogen levels because these tributaries are more heavily forested than those on the south shore. Additionally, total phosphorus and total nitrogen concentrations will often exceed the guidelines during periods of high flow, a situation that is mainly observed during spring snowmelt.

Six (6) stations have been collecting water quality data since 2010: Wolfe Island, Carillon, Lavaltrie, Richelieu, Saint-François and Québec. This section presents the annual phosphorus and nitrogen concentrations at these stations from 2010 to 2025.

Key results

• Over the 2010 to 2025 period,
  • the majority of phosphorus concentration observations were below the guideline at Saint-François, Carillon and Wolfe Island stations, and above the guideline at Richelieu, Québec and Lavaltrie stations
  • the majority of nitrogen concentration observations were below the guideline at Québec, Carillon and Wolfe Island stations, and above the guideline at Richelieu, Lavaltrie and Saint-François stations

Annual phosphorus concentrations of samples for 6 water quality monitoring stations along the St. Lawrence River, 2010 to 2025

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How this indicator was calculated

Note: This figure summarizes annual phosphorus concentrations recorded at a monitoring station per year from 2010 to 2025. Each point is a sample phosphorus concentration (mg/L of phosphorus) recorded for a given year. For more details about the water quality guidelines and categories, please refer to the Data sources and methods section. Samples from the mouth of the Saint-François River from 2010 to 2020 were collected from May to September only.
Source: Environment and Climate Change Canada (2026).

Of the 6 long-term monitoring stations, Wolfe Island and Carillon had the lowest frequency of guideline exceedances, with Wolfe Island recording no phosphorus exceedances from 2020 to 2025. Lavaltrie consistently recorded the most exceedances per year, often with all samples exceeding the phosphorus guideline. Stations located between Montréal and Québec City (Lavaltrie, Richelieu and Saint-François) consistently showed a greater proportion of phosphorus observations above the guideline.

Annual nitrogen concentrations of samples for 6 water quality monitoring stations along the St. Lawrence River, 2010 to 2025

Download data file (Excel/CSV; 2.70 KB)
How this indicator was calculated

Note: This figure summarizes annual nitrogen concentrations recorded at a monitoring station per year from 2010 to 2025. Each point represents nitrogen concentration sample (mg/L of nitrogen) collected in a given year. For more details about the water quality guidelines and categories, please refer to the Data sources and methods section. Samples from the mouth of the Saint-François River from 2010-2020 were collected from May to September only.
Source: Environment and Climate Change Canada (2026).

Nitrogen concentrations varied among stations throughout the monitoring period, and no stations consistently remained below the guideline. The Carillon station had the lowest frequency of guideline exceedances, whereas Saint-François and Lavaltrie stations had the highest. Similar to phosphorus, stations located between Montréal and Québec City (Lavaltrie, Richelieu and Saint-François) consistently showed a greater proportion of nitrogen observations above the guideline.

About the indicators

What the indicator measures

This indicator reports on phosphorus and nitrogen concentrations in the St. Lawrence River and its major tributaries. It measures how often phosphorus and nitrogen concentrations exceed their respective water quality guidelines.

This indicator assumes that water in the St. Lawrence River and its major tributaries would rarely exceed water quality guidelines for phosphorus and nitrogen in the absence of human development. It provides information about how human activity contributes to phosphorus and nitrogen concentrations in the river. The more often the water quality guidelines are exceeded, the greater the risk to the health of the St. Lawrence River.

Why this indicator is important

Clean freshwater is an essential resource. It protects aquatic plant and animal biodiversity. We use it for manufacturing, energy production, irrigation, swimming, boating, fishing and domestic use (for example, drinking and washing). Degraded water quality damages the health of all freshwater ecosystems, such as rivers, lakes, reservoirs and wetlands. It can also disrupt fisheries, tourism and agriculture and make it more expensive to treat to drinking water standards.

Phosphorus and nitrogen are important nutrients that support aquatic life. However, when phosphorus and nitrogen concentrations in water become too high, aquatic plant growth can become excessive and harmful. The decay of excess plant material can reduce the amount of oxygen available for fish and other aquatic animals. High nutrient concentrations can also lead to harmful algal blooms, which can kill animals that use the water and affect human health. Conversely, too little phosphorus or nitrogen can result in not enough plant growth to support a river's food web, which could reduce fish populations and harm local fisheries.

Human activities contribute phosphorus and nitrogen to the St. Lawrence River through sources such as agricultural runoff, urban runoff, wastewater discharges, erosion and air pollution. Over time, excess nutrients can alter the river's ecological condition and affect biodiversity.

This indicator is used to provide information about the state of the St. Lawrence River. Ongoing tracking of phosphorus and nitrogen concentrations allow governments and citizens to better understand an important aspect of the river’s environmental condition.

Related initiatives

This indicator contributes to the Sustainable Development Goals of the 2030 Agenda for Sustainable Development. They are linked to Goal 6, Clean water and sanitation and Target 6.3, "By 2030, improve water quality by reducing pollution, eliminating dumping and minimizing release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally."

In addition, the indicator is being proposed to contribute towards reporting on Target 7 of the draft Canada’s 2030 Nature Strategy: "Pollution and Biodiversity". This target is related to Target 7 of the Kunming-Montreal Global Biodiversity Framework: "Reduce pollution risks and the negative impact of pollution from all sources, by 2030, to levels that are not harmful to biodiversity and ecosystem functions and services, considering cumulative effects, including: reducing excess nutrients lost to the environment by at least half including through more efficient nutrient cycling and use; reducing the overall risk from pesticides and highly hazardous chemicals by at least half including through integrated pest management, based on science, taking into account food security and livelihoods; and also preventing, reducing, and working towards eliminating plastic pollution."

The indicator also supports science priorities in the National Freshwater Science Agenda Section 4: Land use stressors and water pollution. These priorities include evaluating beneficial management practices and mitigation strategies to help avoid or mitigate the impacts of land use activities that impact water quality, improving understanding of multiple stressors and cumulative effects of land use on freshwater ecosystems, and assessing the impacts of changes in land cover and vegetation to water quality. The indicator also supports Section 4 knowledge outcomes and science tools by contributing to broadened and accessible data, strengthening data systems and informing targeted water quality monitoring.

Related indicators

The Water quality in Canadian rivers indicators provide a measure of the ability of river water across Canada to support plants and animals.

The Phosphorus levels in the offshore waters of the Canadian Great Lakes and the Nutrients in Lake Winnipeg indicators report on the status of total phosphorus and total nitrogen concentrations in these 2 ecosystems.

The Phosphorus loading to Lake Erie indicators report on the total phosphorus loadings flowing directly into Lake Erie or from its tributary rivers.

The Municipal wastewater treatment indicators measure the level of wastewater treatment provided to the Canadian population.

Data sources and methods

Data sources

Total phosphorus and total nitrogen data were provided by Environment and Climate Change Canada's Fresh Water Quality Monitoring and Surveillance program. The data can be found on the Saint Lawrence River basin long-term water quality monitoring data and the Great Lakes connecting channels monitoring and surveillance data Open Data web pages.

 Methods

The status of phosphorus and nitrogen concentrations at each monitoring station was determined by the percentage of samples that exceeded their water quality guidelines.

Recent changes

Four (4) stations were retired in 2022 and 2023: Bécancour, Cornwall, Île d’Orléans and Îles-de-Boucherville. Two (2) stations were added in 2024: Contrecœur and Yamachiche River. Observations at Richelieu station restarted in April 2024.

In the previous version of this indicator, percentage of exceedances were reported as a status (Good, Fair and Poor) for each station. The indicator now reports only the percentage of observations exceeding the guidelines.

Trend analysis was previously included in the indicator for stations with more than 10 years of data. Trend analysis was not included in the current version of this indicator because data gaps occurring during COVID-19 limit interpretation.

The nitrogen guideline was updated from 0.65 to 0.64 mg/L of nitrogen using data available as of 2026.

Caveats and limitations

The indicators reflect the state of water quality in the St. Lawrence River based on total phosphorus and total nitrogen concentrations. These concentrations do not reflect the effect of spills or other transient events unless they are frequent or long-lasting.

Caution must be used when comparing these indicators with similar indicators for lakes. In rivers, total phosphorus concentrations are influenced by suspended particles in the water that increase during high-flow events. Elevated total nitrogen concentrations result from high runoff associated with precipitation, which washes nitrogen out of soils. This situation differs in lake ecosystems, as suspended particles generally settle out. Comparisons between lake and river systems may still be useful, provided the methods used to classify water quality are clearly understood.

Resources

References

Canadian Council of Ministers of the Environment (2016) Guidance Manual For Developing Nutrient Guidelines for Rivers and Streams (PDF; 1.95 MB). Retrieved on June 19, 2026.

Chambers PA, Guy M, Dixit SS, Benoy GA, Brua RB, Culp JM, McGoldrick D, Upsdell BL and Vis C (2009) Nitrogen and Phosphorus Standards to Protect the Ecological Condition of Canadian Streams, Rivers and Coastal Waters. National Agri-Environmental Standards Initiative Synthesis Report No. 11. Environment Canada. Gatineau, Quebec. 79 p.

Government of Canada (2008) Technical guidance document for Water Quality Index practitioners reporting under the Canadian Environmental Sustainability Indicators (CESI) initiative 2008. Environment and Climate Change Canada and Statistics Canada. Retrieved on June 19, 2026.

Goyette GO, Bennet EM, Howarth RW and Maranger R. (2016) Changes in anthropogenic nitrogen and phosphorus inputs to the St. Lawrence sub-basin over 110 years and impacts on riverine export. Global Biogeochemical Cycles 30(7): 1000-1014. Retrieved on June 19, 2026.

Hudon C, Gagnon P, Rondeau M, Hébert S, Gilbert D, Hill B, Patoine M and Starr M (2017) Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada). Biogeochemistry 135:251 to 276. Retrieved on June 19, 2026.

Ministère du Développement durable, Environnement et Lutte contre les changements climatiques (2009) Critères de qualité de l'eau de surface : phosphore total (en P) (in French only). Retrieved on June 19, 2026.
Ontario Ministry of the Environment and Energy (1994) Water management policies, guidelines, provincial water quality objectives. Retrieved on June 19, 2026.

United States Environmental Protection Agency (2000a) Nutrient Criteria Development Document: Rivers and Streams. Report No. EPA-822-B-00-002. Retrieved on June 19, 2026.

United States Environmental Protection Agency (2000b) Ecoregional Nutrient Criteria Documents for Rivers and Streams in Nutrient Ecoregion VII: Mostly Glaciated Dairy Region (PDF; 331 kB). Report No. EPA-822-B-00-018. Retrieved on June 19, 2026.

United States Environmental Protection Agency (2001) Ecoregional Nutrient Criteria Documents for Rivers and Streams in Nutrient Ecoregion VIII: Nutrient-Poor, Largely Glaciated Upper Midwest and Northeast (PDF; 2.53 MB). Report No. EPA-822-B-01-015. Retrieved on June 19, 2026.

Related information

Environment and Climate Change Canada (2017) Phosphorus in aquatic ecosystems. Retrieved on June 19, 2026.

Governments of Canada and Quebec (2015) St. Lawrence Action Plan 2011-2026. Retrieved on June 19, 2026.

Environment and Climate Change Canada (2017) St. Lawrence River: phosphorus at the mouths of Lake Saint-Pierre tributaries. Retrieved on June 19, 2026.

Alternative format

Download the alternative format of the Nutrients in the St. Lawrence River indicator (PDF; 960 KB)