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

Map of St. Lawrence River - See long description below
Long description

The map shows the status of phosphorus and nitrogen levels at nine water quality monitoring stations along the St. Lawrence River. The total phosphorus status is green (less than 10% of samples above the guideline) at the Saint-Maurice station, yellow (between 10 and 50% of samples above the guideline) at the Carillon, Saint-François and Quebec City stations, and red (more than 50% of samples above the guideline) at the Lavaltrie, Richelieu, Yamaska, Nicolet and Bécancour stations.

The total nitrogen status is green (less than 10% of samples above the guideline) at the Saint-Maurice station, yellow (between 10 and 50% of samples above the guideline) at the Carillon station, and red (more than 50% of samples above the guideline) at the Lavaltrie, Yamaska, Richelieu, Saint-François, Nicolet, Bécancour and Quebec City stations.

The map shows that phosphorus levels decreased at Carillon and showed no trend at Quebec City between 2005 and 2014.

Data for this map
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
Monitoring station 2012-2014 Total phosphorus guideline exceedance
(percent)
Total phosphorus status 2012-2014 Total nitrogen guideline exceedance
(percent)
Total nitrogen status Trend
1995 to 2014
Carillon 20 Yellow 22 Yellow Phosphorus levels are decreasing
Lavaltrie 89 Red 89 Red n/a
Richelieu 65 Red 51 Red n/a
Yamaska 96 Red 96 Red n/a
Saint-François 44 Yellow 92 Red n/a
Nicolet 84 Red 71 Red n/a
Saint-Maurice 8 Green 3 Green n/a
Bécancour 56 Red 65 Red n/a
Quebec City 36 Yellow 52 Red Phosphorus levels show no trend

Note: n/a = not available.

Download data file (Excel/CSV; 1.82 KB)


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

Annual phosphorus boxplots - see long description below
Long description

The graphic presents annual phosphorus boxplots from 2008 to 2014 for six water quality monitoring stations along the St. Lawrence River (Richelieu, Yamaska, Saint-François, Nicolet, Bécancour and Saint-Maurice), from 2009 to 2014 for the Lavaltrie monitoring station, and from 2005 to 2014 for the Carillon and Quebec City monitoring stations. The boxes are joined by a solid line (red) to give a sense of the direction of change in phosphorus levels over time. A dotted line is also provided in each boxplot representing Quebec's water quality guideline for phosphorus of 0.03 milligrams of phosphorus per litre.

Data for Carillon
Data for Carillon
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Carillon 2005 0.028 0.018 0.150 23
Carillon 2006 0.024 0.016 0.051 20
Carillon 2007 0.021 0.010 0.044 20
Carillon 2008 0.021 0.015 0.065 14
Carillon 2009 0.020 0.016 0.058 17
Carillon 2010 0.019 0.009 0.030 14
Carillon 2011 0.012 0.008 0.021 14
Carillon 2012 0.019 0.008 0.025 14
Carillon 2013 0.024 0.014 0.046 13
Carillon 2014 0.022 0.015 0.034 14
Data for Lavaltrie
Data for Lavaltrie
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Lavaltrie 2009 0.060 0.030 0.075 9
Lavaltrie 2010 0.050 0.032 0.074 12
Lavaltrie 2011 0.055 0.016 0.183 12
Lavaltrie 2012 0.040 0.023 0.088 12
Lavaltrie 2013 0.046 0.032 0.112 13
Lavaltrie 2014 0.040 0.030 0.058 12
Data for Richelieu
Data for Richelieu
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Richelieu 2008 0.030 0.016 0.118 12
Richelieu 2009 0.030 0.018 0.118 10
Richelieu 2010 0.039 0.019 0.072 12
Richelieu 2011 0.043 0.020 0.066 12
Richelieu 2012 0.044 0.017 0.123 12
Richelieu 2013 0.037 0.019 0.192 13
Richelieu 2014 0.030 0.019 0.110 12
Data for Yamaska
Data for Yamaska
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Yamaska 2008 0.106 0.044 0.143 19
Yamaska 2009 0.113 0.066 0.520 17
Yamaska 2010 0.090 0.015 0.164 18
Yamaska 2011 0.122 0.060 0.175 14
Yamaska 2012 0.140 0.093 0.195 7
Yamaska 2013 0.131 0.084 0.156 9
Yamaska 2014 0.108 0.015 0.136 9

Note: Samples from the mouth of the Yamaska River are collected from May until the end of September only.

Data for Saint-François
Data for Saint-François
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Saint-François 2008 0.035 0.021 0.108 15
Saint-François 2009 0.033 0.021 0.105 15
Saint-François 2010 0.027 0.021 0.055 15
Saint-François 2011 0.031 0.021 0.172 14
Saint-François 2012 0.030 0.027 0.035 7
Saint-François 2013 0.031 0.025 0.064 9
Saint-François 2014 0.023 0.019 0.028 9

Note: Samples from the mouth of the Saint-François River are collected from May until the end of September only.

Data for Nicolet
Data for Nicolet
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Nicolet 2008 0.046 0.025 0.102 15
Nicolet 2009 0.053 0.036 0.126 15
Nicolet 2010 0.053 0.042 0.116 15
Nicolet 2011 0.050 0.010 0.073 14
Nicolet 2012 0.071 0.047 0.085 7
Nicolet 2013 0.046 0.035 0.053 9
Nicolet 2014 0.031 0.029 0.039 9

Note: Samples from the mouth of the Nicolet River are collected from May until the end of September only.

Data for Bécancour
Data for Bécancour
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Bécancour 2008 0.037 0.013 0.293 12
Bécancour 2009 0.038 0.024 0.062 12
Bécancour 2010 0.038 0.020 0.172 12
Bécancour 2011 0.041 0.024 0.103 12
Bécancour 2012 0.030 0.013 0.087 12
Bécancour 2013 0.043 0.022 0.136 12
Bécancour 2014 0.031 0.007 0.067 12
Data for Saint-Maurice
Data for Saint-Maurice
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Saint-Maurice 2009 0.015 0.010 0.048 10
Saint-Maurice 2010 0.015 0.009 0.184 12
Saint-Maurice 2011 0.008 0.005 0.015 13
Saint-Maurice 2012 0.014 0.010 0.024 12
Saint-Maurice 2013 0.015 0.012 0.250 13
Saint-Maurice 2014 0.015 0.008 0.147 12
Data for Quebec City
Data for Quebec City
Monitoring station Year Median phosphorus level
(milligrams of phosphorus per litre)
Minimum phosphorus level
(milligrams of phosphorus per litre)
Maximum phosphorus level
(milligrams of phosphorus per litre)
Number of samples
Quebec City 2005 0.033 0.019 0.135 16
Quebec City 2006 0.034 0.019 0.135 16
Quebec City 2007 0.026 0.013 0.072 18
Quebec City 2008 0.029 0.020 0.080 18
Quebec City 2009 0.025 0.008 0.070 17
Quebec City 2010 0.025 0.013 0.062 17
Quebec City 2011 0.030 0.015 0.104 17
Quebec City 2012 0.030 0.013 0.049 20
Quebec City 2013 0.036 0.015 0.075 15
Quebec City 2014 0.033 0.013 0.058 15

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

Annual nitrogen boxplots - see long description below
Long description

The graphic presents annual nitrogen levels from 2009 to 2014 for eight water quality monitoring stations along the St. Lawrence River (Carillon, Lavaltrie, Yamaska, Saint-François, Nicolet, Bécancour, Saint-Maurice and Quebec City) and from 2010 to 2014 for the Richelieu monitoring station. The boxes are joined by a solid line (red) to give a sense of the direction of change in nitrogen levels over time. A dotted line is also provided in each boxplot representing the water quality guideline for nitrogen of 0.63 milligrams of nitrogen per litre derived for the St. Lawrence River.

Data for Carillon
Data for Carillon
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
(milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Carillon 2009 0.492 0.426 0.713 11
Carillon 2010 0.543 0.450 0.897 14
Carillon 2011 0.540 0.440 0.870 14
Carillon 2012 0.530 0.440 0.690 13
Carillon 2013 0.570 0.480 1.06 13
Carillon 2014 0.515 0.400 1.07 14
Data for Lavaltrie
Data for Lavaltrie
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
(milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Lavaltrie 2009 0.900 0.580 1.40 9
Lavaltrie 2010 0.875 0.670 1.44 12
Lavaltrie 2011 0.920 0.580 1.35 12
Lavaltrie 2012 0.910 0.610 1.77 12
Lavaltrie 2013 0.950 0.730 1.86 12
Lavaltrie 2014 0.890 0.540 1.25 12
Data for Richelieu
Data for Richelieu
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
(milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Richelieu 2010 0.780 0.520 1.02 9
Richelieu 2011 0.650 0.430 1.03 12
Richelieu 2012 0.645 0.400 2.03 12
Richelieu 2013 0.705 0.400 2.52 12
Richelieu 2014 0.600 0.410 1.16 12
Data for Yamaska
Data for Yamaska
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
(milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Yamaska 2009 3.58 1.46 6.48 15
Yamaska 2010 2.27 1.25 3.91 15
Yamaska 2011 1.92 1.17 5.70 14
Yamaska 2012 0.75 0.660 1.37 7
Yamaska 2013 1.87 1.07 4.12 9
Yamaska 2014 1.17 0.570 2.60 9

Note: Samples from the mouth of the Yamaska River are collected from May until the end of September only.

Data for Saint-François
Data for Saint-François
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
(milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Saint-François 2009 0.770 0.650 1.12 15
Saint-François 2010 0.800 0.460 1.07 15
Saint-François 2011 0.830 0.590 2.42 14
Saint-François 2012 0.810 0.710 1.04 7
Saint-François 2013 0.760 0.610 1.11 9
Saint-François 2014 0.740 0.600 0.870 9

Note: Samples from the mouth of the Saint-François River are collected from May until the end of September only.

Data for Nicolet
Data for Nicolet
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
(milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Nicolet 2009 1.56 0.730 2.57 15
Nicolet 2010 0.940 0.550 1.81 15
Nicolet 2011 0.990 0.570 2.90 14
Nicolet 2012 0.680 0.400 2.03 16
Nicolet 2013 1.28 0.710 1.94 9
Nicolet 2014 0.670 0.340 1.22 9

Note: Samples from the mouth of the Nicolet River are collected from May until the end of September only.

Data for Bécancour
Data for Bécancour
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
(milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Bécancour 2009 0.855 0.370 1.21 10
Bécancour 2010 0.925 0.470 1.47 12
Bécancour 2011 0.915 0.470 1.42 12
Bécancour 2012 0.665 0.420 1.29 12
Bécancour 2013 0.935 0.490 1.29 12
Bécancour 2014 0.765 0.440 1.39 12
Data for Saint-Maurice
Data for Saint-Maurice
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
(milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Saint-Maurice 2009 0.360 0.270 0.380 9
Saint-Maurice 2010 0.315 0.243 0.630 12
Saint-Maurice 2011 0.340 0.290 0.417 13
Saint-Maurice 2012 0.330 0.270 0.400 12
Saint-Maurice 2013 0.330 0.270 0.760 13
Saint-Maurice 2014 0.340 0.280 0.560 12
Data for Quebec City
Data for Quebec City
Monitoring station Year Median nitrogen level
(milligrams of nitrogen per litre)
Minimum nitrogen level
milligrams of nitrogen per litre)
Maximum nitrogen level
(milligrams of nitrogen per litre)
Number of samples
Quebec City 2009 0.595 0.420 0.900 14
Quebec City 2010 0.630 0.400 0.960 17
Quebec City 2011 0.620 0.430 0.970 17
Quebec City 2012 0.605 0.330 1.02 20
Quebec City 2013 0.715 0.450 0.940 14
Quebec City 2014 0.645 0.480 0.890 14

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.

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