Phosphorus Levels in the St. Lawrence River

Phosphorus levels at the majority of water quality monitoring stations along the St. Lawrence River are above water quality guidelines for 2008 to 2011. Higher phosphorus levels are found in agricultural areas on the south shore of Lake Saint-Pierre.

Phosphorus levels in the St. Lawrence River for 2008-2011

Note: The status of total phosphorus at water quality monitoring stations was determined by comparing water quality monitoring data to Quebec’s phosphorus water quality guideline of 0.03 mg P/L.^[1]
Source: St. Lawrence River Water Quality Monitoring and Surveillance Division (2012) Environment Canada.

Phosphorus levels in the St. Lawrence River increase as the water flows from Montreal to Quebec City. Just downstream of Montreal, phosphorus levels are higher than the water quality guideline because of the release of municipal wastewater into the river. Further down the river, rivers draining agricultural regions bring more phosphorus with them from chemical fertilizers and manure used to help crops grow. The levels drop again as the river gets larger at Quebec City because water with lower phosphorus levels from rivers draining the north shore of the river is added to the river. These tributary rivers have lower phosphorus levels because they drain is a more forested area than the land on the south shore of the river. Over the last 15 years, phosphorus levels in the St. Lawrence River at Quebec City have remained stable.

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 lakes and freshwater reaches, a long estuary and a gulf with marine features. It is home to many different habitats as well as a diverse collection of plants, fish and animals.

Phosphorus is a key nutrient for plant growth in lakes and rivers; but, too much or too little in the environment is harmful. Phosphorus from human activity enters the St. Lawrence River in municipal and industrial wastewaters, agricultural runoff and air pollution. Natural sources of phosphorus are rock weathering and the rotting of dead plants and animals. When phosphorus levels in a river become too high, it can result in excessive growth of aquatic plants and cause water quality to get worse.

Related indicators

• Phosphorus Levels in the Great Lakes 
• Canada’s Freshwater Quality by Region
• Freshwater Quality at Monitoring Stations across Canada

Other information

• Information for this indicator comes from The State of the St. Lawrence Monitoring Program which was created under the St. Lawrence Action Plan 2011-2026. The monitoring program brings together six 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.
• Phosphorus at the Mouths of Lake Saint-Pierre Tributaries
• Phosphorus in Canada's Aquatic Ecosystems

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[1] Ministère du Développement durable, Environnement et Parcs (2009) Critères de qualité de l’eau de surface. Retrieved on 27 September, 2011.