Nutrients in the St. Lawrence River

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Phosphorus and nitrogen levels at the majority of water quality monitoring stations along the St. Lawrence River were above water quality guidelines more than 50% of the time during the 2012-2014 period. Higher phosphorus and nitrogen levels were found at stations next to agricultural areas along the south shore of the river between Richelieu and Bécancour.

Compared with the 2010-2012 period, two monitoring stations saw their status in terms of phosphorus levels change:

• The status at Saint-François improved from > 50% of samples above the guideline to 10-50% above
• The status at Carillon deteriorated from < 10% of samples above the guideline to 10-50% above

For nitrogen, a more precise guideline of 0.63 milligrams per litre was established specifically for the St. Lawrence River in 2016. As a result, the water quality at seven stations appears to have changed, even though there were no changes in nitrogen levels over the same period. The change in the guideline value did not impact the nitrogen status at the Saint-Maurice or Yamaska stations.

Only the stations at Carillon and Quebec City have data for phosphorus starting in 2005, which allows for an assessment of trends. Phosphorus levels in water entering the St. Lawrence River at Carillon decreased between 2005 and 2014 and showed no trend at Quebec City over the same period

Status of total phosphorus and total nitrogen levels in the St. Lawrence River, Canada, 2012 to 2014, and total phosphorus level trends at Carillon and Quebec City, 2005 to 2014

How this indicator was calculated

Note: Water quality at a monitoring station is considered green when ambient water quality exceeds the guideline less than 10% of the time. A yellow status is applied when the guideline is exceeded 10-50% of the time. Red status is applied when exceedances occur in over 50% of samples. The status of total phosphorus and total nitrogen at water quality monitoring stations was determined by comparing water quality monitoring data to Quebec's total phosphorus water quality guideline of 0.03 milligrams of phosphorus per litre (mg P/L) ^{Footnote [1]} and total nitrogen water quality guideline of 0.63 milligrams of nitrogen per litre (mg N/L). The nitrogen guideline was changed from 1 mg N/L to 0.63 mg N/L for this update; for more details about the water quality guidelines, please refer to the Data Sources and Methods.
Source: St. Lawrence River Water Quality Monitoring and Surveillance Division (2015) Environment and Climate Change Canada.

The St. Lawrence River links the Great Lakes with the Atlantic Ocean and is among the world's most important commercial waterways. It is a complex ecosystem that includes freshwater lakes and river reaches, a long estuary, and a salt-water gulf. Its many different habitats are home to a diverse range of plants, fish and animals.

Phosphorus and nitrogen levels in the St. Lawrence River are affected by a variety of human activities along the river. Just downstream of Montreal, at Lavaltrie, phosphorus and nitrogen levels exceeded the water quality guidelines because of the release of municipal wastewater into the river. Further along, tributary rivers draining agricultural regions bring more phosphorus and nitrogen with them from the chemical fertilizers and manure used to grow crops. Just upstream of Quebec City, water from tributary rivers draining the north shore have lower phosphorus and nitrogen levels because they run through a more forested area than the land on the south shore of the river. The cleaner water allows phosphorus and nitrogen levels in the water to decline to levels closer to the guidelines. Past Quebec City, the St. Lawrence River flows into the Gulf of the St. Lawrence, where the nitrogen and phosphorus levels contribute to harmful algal blooms.

Phosphorus and nitrogen are key nutrients for plant growth in lakes and rivers. They are both naturally occurring and manufactured by human activity. Natural sources of phosphorus include weathering of rocks and decomposition of plants and animals. Nitrogen is added to the environment by bacteria that convert nitrogen gas in the air into forms plants can use for growth. Phosphorus and nitrogen from human activity enters the St. Lawrence River through municipal and industrial wastewaters, agricultural runoff, and air pollution. Problems arise when too much, or too little, nitrogen and phosphorus enters the environment. When phosphorus and nitrogen levels in water become too high, aquatic plant growth thrives and can cause excessive blooms. The decay of excess plant material can reduce the amount of oxygen available for fish and other aquatic animals. Further, high nutrient levels can also lead to harmful algal blooms, which can kill animals that use the water and affect human health.

Phosphorus levels by water quality monitoring stations

Plotting phosphorus data for each station for each year provides a general view of how phosphorus levels are changing along the St. Lawrence River. Each boxplot summarizes annual phosphorus levels at a monitoring station and shows the range of values measured. The boxes are joined by a solid line (red) to give a sense of the direction of change in phosphorus levels over time.

Trend analysis can be performed for Carillon and Quebec City, where data are available from 2005. The analysis indicates a decreasing trend at Carillon, and no change at Quebec City. While this analysis cannot be performed for other stations, the available data suggest that phosphorus levels are slowly falling at the Saint-François station and are not changing at any of the other stations along the river.

Annual total phosphorus boxplots for nine water quality monitoring stations along the St. Lawrence River

Download data file (Excel/CSV; 3.16 KB)
How this indicator was calculated

Note: The dotted line shows the guideline value of 0.03 mg P/L. The solid (red) line is drawn through the median to give a sense of the changes in concentrations over time. Only the Carillon and Quebec City stations have enough data for a seasonal Mann-Kendall trend analysis for phosphorus.
Source: St. Lawrence River Water Quality Monitoring and Surveillance Division (2015) Environment and Climate Change Canada.

Nitrogen levels by water quality monitoring stations

Plotting nitrogen data for each station for each year provides a general view of how nitrogen levels are changing over time along the St. Lawrence River. Each boxplot below summarizes annual nitrogen levels for a monitoring station and shows the range of values measured. The boxes are joined by a solid line (red) to give a sense of the direction of change in nitrogen levels over time.

None of the stations have enough data to perform a trend analysis. However, the information available suggest nitrogen levels have declined at the Richelieu, Yamaska and Nicolet stations and are not changing at any of the other stations along the river.

Annual total nitrogen boxplots for nine water quality monitoring stations along the St. Lawrence River

Download data file (Excel/CSV; 2.51 KB)
How this indicator was calculated

Note: The dotted line shows the guideline value of 0.63 mg N/L. The solid line (red) is drawn through the median to give a sense of trends in concentration.
Source: St. Lawrence River Water Quality Monitoring and Surveillance Division (2015) Environment and Climate Change Canada.

Related indicators

• Local Freshwater Quality in Canada
• Nutrients in Lake Winnipeg
• Phosphorus Levels in the Great Lakes
• Regional Freshwater Quality in Canadian Rivers

Other information

• Information for this indicator comes from the State of the St. Lawrence Monitoring Program under the St. Lawrence Action Plan 2011-2026. The monitoring program brings together four government partners and one non-governmental organization to provide integrated, scientific information in support of the protection and sustainability of the ecosystem for current and future generations.
• More information about water quality on the St. Lawrence River can be found in the Canadian Environmental Sustainability Indicators mapping application.
• Phosphorus at the Mouths of Lake Saint-Pierre Tributaries.
• Phosphorus in Canada's Aquatic Ecosystems.

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