St. Lawrence River: phosphorus at the mouths of Lake Saint-Pierre tributaries

Phosphorus is used liberally in the manufacture of chemical fertilizers. Through erosion and leaching from farmlands, this chemical substance is being found in aquatic ecosystems at concentrations that lead to the degradation of the ecosystem by eutrophication, a phenomenon caused by excessive contributions of nutrients (phosphorus and nitrogen). Eutrophic conditions are likely to occur in Lake Saint-Pierre, where major tributaries, the Yamaska, Saint-François and Nicolet rivers, drain agricultural regions.

Location of sample stations at the mouths of Yamaska, Saint-François and Nicolet rivers
Location of sample stations at the mouths of Yamaska, Saint-François and Nicolet rivers

Map shows the water quality sampling sites at the mouth of the Yamaska, Nicolet and St. Francois rivers in their each watershed of 4861 square kilometers, 8994 square kilometers and 3626 square kilometers. The land use for agricultural is shown in orange.

The symptoms of eutrophication in an aquatic ecosystem are:

  • proliferation of macroalgae and phytoplankton;
  • reduction in dissolved oxygen concentrations;
  • reduction and disappearance of submergent vegetation;  
  • appearance of harmful or toxic aquatic flowers;
  • contamination or mass mortality of fish and shellfish
Phosphorus concentrations
Milligrams per liter (mg/L) State
less than 0.004 Ultra-oligotrophic
0.004-0.010 Oligotrophic
0.010-0.020 Mesotrophic
0.020-0.035 Meso-eutrophic
0.035-0.100 Eutrophic
more than 0.100 Hypereutrophic

Source: Canadian Council of Ministers of the Environment, 2007.

Did you know? The concentration of phosphorus is an indicator of the trophic status of an ecosystem. An aquatic environment rich in phosphorus is considered to be eutrophic, whereas a phosphorus-poor area is considered oligotrophic.

Median concentration of Phosphorus
Median Concentrations of Phosphorus at the mouths of Lake Saint-Pierre tributaries, 2005-2014

The graph shows median concentrations of phosphorus for the Yamaska River of 0.16 mg / L in 2005, 0.14 mg / L in 2006, 0.1 mg / L in 2007, 0.11 mg / L in 2008, 0.08 in 2009, 0.73 mg / L in 2010, 0.12 mg / L in 2011, 0.14 mg / L in 2012, 0.13 mg / L in 2013, and 0.11 mg / L in 2014. In the St. François River, it is between 0.2 mg / L and 0.4 mg / L from 2005 to 2014. In the Nicolet River, it was 0.03 mg / L in 2005, 0.07 in 2006, 0.02 in 2007, 0.05 in 2008, 0, 04 in 2009, 0.05 in 2010 and 2011, 0.07 in 2012, 0.05 in 2013 and 0.03 in 2014.

Between 2005 and 2014, the median concentrations of phosphorus measured at the mouths of the Saint-François and Nicolet rivers oscillated around values corresponding to a eutrophic environment. However, it was at the mouth of the Yamaska River that the highest concentrations were observed and where conditions were hypereutrophic.

Greenish hue of the Yamaska River caused by the proliferation of cyanobacteria in August 2005

© Nathalie Gratton, Environment Canada.
Greenish hue of the Yamaska River caused by the proliferation of cyanobacteria in August 2005

Phosphorus contributes to phytoplancton bloom such as algae who produce chlorophyll a, a pigment linked to photosynthesis. In 2005, the Yamaska River experienced an abnormally high influx of phosphorus and nitrogen, which resulted in the high proliferation of cyanobacteria, commonly referred to as blue-green algae. Cholorophyll concentrations measured in the Yamaska River in August 2005 reached 257 µg/L, which is 10 times greater than the observed average for the month of August.

Concentration of phosphorus at the mouth of the Yamaska river in 2005
Concentration of phosphorus at the mouth of the Yamaska river in 2005

The graph shows phosphorus concentrations observed at the mouth of the Yamaska River in the summer of 2005. The concentrations are about 0.06 milligrams per liter 18th and 25th May, from 0.07 milligrams per liter on 1st June, from 0.1 to 0.14 milligrams per liter in June and 0.16 to 0.27 milligrams per liter in July and August.

Phosphorus concentrations vary from season to season. The highest concentrations are generally noted in the summer. Episodes of high phosphorus concentrations often correspond to an increase in suspended particulate matter resulting from the erosion of farmland and high river discharge. This is what was observed in the Yamaska River in 2005, where the average flow recorded between July and August, a period corresponding to an increase in phosphorus concentrations, doubled to 65 m3/s compared to a flow of 30 m3/s recorded between May and July.

Did you know? It is estimated that at least 67% of the phosphorus at the mouth of the Yamaska River comes from farmland, 15% from urban effluent, and 10% from wastewater dumped untreated into the aquatic environment. The quantity of phosphorus exported to the mouth of the Yamaska has gone from approximately 646 tonnes per year in 1994 to about 329 tonnes per year in 2001-2003 (Berryman, 2008).

Lake Saint-Pierre receives excessive inputs of phosphorus and suspended particulate matter from the tributaries feeding into it. Downstream of the mouth of the Saint-François and Yamaska rivers, benthic cyanobacteria proliferate (Hudon and Carignan, 2008). The quality of the water in Lake Saint-Pierre is affected in its shallow riparian areas that receive water from the tributaries draining farmlands, especially those with high flows.

Related Link:


Berryman, D. 2008. État de l’écosystème aquatique du bassin versant de la rivière Yamaska : faits saillants 2004-2006. [Status of the Aquatic Ecosystem in the Yamaska River Watershed], Quebec, Ministère du Développement durable, de l’Environnement et des Parcs, Direction du suivi de l’état de l’environnement. ISBN 978-2-550-53592-8 (PDF), 22 pages.

Canadian Council of Ministers of the Environment. 2007. “Canadian Water Guidelines for the Protection of Aquatic Life”, Summary Table, updated December 2007, in Canadian Environmental Quality Guidelines. Winnipeg, the Council.

Environment Canada. 2011. Water quality status and trends of nutrients in major drainage areas of Canada. Technical Summary. 38 pages + Appendix.

Environment Canada. 2010 (2001, updated on 17 August 2010). Water Contaminants (database). Contains measurements of water quality, concentrations of chemical and bacteriological contaminants, levels of nutritional elements as well as the chemical and physical properties of surface water. Montréal, Environment Canada, Science and Technology Branch, Water Quality Monitoring and Surveillance.

Hudon, C., and R. Carignan. 2008. Cumulative impacts of hydrology and human activities on water quality in the St. Lawrence River (Lake Saint-Pierre, Quebec, Canada). Canadian Journal of Fisheries and Aquatic Sciences 65: 1165-1180.

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