Phosphorus levels in the offshore waters of the Great Lakes

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Phosphorus is an essential plant nutrient. When phosphorus levels are too high or too low, they can have harmful impacts on a lake's food web. It is one feature of the health of the offshore waters of the Great Lakes. Conditions may be very different in nearshore areas.

Key results

  • Phosphorus levels are too high in the offshore waters of Lake Erie. While there has been an overall decrease in phosphorus levels over the past 40 years, recent levels vary greatly from year to year. They are no longer showing a declining trend.
  • Phosphorus levels are too low in the offshore waters of Lake Ontario, Lake Huron and Georgian Bay. Since 1972, levels have decreased to a point where preyfish populations are declining.
  • Offshore phosphorus levels in Lake Superior are at the level they should be and not changing.

Status and trends of phosphorus levels in the offshore waters of the Canadian Great Lakes, 1972 to 2013

Map of  Canadian Great Lakes showing the status of phosphorus levels in the offshore waters - long description below.
Long description

The graphic shows the status of phosphorus levels in the offshore waters of the Canadian Great Lakes (Lake Superior, Lake Huron, Georgian Bay, Lake Ontario and the western, central and eastern basins of Lake Erie). Phosphorus levels in offshore areas of Lake Superior are classified as good. In Lake Huron, Lake Ontario and Georgian Bay, phosphorus levels have gotten too low in offshore waters and the lakes are given a classification of fair. Levels are above water quality objectives in the eastern, western and central basins of Lake Erie and are classified as poor. Since 1972, phosphorus levels have decreased in all the lakes, except Lake Superior where there has been no change.

Data for this map
Status and trends of phosphorus levels in the offshore waters of the Canadian Great Lakes, 1972 to 2013
Lake Phosphorus water quality objective (micrograms of phosphorus per litre) Spring phosphorus level (micrograms of phosphorus per litre) Year of most recent measurement Status for offshore waters Long-term trend (1972 to 2013)
Superior 5 2.1 2013 Good No trend
Huron 5 4.2 2012 Fair Decreasing
Georgian Bay 5 2.0 2012 Fair Decreasing
Erie - western basin 12Footnote [A] 25.4 2013 Poor DecreasingFootnote [B]
Erie - central basin 6Footnote [A] 15.8 2013 Poor DecreasingFootnote [B]
Erie - eastern basin 6Footnote [A] 13.4 2013 Poor DecreasingFootnote [B]
Ontario 10 6.0 2013 Fair Decreasing

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How this indicator was calculated

Note: Water quality in the offshore regions of a lake is considered good when it can support a healthy food web. Lakes where phosphorus levels are below objectives and negative impacts to the offshore food web have been observed are given a classification of fair. Where phosphorus levels are above a lake's phosphorus objectives, lakes are classified as poor. Long-term trends to explore how phosphorus levels in the offshore areas of the lakes have changed since 1972 were assessed using linear regression.
Source: Environment and Climate Change Canada (2016) Great Lakes Surveillance Program.

More information

Phosphorus levels remain an issue in the offshore areas of 3 of the 4 Canadian Great Lakes. Only in Lake Superior are plankton and preyfish populations healthy, resulting in a good phosphorus status in of the lake's offshore waters.Footnote [1],Footnote [2]

For Lake Superior, spring average phosphorus levels in offshore waters have declined very slowly since 1972. Over the 40-year period, phosphorus levels have remained consistently below the lake's water quality objective of 5 micrograms of phosphorus per litre.

In the offshore waters of Lake Huron and Georgian Bay, phosphorus levels were close to their phosphorus objective of 5 micrograms of phosphorus per litre from 1972 until the late 1990s when they started to decline. For 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. 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],Footnote [2] These 3 systems are given the fair designation.

In recent years, there has been an increase in toxic and nuisance algae in Lake Erie that may be linked 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. Phosphorus levels decreased from 1972 to 2013. Recent changes are difficult to identify because of the highly variable nature of the data. For example in 2012, a majority of samples taken from the western basin of the lake met the expected level. In contrast, in 2011 and 2013, samples from the same stations were above the expected level.Footnote [3]

While offshore phosphorus levels are reaching unprecedented lows in some lakes, many nearshore regions of the Great Lakes are experiencing nuisance algae problems due to excessive concentrations of nutrients in these areas.Footnote [3]

About the indicator

What does the indicator measure

This indicator reports total phosphorus levels in the offshore waters of the 4 Canadian Great Lakes.

The indicator assumes water in the Great Lakes would never be above phosphorus water quality objectives in the absence of human development. It provides information on how human activity contributes to phosphorus levels in lakes.

A lake's phosphorus status is determined by comparing spring total phosphorus levels to its water quality objectives and the health of the lake's food web. Failure to meet a water quality objective for phosphorus suggests a greater risk to the health of the lake ecosystems.

Why is this indicator important

Clean freshwater is an essential resource. It protects the biodiversity of aquatic plants and animals. We use it for drinking, manufacturing, energy production, irrigation, swimming, boating and fishing. Degraded water quality damages the health of freshwater ecosystems and can disrupt economic activities, such as fisheries, tourism and agriculture. When phosphorus 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 can result in not enough plant growth to support a lake's food web, which can result in a collapse of the fishery.

This indicator is used to provide information about the state of the Great Lakes and Canadian environment. It is also used to assess progress towards implementing the 2016-2019 Federal Sustainable Development Strategy.

What are the related indicators

The Restoring the Great Lakes areas of concern indicator assesses progress towards the restoration of Canada's 17 Great Lakes areas of concern.

The Nutrients in Lake Winnipeg and the Nutrients in the St. Lawrence River indicators report the status of total phosphorus and total nitrogen levels in those two ecosystems.

The Water quality in Canadian rivers indicators rank water quality at monitoring sites across Canada where human activity is likely to impair water quality in its ability to support aquatic life.

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Pristine lakes and rivers

This indicator supports the measurement of progress towards the following 2016-2019 Federal Sustainable Development Strategy long-term goal: Clean and healthy lakes and rivers support economic prosperity and the well-being of Canadians.

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