Nutrients in the St. Lawrence River
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Phosphorus and nitrogen are essential plant nutrients. However, when phosphorus or nitrogen levels are too high or too low, they can have harmful effects on the food web of a river. They are an important measure of the health of the river and its surrounding watersheds. These indicators provide the status of phosphorus and nitrogen levels along the St. Lawrence River.
Key results
Key results
- For the 2017 to 2019 period,
- phosphorus and nitrogen levels exceeded water quality guidelines at most monitoring stations
- only at Saint-Maurice did nitrogen level exceedances occur in less than 10% of samples
- From 2010 to 2019, Yamaska had decreasing nitrogen levels
Status of total phosphorus and total nitrogen levels for the 2017 to 2019 period and total phosphorus and total nitrogen level trends in the St. Lawrence River, Canada, 2010 to 2019

Data table for the long description
Monitoring station | 2017 to 2019 total phosphorus guideline exceedance (percentage) |
Total phosphorus status | 2010 to 2019 total phosphorus trend | 2017 to 2019 total nitrogen guideline exceedance (percentage) |
Total nitrogen status | 2010 to 2019 total nitrogen trend |
---|---|---|---|---|---|---|
Wolfe Island | 19 | Fair | Phosphorus levels show no trend | 48 | Fair | Nitrogen levels show no trend |
Carillon | 31 | Fair | Phosphorus levels show no trend | 38 | Fair | Nitrogen levels show no trend |
Lavaltrie | 97 | Poor | Phosphorus levels show no trend | 92 | Poor | Nitrogen levels show no trend |
Richelieu | 69 | Poor | Phosphorus levels show no trend | 64 | Poor | Nitrogen levels show no trend |
Yamaska | 100 | Poor | Phosphorus levels show no trend | 98 | Poor | Nitrogen levels are decreasing |
Saint-François | 29 | Fair | Phosphorus levels show no trend | 94 | Poor | Nitrogen levels show no trend |
Nicolet | 90 | Poor | Phosphorus levels show no trend | 76 | Poor | Nitrogen levels show no trend |
Saint-Maurice | 14 | Fair | Phosphorus levels show no trend | 3 | Good | Nitrogen levels show no trend |
Bécancour | 69 | Poor | Phosphorus levels show no trend | 69 | Poor | Nitrogen levels show no trend |
Quebec City | 61 | Poor | Phosphorus levels show no trend | 53 | Poor | Nitrogen levels show no trend |
Download data file (Excel/CSV; 2.75 kB)
How this indicator was calculated
Note: The nutrient status at a monitoring station is considered Good when nutrient levels (phosphorus or nitrogen) exceed the guideline in less than 10% of samples. A Fair status is applied when the guideline is exceeded in 10% to 50% of samples. A Poor 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 Ontario and Quebec's total phosphorus water quality guideline of 0.03 milligrams of phosphorus per litre (mg P/L)Footnote 1 and a derived total nitrogen water quality guideline of 0.63 milligrams of nitrogen per litre (mg N/L). For more details about the water quality guidelines, please refer to the Data sources and methods. Samples from the mouths of the Yamaska, Saint-François and Nicolet rivers are collected from May to September only.
Source: Environment and Climate Change Canada (2020) Saint Lawrence River basin long-term water quality monitoring data and Great Lakes connecting channels monitoring and surveillance data.
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 other 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. Farther downstream, tributary rivers draining agricultural regions transport higher concentrations of phosphorus and nitrogen which result from the chemical fertilizers and manure used to grow crops. Upstream of Quebec City, water from tributary rivers, such as the Saint-Maurice, which drain the north shore have lower phosphorus and nitrogen levels because they run through an area with more forest cover than that found on the south shore of the river. Beyond Quebec City, the St. Lawrence River flows into the Gulf of St. Lawrence, where the nitrogen and phosphorus levels contribute to harmful algal blooms.
For the St. Lawrence River, nutrient status at a monitoring station is considered Good when fewer than 10% of samples exceed the water quality guidelines for total phosphorus or total nitrogen. The 10% cut-off limit allows for 1 sample per year to exceed the guideline. In rivers, total phosphorus and total nitrogen concentrations will often exceed the guidelines when water levels are high, a situation that is mainly observed when the snow melts in the spring. When 10% to 50% of the samples exceed the guidelines, the nutrient status is considered Fair. In contrast, nutrient status is Poor if more than 50% of the samples exceed the water quality guidelines.
During the 2017 to 2019 period, status of phosphorus and nitrogen levels at the majority of water quality monitoring stations along the St. Lawrence River was rated as Poor. Over the last 10 years, 2010 to 2019, the Yamaska station had a slight decreasing trend in nitrogen levels, while the remaining stations showed no detectable trends. There were no phosphorus level trends at any station.
Phosphorous
Phosphorus levels by water quality monitoring station
Key results
- A trends analysis from 2010 to 2019 showed there were no detectable trends at any station
Annual total phosphorus levels for 10 water quality monitoring stations along the St. Lawrence River, 2010 to 2019

Data table for the long description
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 |
---|---|---|---|---|---|
Wolfe Island | 2010 | 0.010 | 0.005 | 0.061 | 26 |
Wolfe Island | 2011 | 0.007 | 0.005 | 0.010 | 13 |
Wolfe Island | 2012 | 0.013 | 0.005 | 0.401 | 52 |
Wolfe Island | 2013 | 0.008 | 0.005 | 0.412 | 65 |
Wolfe Island | 2014 | 0.008 | 0.005 | 0.038 | 28 |
Wolfe Island | 2015 | 0.009 | 0.003 | 0.133 | 33 |
Wolfe Island | 2016 | 0.011 | 0.003 | 0.246 | 63 |
Wolfe Island | 2017 | 0.022 | 0.002 | 0.461 | 55 |
Wolfe Island | 2018 | 0.008 | 0.002 | 0.130 | 48 |
Wolfe Island | 2019 | 0.008 | 0.004 | 0.145 | 51 |
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 |
Carillon | 2015 | 0.020 | 0.014 | 0.092 | 14 |
Carillon | 2016 | 0.022 | 0.014 | 0.077 | 14 |
Carillon | 2017 | 0.021 | 0.017 | 0.083 | 14 |
Carillon | 2018 | 0.020 | 0.014 | 0.054 | 14 |
Carillon | 2019 | 0.024 | 0.015 | 0.109 | 14 |
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 |
Lavaltrie | 2015 | 0.046 | 0.031 | 0.135 | 12 |
Lavaltrie | 2016 | 0.043 | 0.027 | 0.165 | 12 |
Lavaltrie | 2017 | 0.043 | 0.033 | 0.098 | 12 |
Lavaltrie | 2018 | 0.045 | 0.031 | 0.074 | 12 |
Lavaltrie | 2019 | 0.037 | 0.029 | 0.090 | 12 |
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 | 0.041 | 0.019 | 0.192 | 12 |
Richelieu | 2014 | 0.030 | 0.019 | 0.110 | 12 |
Richelieu | 2015 | 0.039 | 0.018 | 0.133 | 12 |
Richelieu | 2016 | 0.045 | 0.026 | 0.111 | 12 |
Richelieu | 2017 | 0.033 | 0.020 | 0.253 | 12 |
Richelieu | 2018 | 0.044 | 0.020 | 0.089 | 12 |
Richelieu | 2019 | 0.039 | 0.022 | 0.087 | 12 |
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 |
Yamaska | 2015 | 0.099 | 0.040 | 0.197 | 12 |
Yamaska | 2016 | 0.113 | 0.041 | 0.186 | 16 |
Yamaska | 2017 | 0.087 | 0.035 | 0.125 | 17 |
Yamaska | 2018 | 0.122 | 0.041 | 0.312 | 17 |
Yamaska | 2019 | 0.119 | 0.056 | 0.196 | 17 |
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 |
Saint-François | 2015 | 0.029 | 0.018 | 0.045 | 12 |
Saint-François | 2016 | 0.028 | 0.020 | 0.040 | 16 |
Saint-François | 2017 | 0.023 | 0.017 | 0.048 | 17 |
Saint-François | 2018 | 0.026 | 0.020 | 0.045 | 17 |
Saint-François | 2019 | 0.026 | 0.017 | 0.049 | 17 |
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 |
Nicolet | 2015 | 0.040 | 0.023 | 0.149 | 12 |
Nicolet | 2016 | 0.052 | 0.026 | 0.144 | 16 |
Nicolet | 2017 | 0.042 | 0.027 | 0.094 | 17 |
Nicolet | 2018 | 0.039 | 0.021 | 0.101 | 17 |
Nicolet | 2019 | 0.039 | 0.029 | 0.064 | 17 |
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 |
Saint-Maurice | 2015 | 0.013 | 0.009 | 0.019 | 12 |
Saint-Maurice | 2016 | 0.014 | 0.010 | 0.018 | 12 |
Saint-Maurice | 2017 | 0.015 | 0.011 | 0.040 | 12 |
Saint-Maurice | 2018 | 0.011 | 0.009 | 0.056 | 12 |
Saint-Maurice | 2019 | 0.010 | 0.007 | 0.041 | 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 |
Bécancour | 2015 | 0.045 | 0.020 | 0.091 | 12 |
Bécancour | 2016 | 0.050 | 0.027 | 0.293 | 12 |
Bécancour | 2017 | 0.043 | 0.024 | 0.240 | 12 |
Bécancour | 2018 | 0.044 | 0.009 | 0.117 | 12 |
Bécancour | 2019 | 0.029 | 0.021 | 0.104 | 12 |
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 | |
Quebec City | 2013 | 0.036 | 0.015 | 0.075 | 15 |
Quebec City | 2014 | 0.033 | 0.013 | 0.058 | 15 |
Quebec City | 2015 | 0.034 | 0.016 | 0.137 | 17 |
Quebec City | 2016 | 0.042 | 0.019 | 0.114 | 17 |
Quebec City | 2017 | 0.032 | 0.022 | 0.142 | 17 |
Quebec City | 2018 | 0.036 | 0.006 | 0.069 | 17 |
Quebec City | 2019 | 0.034 | 0.016 | 0.088 | 17 |
Download data file (Excel/CSV; 4.56 kB)
How this indicator was calculated
Note: Each boxplot summarizes annual phosphorus levels at a monitoring station and shows the range of values measured. The dotted line shows Ontario and Quebec's total phosphorus water quality guideline value of 0.03 milligrams of phosphorus per litre (mg P/L). The solid line is drawn through the median to give a sense of the changes in concentrations over time. A Seasonal Kendall trend analysis for phosphorus was calculated for each station from 2010 to 2019. Samples from the mouths of the Yamaska, Saint-François and Nicolet rivers are collected from May to September only.
Source: Environment and Climate Change Canada (2020) Saint Lawrence River basin long-term water quality monitoring data and Great Lakes connecting channels monitoring and surveillance data.
Plotting phosphorus data for each station by year provides a general view of how phosphorus levels are changing along the St. Lawrence River. From 2010 to 2019, median phosphorus levels were below the guideline at Saint-Maurice, Wolfe Island and Carillon. Over the same time period, median phosphorus levels were above the guideline at Yamaska, Nicolet and Lavaltrie. At Saint-François, Bécancour, Quebec City and Richelieu, median phosphorus levels fluctuated above and below the guideline.
Nitrogen
Nitrogen levels by water quality monitoring station
Key results
- A trends analysis from 2010 to 2019 showed:
- Yamaska had a slight decrease in nitrogen levels
- there were no detectable trends at the other 9 stations
Annual total nitrogen levels for 10 water quality monitoring stations along the St. Lawrence River, 2010 to 2019

Data table for the long description
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 |
---|---|---|---|---|---|
Wolfe Island | 2010 | 0.605 | 0.387 | 1.326 | 24 |
Wolfe Island | 2011 | 0.523 | 0.398 | 0.606 | 13 |
Wolfe Island | 2012 | 0.520 | 0.315 | 1.252 | 52 |
Wolfe Island | 2013 | 0.535 | 0.377 | 1.646 | 65 |
Wolfe Island | 2014 | 0.608 | 0.526 | 1.056 | 28 |
Wolfe Island | 2015 | 0.674 | 0.423 | 2.885 | 31 |
Wolfe Island | 2016 | 0.785 | 0.360 | 3.538 | 63 |
Wolfe Island | 2017 | 0.663 | 0.375 | 4.192 | 55 |
Wolfe Island | 2018 | 0.608 | 0.374 | 1.207 | 48 |
Wolfe Island | 2019 | 0.562 | 0.021 | 1.887 | 51 |
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 | 1.060 | 13 |
Carillon | 2014 | 0.515 | 0.400 | 1.070 | 14 |
Carillon | 2015 | 0.520 | 0.340 | 1.130 | 14 |
Carillon | 2016 | 0.545 | 0.380 | 0.780 | 14 |
Carillon | 2017 | 0.605 | 0.440 | 1.050 | 14 |
Carillon | 2018 | 0.555 | 0.430 | 1.050 | 14 |
Carillon | 2019 | 0.515 | 0.460 | 0.920 | 14 |
Lavaltrie | 2010 | 0.875 | 0.670 | 1.440 | 12 |
Lavaltrie | 2011 | 0.920 | 0.580 | 1.350 | 12 |
Lavaltrie | 2012 | 0.910 | 0.610 | 1.770 | 12 |
Lavaltrie | 2013 | 0.950 | 0.730 | 1.860 | 12 |
Lavaltrie | 2014 | 0.890 | 0.540 | 1.250 | 12 |
Lavaltrie | 2015 | 0.940 | 0.390 | 1.520 | 11 |
Lavaltrie | 2016 | 1.035 | 0.540 | 2.220 | 12 |
Lavaltrie | 2017 | 0.990 | 0.740 | 1.560 | 12 |
Lavaltrie | 2018 | 0.955 | 0.640 | 1.620 | 12 |
Lavaltrie | 2019 | 0.825 | 0.530 | 1.260 | 12 |
Richelieu | 2010 | 0.780 | 0.520 | 1.020 | 9 |
Richelieu | 2011 | 0.650 | 0.430 | 1.030 | 12 |
Richelieu | 2012 | 0.645 | 0.400 | 2.030 | 12 |
Richelieu | 2013 | 0.705 | 0.400 | 2.520 | 12 |
Richelieu | 2014 | 0.600 | 0.410 | 1.160 | 12 |
Richelieu | 2015 | 0.610 | 0.500 | 2.440 | 12 |
Richelieu | 2016 | 0.850 | 0.390 | 3.720 | 12 |
Richelieu | 2017 | 0.665 | 0.390 | 1.590 | 12 |
Richelieu | 2018 | 0.765 | 0.400 | 2.320 | 12 |
Richelieu | 2019 | 0.680 | 0.380 | 2.020 | 12 |
Yamaska | 2010 | 2.270 | 1.250 | 3.910 | 15 |
Yamaska | 2011 | 1.920 | 1.170 | 5.700 | 14 |
Yamaska | 2012 | 0.750 | 0.660 | 1.370 | 7 |
Yamaska | 2013 | 1.870 | 1.070 | 4.120 | 9 |
Yamaska | 2014 | 1.170 | 0.570 | 2.600 | 9 |
Yamaska | 2015 | 3.055 | 0.560 | 5.094 | 12 |
Yamaska | 2016 | 1.750 | 0.580 | 7.300 | 16 |
Yamaska | 2017 | 1.840 | 1.200 | 4.970 | 17 |
Yamaska | 2018 | 1.250 | 0.620 | 5.320 | 17 |
Yamaska | 2019 | 1.510 | 0.780 | 2.520 | 17 |
Saint-François | 2010 | 0.800 | 0.460 | 1.070 | 15 |
Saint-François | 2011 | 0.830 | 0.590 | 2.420 | 14 |
Saint-François | 2012 | 0.810 | 0.710 | 1.040 | 7 |
Saint-François | 2013 | 0.760 | 0.610 | 1.110 | 9 |
Saint-François | 2014 | 0.740 | 0.600 | 0.870 | 9 |
Saint-François | 2015 | 0.770 | 0.410 | 0.969 | 12 |
Saint-François | 2016 | 0.920 | 0.650 | 1.240 | 16 |
Saint-François | 2017 | 0.830 | 0.580 | 3.040 | 17 |
Saint-François | 2018 | 0.800 | 0.620 | 0.950 | 17 |
Saint-François | 2019 | 0.810 | 0.560 | 1.000 | 17 |
Nicolet | 2010 | 0.940 | 0.550 | 1.810 | 15 |
Nicolet | 2011 | 0.990 | 0.570 | 2.900 | 14 |
Nicolet | 2012 | 0.680 | 0.400 | 2.030 | 16 |
Nicolet | 2013 | 1.280 | 0.710 | 1.940 | 9 |
Nicolet | 2014 | 0.670 | 0.340 | 1.220 | 9 |
Nicolet | 2015 | 1.390 | 0.170 | 3.070 | 12 |
Nicolet | 2016 | 1.240 | 0.640 | 3.070 | 15 |
Nicolet | 2017 | 1.340 | 0.620 | 2.540 | 17 |
Nicolet | 2018 | 0.680 | 0.380 | 3.960 | 17 |
Nicolet | 2019 | 0.920 | 0.460 | 1.740 | 17 |
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 |
Saint-Maurice | 2015 | 0.320 | 0.210 | 0.490 | 12 |
Saint-Maurice | 2016 | 0.320 | 0.190 | 0.360 | 12 |
Saint-Maurice | 2017 | 0.320 | 0.260 | 0.380 | 12 |
Saint-Maurice | 2018 | 0.320 | 0.270 | 0.780 | 12 |
Saint-Maurice | 2019 | 0.290 | 0.260 | 0.350 | 12 |
Bécancour | 2010 | 0.925 | 0.470 | 1.470 | 12 |
Bécancour | 2011 | 0.915 | 0.470 | 1.420 | 12 |
Bécancour | 2012 | 0.665 | 0.420 | 1.290 | 12 |
Bécancour | 2013 | 0.935 | 0.490 | 1.290 | 12 |
Bécancour | 2014 | 0.765 | 0.440 | 1.390 | 12 |
Bécancour | 2015 | 0.825 | 0.440 | 1.910 | 12 |
Bécancour | 2016 | 0.850 | 0.310 | 1.800 | 12 |
Bécancour | 2017 | 0.950 | 0.440 | 1.490 | 12 |
Bécancour | 2018 | 0.825 | 0.320 | 2.060 | 12 |
Bécancour | 2019 | 0.760 | 0.390 | 1.170 | 12 |
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 | 1.020 | 20 |
QuebecCity | 2013 | 0.715 | 0.450 | 0.940 | 14 |
Quebec City | 2014 | 0.645 | 0.480 | 0.890 | 14 |
Quebec City | 2015 | 0.670 | 0.270 | 1.180 | 17 |
Quebec City | 2016 | 0.700 | 0.370 | 0.960 | 17 |
Quebec City | 2017 | 0.650 | 0.440 | 1.170 | 17 |
Quebec City | 2018 | 0.630 | 0.400 | 0.940 | 17 |
Quebec City | 2019 | 0.610 | 0.420 | 1.130 | 17 |
Download data file (Excel/CSV; 4.32 kB)
How this indicator is calculated
Note: Each boxplot summarizes annual nitrogen levels for a monitoring station and shows the range of values measured. The dotted line shows the guideline value of 0.63 milligrams of nitrogen per litre (mg N/L). The solid line is drawn through the median to give a sense of trends in concentration. A Seasonal Kendall trend analysis for nitrogen was calculated for each station from 2010 to 2019. Samples from the mouths of the Yamaska, Saint-François and Nicolet rivers are collected from May to September only.
Source: Environment and Climate Change Canada (2020) Saint Lawrence River basin long-term water quality monitoring data and Great Lakes connecting channels monitoring and surveillance data.
Plotting nitrogen data for each station by year provides a general view of how nitrogen levels are changing over time along the St. Lawrence River. Nitrogen levels tend to be lower at stations situated near forested areas with smaller urban populations, such as Carillon and Saint-Maurice. From 2010 to 2019, median nitrogen levels were below the guideline at Saint-Maurice and Carillon. Over the same time period, median nitrogen levels were above the guideline at Lavaltrie, Yamaska, Nicolet, Bécancour and Saint-François. At Wolfe Island, Quebec City and Richelieu, median nitrogen levels fluctuated above and below the guideline.
About the indicator
About the indicators
What the indicators measure
The indicators report on the status of total phosphorus and total nitrogen levels along the St. Lawrence River. It ranks the status based on how often total phosphorus and total nitrogen levels exceed their respective water quality guidelines.
These indicators assume that water in the St. Lawrence River would rarely exceed water quality guidelines for phosphorus and nitrogen in the absence of human development. They provide information about how human activity contributes to phosphorus and nitrogen levels in the river. The more often the water quality guidelines are exceeded, the greater the risk to the health of the St. Lawrence River. The phosphorus and nitrogen trend analysis provides information about how concentrations are changing over time.
Why thes indicators are 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 for domestic use (for example, drinking, 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. When phosphorus and nitrogen levels 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 levels 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 hrm local fisheries.
Phosphorus and nitrogen used in chemical fertilizers reach the river through erosion, leaching from urban areas, farmland runoff, municipal and industrial wastewater discharges, and air pollution. Over time, excess phosphorus and nitrogen levels in the river can alter its food web.
These indicators are used to provide information about the state of the St. Lawrence River. Ongoing tracking of phosphorus and nitrogen levels allows governments and citizens to remain aware of an important aspect of the environmental condition of the river.

Pristine lakes and rivers
These indicators support the measurement of progress towards the following 2019 to 2022 Federal Sustainable Development Strategy long-term goal: Clean and healthy lakes and rivers support economic prosperity and the well-being of Canadians.
In addition, the indicators contribute 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."
The indicators also contribute towards reporting on Target 10 of the 2020 Biodiversity goals and targets for Canada: "By 2020, pollution levels in Canadian waters, including pollution from excess nutrients, are reduced or maintained at levels that support healthy aquatic ecosystems."
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 levels 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 Household use of chemical pesticides and fertilizers indicator reports on how many people in Canada use pesticides and fertilizers on their lawns and gardens.
The Municipal wastewater treatment indicators measure the level of wastewater treatment provided to the Canadian population.
Data sources and methods
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.
More information
Sampling
The status of total phosphorus and total nitrogen levels are based on measurements recorded between January 2017 and December 2019. The trend analysis uses data from 2010 to 2019.
The sampling frequency at the water quality monitoring stations included in these indicators is not uniform. Sampling at the Carillon, Lavaltrie, Richelieu, Saint-Maurice, Bécancour and Quebec City stations is conducted on a monthly basis. At monitoring stations at the mouths of the Yamaska, Saint-François and Nicolet rivers, samples are typically collected on a weekly basis from May to September. Sampling at the Wolfe Island station is typically conducted on a weekly basis year round. Gaps exist in the data due to program changes, field laboratory updates, weather and mechanical issues with the equipment used to collect the data.
Water quality monitoring station locations
Data were obtained from 10 monitoring stations along the St. Lawrence River from the outlet of Lake Ontario at Wolfe Island near Kingston in the west to Quebec City in the east (Table 1). The stations are sited so as to monitor the principal water sources entering the St. Lawrence River and are sometimes installed at the mouths of tributary rivers.
Monitoring station | Station code | Station name | Latitude | Longitude |
---|---|---|---|---|
Wolfe Island | ON02MA0030 | St. Lawrence River (South Channel) | 44.2078 | -76.2368 |
Carillon | QU02LB9001 | Ottawa River at Carillon | 45.5676 | -74.3799 |
Lavaltrie | QU02OB9004 | St. Lawrence River, water intake at Lavaltrie water treatment plant | 45.8744 | -73.2806 |
Richelieu | QU02OJ0052 | Richelieu River, water intake of Sorel's filtration plant | 46.0340 | -73.1176 |
Yamaska | QU02OG3007 | Yamaska River, Route 132 bridge | 46.0051 | -72.9101 |
Saint-François | QU02OF3004 | Saint-François Riverat Pierreville | 46.0664 | -72.8122 |
Nicolet | QU02OD3004 | Nicolet River at Nicolet | 46.2454 | -72.6512 |
Saint-Maurice | QU02NG3013 | Saint-Maurice River, water intake at Trois-Rivières water treatmentplant | 46.3820 | -72.6105 |
Bécancour | QU02OD9009 | St. Lawrence River, water intake of Bécancour's filtration plant | 46.3116 | -72.5460 |
Quebec City | QU02PH9024 | St. Lawrence River at Lévis | 46.8071 | -71.1900 |
Methods
The status of phosphorus and nitrogen levels at each monitoring station was calculated on the basis of how often levels were above their water quality guidelines.
A Seasonal Kendall test with Seasonal Kendall slope was used to test for the presence of a statistically significant increasing or decreasing trend in total phosphorus and total nitrogen over the last 10 years.Footnote 2
More information
Water quality guidelines
Total phosphorus
Ontario and Quebec's total phosphorus water quality guideline for the protection of aquatic life, specifically 0.03 milligrams of phosphorus per litre (mg P/L) was used.Footnote 1
Total nitrogen
Neither Ontario, Quebec nor the Canadian Council of Ministers of the Environment (CCME) has a water quality guideline for total nitrogen. Accordingly, a total nitrogen guideline for the St. Lawrence River was derived in keeping with the CCME's lines-of-evidence approach (PDF; 1.95 MB). A total nitrogen guideline of 0.63 milligrams of nitrogen per litre (mg N/L) was selected for calculation of the indicators. This coincides with the ideal performance standardFootnote 3 of 0.63 mg N/L for large rivers in the Mixedwood Plains Ecozone as recommended during Environment and Climate Change Canada's National Agri‑Environmental Standards Initiative.Footnote 4
See Annex A for more detail about how the total nitrogen guideline was derived.
Calculation of phosphorus and nitrogen status for the St. Lawrence River
The phosphorus status at each of the 10 water quality monitoring stations was computed by comparing total phosphorus concentrations at each station with the total phosphorus water quality guideline for the protection of aquatic life of 0.03 mg P/L.Footnote 5 Similarly, the nitrogen status at each water quality monitoring station was determined by comparing the total nitrogen concentrations at each station to the St. Lawrence-specific total nitrogen water quality guideline for the protection of aquatic life of 0.63 mg N/L (see Annex A).
The number of times total phosphorus and total nitrogen concentrations exceeded the guidelines were summed from 2017 to 2019, and the results were divided by the total number of samples collected over the same time period. The status of each station was determined by calculating the percentage of samples exceeding the guidelines.
- Good nutrient status = fewer than 10% of samples exceed the guidelines
- Fair nutrient status = 10% to 50% of samples exceed the guidelines
- Poor nutrient status = more than 50% of samples exceed the guidelines
Trend Analysis
Data requirements
With environmental trend analysis, the more data available, the more statistical power the test has. For a station to be included in trend analysis reporting, at least 10 years of data were required. These data requirements were met by all stations for total phosphorus and total nitrogen. Total phosphorus concentrations are strongly correlated with the river's flows because high flows transport more suspended sediment with bound phosphorus. For example, phospohorus and nitrogen loads at Quebec City and the Ottawa River at Carillon were higher due to the inflow from tributary rivers at these stations, compared to the outflow from Lake Ontario into the St. Lawrence River at Wolfe Island.Footnote 6
Stations sampled throughout the year
With the exception of Wolfe Island, which was sampled weekly, stations were typically sampled monthly throughout the entire year. Within the dataset for each station, data were sorted by sampling date from oldest to most current. Duplicate (replicate) values were removed and each sample was assigned to a month based on the sampling date. To correct sampling frequency variation in the data, and to minimize analytical issues associated with serial correlation in the data, one sample per month (approximate 30-day interval) was selected for the analysis. An Excel function was run to count the number of days between sampling dates. If there were more than one sample in the same month, the extra samples were removed from the dataset based on the desired 30-day interval between samples. The analysis was run using the Kendall package within the R software environment.
Stations sampled seasonally
The samples from the mouths of the Yamaska, Saint-François and Nicolet rivers were typically collected on a weekly basis from May to September. Within the dataset for each station, duplicate (replicate) values were removed and each sample was assigned to 1 of 22 weeks from May 1 to October 1. To correct sampling frequency variation in the data, and to minimize analytical issues associated with serial correlation in the data, a single sample taken approximately every 7 days was selected for the analysis. Only weeks 9 through 17 (June 26 to August 27) had enough samples over the 10-year period to be used for the trend analysis. The analysis was run using the Kendall package within the R software environment.
Monitoring station | Parameter | Tau | 2-sided p-value | Seasonal Kendall slope |
---|---|---|---|---|
Wolfe Island | Total phosphorus | 0.029 | 0.721 | 0.000 |
Carillon | Total phosphorus | 0.228 | 0.001 | 0.001 |
Lavaltrie | Total phosphorus | -0.117 | 0.109 | 0.001 |
Richelieu | Total phosphorus | 0.028 | 0.716 | 0.000 |
Yamaska | Total phosphorus | 0.098 | 0.260 | 0.002 |
Saint-François | Total phosphorus | -0.077 | 0.378 | 0.000 |
Nicolet | Total phosphorus | -0.108 | 0.210 | -0.001 |
Saint-Maurice | Total phosphorus | -0.093 | 0.202 | 0.000 |
Bécancour | Total phosphorus | 0.070 | 0.337 | 0.000 |
Quebec City | Total phosphorus | 0.177 | 0.016 | 0.001 |
Monitoring station | Parameter | Tau | 2-sided p-value | Seasonal Kendall slope |
---|---|---|---|---|
Wolfe Island | Total nitrogen | 0.134 | 0.085 | 0.006 |
Carillon | Total nitrogen | 0.004 | 0.979 | 0.000 |
Lavaltrie | Total nitrogen | 0.041 | 0.582 | 0.003 |
Richelieu | Total nitrogen | -0.008 | 0.936 | 0.000 |
Yamaska | Total nitrogen | -0.222 | 0.009 | -0.054 |
Saint-François | Total nitrogen | 0.151 | 0.079 | 0.007 |
Nicolet | Total nitrogen | 0.059 | 0.498 | 0.010 |
Saint-Maurice | Total nitrogen | -0.128 | 0.076 | -0.002 |
Bécancour | Total nitrogen | -0.037 | 0.622 | -0.005 |
Quebec City | Total nitrogen | -0.012 | 0.892 | 0.000 |
Interpretation of the trends
The analysis was run using the Kendall package (version 2.2, 2011) of the statistical software R (version 3.4.4, 2018) to detect the presence of statistically significant trends in total phosphorus and total nitrogen levels from 2010 to 2019. The Seasonal Kendall analysis statistical outputs from R are shown in Table 2 for total phosphorus and Table 3 for total nitrogen.
Kendall's tau was used to measure the strength of the relationship between total phosphorus or total nitrogen and the sampling date. The tau values in tables 2 and 3 are all close to 0, which indicates there is negligible correlation between the nutrient samples and the sampling date.
The observed significance level or 2-sided p-value statistic was used to determine whether a statistically significant trend through time was present in the data. A p-value statistic of 0.05 or less indicates there is sufficient evidence in the data to signal the presence of a trend. Further, a p-value statistic of less than 0.01 indicates strong evidence of a trend in the data. A p-value statistic greater than 0.05 indicates the absence of a trend.
Where the p-value indicated a trend, the Seasonal Kendall slope was used to determine whether the trend was increasing or decreasing. A positive slope value indicates an upward trend or increasing phosphorus or nitrogen levels. A negative slope value indicates a downward trend or decreasing phosphorus or nitrogen levels. Trends were only reported if the slope was greater than 0. In the case of the total phosphorus trends for these indicators, the significant slope at Carillon and Quebec City was 0.001 and thus too small and close to 0 to give a direction.
The boxplot charts within figures 2 and 3 can also give a sense of trends in total phosphorus or total nitrogen levels over time. The changes in concentrations from one year to the next can be viewed using the solid line drawn through the median.
Recent changes
A 10th station was added to the indicators. This station is located at the outflow of Lake Ontario into the St. Lawrence River at Wolfe Island near Kingston, Ontario.
In the previous version of the indicators, only 7 of 9 stations met the minimum data requirements for a phosphorus trends analysis and none of the stations met the data requirements for a nitrogen trends analysis. In the current version, enough data was available for all monitoring stations (10) for both the phosphorus and nitrogen trends analyses. Refer to the Methods for more information on the trend analysis.
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 exercised 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. However, it is still reasonable to compare lake and river systems as long as the methods used to determine the water quality classifications are clear.
Resources
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 January 8, 2021.
Chambers PA, Guy M, Dixit SS, Benoy GA, Brua RB, Culp JM, McGoldrick D, Upsdell BL, 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 January 8, 2021.
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 January 8, 2021.
Ministère du Développement durable, de l'Environnement et de la Lutte contre les changements climatiques (2009) Critères de qualité de l'eau de surface : phosphore total (en P) (in French only). Retrieved on January 8, 2021.
Ontario Ministry of the Environment and Energy (1994) Water Management Policies, Guidelines, Provincial Water Quality Objectives. Retrieved on January 8, 2021.
United States Environmental Protection Agency (2000a) Nutrient Criteria Technical Guidance Manual: Rivers and Streams. Report No. EPA-822-B-00-002. Retrieved on January 8, 2021.
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 January 8, 2021.
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 January 8, 2021.
Related information
Environment and Climate Change Canada (2015) Phosphorus in aquatic ecosystems. Retrieved on January 8, 2021.
Governments of Canada and Quebec (2015) St. Lawrence Action Plan 2011-2026. Retrieved on January 8, 2021.
Environment and Climate Change Canada (2017) St. Lawrence River: phosphorus at the mouths of Lake Saint-Pierre tributaries. Retrieved on January 8, 2021.
Annex
Annex A. A total nitrogen guideline to protect the ecological condition of the St. Lawrence
Neither the governments of Ontario and Quebec nor the Canadian Council of Ministers of the Environment (CCME) has a water quality guideline for total nitrogen. In order to develop a guideline for the indicator, research and analysis was performed following the lines-of-evidence approach outlined in the CCME's Guidance manual for developing nutrient guidelines for rivers and streams (PDF; 1.95 MB). This approach recommends a number of consecutive steps to formulate a final guideline. A summary of the key steps followed to develop the guideline of 0.63 mg N/L for the calculation of the Nutrients in the St. Lawrence River indicators are set-out below.
It is important to note that this guideline has been designed for use in this indicator and may not include all possible data. Should an official total nitrogen guideline be developed for the St. Lawrence River, it will replace the guideline derived here.
Step 1. Definition of the area of interest
For the purpose of the indicators and the analysis performed, the St. Lawrence River is defined as extending from the outflow of Lake Ontario at Wolfe Island in the west to Quebec City in the east.
Site Description
The St. Lawrence River is a very large river with a catchment area of 1 610 000 km2. It is situated in the St. Lawrence Lowlands ecoregion of the Mixedwood Plains Ecozone. About 60% of the region is intensively cultivated farmland, with dairy and mixed farming systems prevailing. Urban development is extensive. Intensive land use is increasing, with a trend toward rising nutrient loads to streams and rivers. The St. Lawrence Lowlands ecoregion has a humid, continental climate with very cold winters and very hot summers. Rivers in humid regions tend to have more water throughout the year.
The river was formed around the end of the last ice age when faulting led to the sinking of the area around the river (a rift valley), which was then flooded with water from the Atlantic Ocean. It forms much of the southwestern outline of the Canadian Shield in Quebec.
Step 2. Establishment of the desired outcomes and selection of the guideline variables
The desired outcome of this nitrogen guideline is to prevent eutrophication in the St. Lawrence River and the Gulf of St. Lawrence caused by total nitrogen.
Step 3. Classification of streams
The St. Lawrence River is a very large river ecosystem. In such systems, the relationships between aquatic communities and nutrients may be confounded by physical factors that exert their influence temporally and spatially at the local scale, as well as along a continuum of river size from small streams to large rivers. Water quality in streams is more subject to sudden changes in hydrology than is the case for rivers, and plant and animal community abundance and composition varies with river size. For these reasons, separate standards to protect the ecological condition of different rivers are necessary.
The river was not subdivided into separate subregions for this guideline derivation because of the need for a single value that would apply along the whole river to allow comparability among stations.
Step 4. Collection and analysis of data
Total phosphorus and total nitrogen data were provided by Environment and Climate Change Canada's Freshwater Quality Monitoring and Surveillance program. The data can be found on the Freshwater quality monitoring: online data web page.
Observed spatial patterns in the data (Figure A.1; Table A.3):
- total nitrogen concentrations in the river tend to be lowest in summer and highest in winter
- total nitrogen concentrations increase from Carillon to Lavaltrie and then decrease to Bécancour and Quebec City
- total nitrogen concentrations at Lavaltrie are influenced by the region of Montreal's sewage outfall
- at Bécancour, the influence of nitrogen inflow from tributaries draining the agricultural regions on the south shore of Lake Saint-Pierre can be seen
Figure A.1. Total nitrogen data for 4 water quality monitoring stations on the St. Lawrence River (stations are presented in order from Carillon in the west to Quebec City in the east)

Long description
The line charts show the total nitrogen concentrations measured at 4 water quality monitoring stations (Carillon, Lavaltrie, Bécancour and Quebec City) on the St. Lawrence River between January 1, 2009 and January 1, 2015. All samples collected were measured in milligrams of nitrogen per litre.
Step 5. Literature review
Existing suggested guidelines for the St. Lawrence River were found in the primary and grey literature. The examples below were the most applicable.
Chambers et al. 2009
Ideal performance standards for medium and large rivers draining agricultural regions in Canada were developed following 2 lines of data analysis. The first method involved approximating background nutrient concentrations by calculating 25th percentiles for total phosphorus and total nitrogen following the United States Environmental Protection Agency's (U.S. EPA) nutrient criteria methodology (U.S. EPA 2000a). The second method involved exploring relationships between total nitrogen and total phosphorus and either benthic or sestonic algal biomass expressed as chlorophyll a using stepwise multiple linear regression on log10-transformed data.
The results of the analysis produced a suggested total nitrogen guideline of 0.63 mg N/L for large rivers in the Mixedwood Plains. Chambers et al. also recommended an ideal performance standard of 0.100 mg N/L for total nitrogen for Prince Edward Island coastal waters. This value is 6 times lower than the concentrations currently seen at Quebec City.
Caveats
Rivers with drainage basins larger than 10 000 km2 were considered too large to be included in the analysis.
The methods deviated from the U.S. EPA approach by only using 25th percentiles for 2 reasons. First, given the amount of data in the freshwater database and the number of disparate sources of data, it was not possible to determine whether a site could be considered reference or low-impact. Second, the data came from rivers draining agricultural areas, signifying that they are impacted. The methods also deviated from the U.S. EPA method by analyzing data for large rivers collected for a 20‑year period between 1985 and 2005 rather than the recommended 10‑year period.
United States Environmental Protection Agency 2000b
The U.S. EPA's ecoregional nutrient criteria are intended to address cultural eutrophication. The criteria, or guidelines, are empirically derived to represent surface water conditions that are minimally impacted by human activities and protective of aquatic life and recreational uses.
This document sets out the U.S. EPA's recommended criteria for total nitrogen for rivers and streams in Nutrient Ecoregion VII (Mostly Glaciated Dairy Region) derived following procedures described in U.S. EPA 2000a. Reference condition criteria are based on the 25th percentiles of all nutrient data including a comparison of reference conditions for the aggregate ecoregion and the sub-ecoregions.
The analysis resulted in suggested total nitrogen guidelines for the whole ecoregion, as well as the sub-ecoregions closest to the St. Lawrence River (Table A.1).
Name | Suggested total nitrogen guideline (milligrams of nitrogen per litre) |
---|---|
Aggregate ecoregion VII | 0.54 (reported) |
Aggregate ecoregion VII | 0.54 (calculated) |
Sub-ecoregion 83 - Eastern Great Lakes and Hudson Lowlands | 0.48 (reported) |
Sub-ecoregion 83 - Eastern Great Lakes and Hudson Lowlands | 0.50 (calculated) |
Caveats
Nutrient criteria are derived for wadeable streams in the U.S. only, which generally have basins much smaller than 10 000 km2.
United States Environmental Protection Agency 2001
The analysis in U.S. EPA 2001 is the same as that in U.S. EPA 2000b, except that it encompasses Nutrient Ecoregion VIII (Nutrient-Poor Largely Glaciated Upper Midwest and Northeast) (Table A.2).
Name | Suggested total nitrogen guideline (milligrams of nitrogen per litre) |
---|---|
Aggregate ecoregion VIII | 0.38 (reported) |
Sub-ecoregion 58 - Northeastern Highlands | 0.42 (reported) |
Sub-ecoregion 58 - Northeastern Highlands | 0.26 (calculated) |
Step 6. Collection and analysis of data
The following guideline calculation techniques were applied to the data for the 4 St. Lawrence River water quality monitoring stations. The U.S. EPA recommends the use of 10 years of data for its analysis; however, there were only 6 years of data available for the St. Lawrence River at the time of calculation.
United States Environmental Protection Agency 2000a
To derive nutrient criteria, the U.S. EPA recommends using the 75th percentile of 10 years of monitoring data from reference or low-impact sites. In the absence of adequate reference data, the 25th percentile of all monitoring sites can be used (Table A.3).
For the 25th percentile analysis for the St. Lawrence River, all total nitrogen data for each station were combined into a single median value for each season. The 25th percentile of all station medians was then calculated for each season (Table A.3). The median value from the 4 seasonal 25th percentile values is considered the standard. This analysis generated a guideline of 0.65 mg N/L (Table A.4).
Monitoring station | Season | Number of records for total nitrogen | Minimum (milligrams of nitrogen per litre) |
25th percentile (milligrams of nitrogen per litre) |
Median (milligrams of nitrogen per litre) |
75th percentile (milligrams of nitrogen per litre) |
Maximum (milligrams of nitrogen per litre) |
---|---|---|---|---|---|---|---|
Carillon | Whole year | 79 | 0.400 | 0.490 | 0.530 | 0.600 | 1.070 |
Carillon | Spring | 31 | 0.440 | 0.499 | 0.550 | 0.625 | 1.070 |
Carillon | Summer | 17 |
0.400 | 0.470 | 0.490 | 0.510 | 0.670 |
Carillon | Fall | 16 | 0.434 | 0.494 | 0.510 | 0.607 | 0.770 |
Carillon | Winter | 15 |
0.470 | 0.533 | 0.560 | 0.624 | 0.897 |
Lavaltrie | Whole year | 69 | 0.540 | 0.780 | 0.900 | 1.070 | 1.860 |
Lavaltrie | Spring | 19 | 0.650 | 0.795 | 0.890 | 1.240 | 1.860 |
Lavaltrie | Summer | 15 | 0.540 | 0.615 | 0.750 | 0.825 | 0.900 |
Lavaltrie | Fall | 21 |
0.690 | 0.790 | 0.940 | 1.040 | 1.660 |
Lavaltrie | Winter | 14 | 0.930 | 0.973 | 1.045 | 1.158 | 1.440 |
Bécancour | Whole year | 69 | 0.370 | 0.610 | 0.780 | 1.060 | 1.470 |
Bécancour | Spring | 18 | 0.600 | 0.705 | 0.780 | 1.033 |
1.320 |
Bécancour | Summer | 17 | 0.420 | 0.490 | 0.610 | 0.720 |
1.420 |
Bécancour | Fall | 19 | 0.370 | 0.580 | 0.700 | 1.060 |
1.470 |
Bécancour | Winter | 15 | 0.750 | 0.840 | 1.010 | 1.125 |
1.390 |
Quebec City | Whole year | 96 | 0.330 | 0.540 | 0.630 | 0.735 |
1.020 |
Quebec City | Spring | 29 | 0.540 | 0.620 | 0.680 | 0.840 |
1.020 |
Quebec City | Summer | 30 | 0.400 | 0.480 | 0.520 | 0.660 | 0.890 |
Quebec City | Fall | 23 |
0.330 | 0.515 | 0.570 | 0.660 | 0.930 |
Quebec City | Winter | 14 | 0.610 | 0.653 | 0.720 | 0.780 | 0.960 |
Whole river | Whole year | 313 | 0.330 | 0.540 | 0.670 | 0.890 | 1.860 |
Whole river | Spring | 97 | 0.440 | 0.590 | 0.690 | 0.890 | 1.860 |
Whole river | Summer | 79 | 0.400 | 0.480 | 0.540 | 0.695 | 1.420 |
Whole river | Fall | 79 | 0.330 | 0.550 | 0.680 | 0.915 | 1.660 |
Whole river | Winter | 58 | 0.470 | 0.653 | 0.810 | 0.018 |
1.440 |
Monitoring station | 25th percentile of seasonal medians (milligrams of nitrogen per litre) |
---|---|
Carillon | 0.505 |
Lavaltrie | 0.855 |
Bécancour | 0.678 |
Quebec City | 0.558 |
Whole river | 0.645 |
The U.S. EPA also suggests using reference reaches to establish criteria. For this approach, it recommends using the 75th percentile of the nutrient frequency distribution for reference sites. As Carillon is the most upstream station,Footnote 7 it can be considered the reference site for the dataset, even though technically its water quality is not degraded, as it is situated at the mouth of the Ottawa River. Total nitrogen is at its lowest here until the water reaches Quebec City. The 75th percentile of Carillon's total nitrogen concentrations is 0.60 mg N/L (Table A.3)..
Step 7. Establishment of guidelines
In the absence of more detailed analyses to assess the relationship between nitrogen and aquatic plant growth in the St. Lawrence River, the analysis presented here helps point toward a total nitrogen guideline. Based on the recommended total nitrogen guideline values summarized in the table below, the values calculated using Canadian data for the area result in a total nitrogen guideline in the 0.60 to 0.65 mg N/L range (Table A.5). The mid-point of the range, 0.63 mg N/L, is the value used to calculate of the Nutrients in the St. Lawrence River indicator.
Value type | Guideline analysis reference | Recommended total nitrogen guideline (milligrams of nitrogen per litre) |
Notes or comments |
---|---|---|---|
Calculated value | U.S. EPA 2000a | 0.65 | 25th percentile of seasonal medians for all sites in an ecoregion |
Calculated value | U.S. EPA 2000a | 0.60 | 75th percentile of reference site (Carillon) |
Literature value | Chambers et al. 2009 | 0.63 | For large rivers in the Mixedwood Plains Ecozone |
Literature value | U.S. EPA 2000b | 0.54 | Streams in Aggregate ecoregion VII – Mostly Glaciated Dairy Region |
Literature value | U.S. EPA 2001 | 0.38 | Streams in Aggregate ecoregion VIII – Nutrient Poor Largely Glaciated Upper Midwest and Northeast |
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