Nutrients in the St. Lawrence River

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

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. 

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

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 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

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 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

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.

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}

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

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 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)

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).

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).

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).

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.