Phosphorus levels in the offshore waters of the Great Lakes

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Note: ^[C] Interim phosphorus water quality objectives for Lake Erie basins are based on the recommended binational phosphorus targets: https://www.epa.gov/glwqa/recommended-binational-phosphorus-targets.
Source: Environment and Climate Change Canada (2020) and interim phosphorus water quality objectives for Lake Erie basins are based on the recommended binational phosphorus targets: https://www.epa.gov/glwqa/recommended-binational-phosphorus-targets.

Phosphorus is found in several products including detergents, fertilizers, manure, human waste and decaying plants. Phosphorus reaches rivers and lakes through erosion and leaching from urban areas, farmland runoff, municipal and industrial wastewater discharges, and air pollution. Although governments, municipalities, farmers and citizens are making efforts to reduce releases to water, phosphorus levels continue to be an issue in the offshore areas for 3 of the 4 Canadian Great Lakes.

For Lake Superior, spring average phosphorus levels in offshore waters have declined very slowly since 1972. Over that period, phosphorus levels have remained consistently below the lake's water quality objective of 5 micrograms of phosphorus per litre. However, the phosphorus levels status for Lake Superior is still considered Good, as the lake continues to support a healthy food web, including healthy plankton and preyfish populations.

In the offshore waters of Lake Huron and Georgian Bay, phosphorus levels were close to the objective of 5 micrograms of phosphorus per litre from 1972 until the late 1990s when they started to decline, dropping below the objective. In Lake Ontario's offshore waters, levels have declined from very high levels in 1972, dropped below the phosphorus objective of 10 micrograms of phosphorus per litre in the late 1980s, and continue to decline to historic lows. These 3 systems are given the designation of Fair. This designation means that rather than exceeding the objectives, phosphorus levels fall below objectives. This lack of offshore nutrients is likely having a negative impact on the lake's productivity. For example, open-water plankton, algae and preyfish populations in Lake Huron, Georgian Bay and Lake Ontario are showing signs of the impacts of these declines and low phosphorus levels are contributing to this stress.^{Footnote 1}

In recent years, there has been an increase in toxic and nuisance algae in Lake Erie that has been linked, at least in part, to phosphorus levels. The offshore waters of Lake Erie's eastern, central and western basins continue to have levels exceeding each basin's expected level, giving it a Poor status. While there has been an overall decrease in phosphorus levels from 1972 to 2019, recent changes are difficult to interpret because of the highly variable nature of the data. For example in 2019, a majority of samples taken from the western basin of the lake met the expected level. In contrast, in 2017 and 2013, samples from the same stations were above the expected level. Variations like these may be attributed to changes in the amount and frequency of precipitation from one year to another, which in turn affects runoff from nearby lands into the lakes.

Lake ecosystems are complex, and phosphorus levels in offshore environments can be different from levels measured in nearshore environments. While offshore phosphorus levels are reaching unprecedented lows in some of the lakes, which may be linked to invasive mussel species colonizing the lakes and intercepting the phosphorus, many nearshore regions of the Great Lakes are experiencing nuisance algae problems due to excessive concentrations of nutrients in these areas.^{Footnote 1}

Total phosphorus ratings reported in the indicator are based on spring measurements taken in 2017 for Lake Huron and Georgian Bay, 2018 for Lake Ontario, and in 2019 for Lake Superior and the western, central and eastern basins of Lake Erie.

The total phosphorus objectives used in this indicator are the interim substance objectives for total phosphorus concentration in open waters published in the 2012 Great Lakes Water Quality Agreement.

For the trend analysis, total phosphorus data from 1972 to 2017 are used for Lake Huron and Georgian Bay, from 1972 to 2018 for Lake Ontario, and from 1972 to 2019 for Lake Superior and the western, central and eastern basins of Lake Erie.

The Great Lakes are monitored by Environment and Climate Change Canada on a schedule that permits the assessment of status and trends. In general, Lake Ontario and Lake Erie tend to be monitored every year or every 2 years, and Lake Huron, Georgian Bay and Lake Superior are monitored every 2 to 3 years. Sampling is typically conducted in both spring and summer. There are gaps in the data collected since the 1970s due to program changes or operational issues, such as weather and mechanical problems with the ships used to collect the data.

Calculation of phosphorus status for the Great Lakes

Spring (late March to late May) phosphorus concentrations are compared to water quality objectives because they typically represent the annual maximum concentration of phosphorus in the lakes. The status categories for this indicator are determined by comparing the most recent average spring total phosphorus concentration to:

• the water quality objectives
• the long-term trends for phosphorus concentrations in the lake
• the trends for related State of the Great Lakes indicators,^{Footnote 4} especially abundance trends for algae and preyfish, as these reflect the lake's ecological health

Phosphorus levels categories are defined as:

• Good (at or near objective), if concentrations are below or close to the lake's objective, and no long-term changes to the lake ecosystems are observed
• Fair (below objective), if concentrations are below the lake's objective and recent deterioration in algal, zooplankton and preyfish populations caused by low phosphorus concentrations are observed
• Poor (above objective), if concentrations are above the lake's objective

Trend Analysis

To calculate the long-term trends, the data are restricted to surface water samples collected at offshore locations, because offshore waters are less influenced by local pollutant discharges than nearshore, shallow waters. For Lake Huron and Georgian Bay, these samples are taken from stations with depths greater than or equal to 50 metres. Samples taken from stations with depths greater than or equal to 100 metres are used for Lake Ontario and depths greater than or equal to 150 metres for Lake Superior. Lake Erie is shallow relative to the other lakes and is instead divided into its 3 basins. Least squares regression is used to examine changes in mean phosphorus concentrations over the entire length of the data record.^{Footnote 1} Recent trends are additionally assessed on a lake-by-lake basis.

Although some gaps exist in the data collected since the 1970s, these have no major impact on the statistical trend analysis because of the length of the phosphorus monitoring record for the Great Lakes.