Phosphorus levels in the offshore waters of the Great Lakes: data sources and methods, chapter 2


Data sources and methods

What are the data sources

Environment and Climate Change Canada's Great Lakes Surveillance Program collects the total phosphorus data used to calculate the status and trends in the offshore waters of the four Canadian Great Lakes. The indicator is calculated using the most recent data available for each lake.

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Total phosphorus ratings reported in the indicator are based on measurements taken in 2012 for Lake Huron and Georgian Bay and in 2013 for Lake Superior, Lake Ontario 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 2012 are used for Lake Huron and Georgian Bay. All other lakes use total phosphorus data from 1972 to 2013.

Each lake is monitored by Environment and Climate Change Canada every two years with several monitoring cruises typically conducted within that year. There are gaps in the data collected since the 1970s because of program changes, weather and mechanical problems with the ships used to collect the data.

How is this indicator calculated

Average open-water, spring-time total phosphorus concentrations in each lake are compared to water quality objectives. The status of phosphorus levels in the offshore waters of the Great Lakes were categorized as good (at or near the objective), fair (below the objective) or poor (above the objective).

For the trend analysis, linear regression was used to examine changes in mean total phosphorus levels over the entire length of the data record.

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Calculation of phosphorus status for the Great Lakes

Spring (late March to late May) phosphorus concentrations are compared to water quality objectives because they 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 as the maximum acceptable level
  • the long-term trends for phosphorus levels in the lake
  • the trends for related State of the Great Lakes indicators, especially abundance trends for algaeFootnote [1] and prey fishFootnote [2]

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 prey fish 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 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 divided into three basins. Least squares regression was used to examine changes in mean phosphorus concentrations over the entire length of the data record.Footnote [3]

There are gaps in the data collected since the 1970s because of program changes, weather and mechanical problems with the ships used to collect the data. These data gaps have no major impact on the statistical trend analysis because of the length of the phosphorus monitoring record for the Great Lakes.

What has recently changed

Since this indicator was last published in 2011, the starting year for the trend analysis changed from 1970 to 1972 due to uncertainty in the early laboratory results. As well, 1972 corresponds to the date when the first Canada-U.S. Great Lakes Water Quality Agreement was signed.

What are the caveats and limitations

The indicator reflects the overall state of phosphorus levels in the offshore waters of the Great Lakes and only includes data collected by Environment and Climate Change Canada.

The indicator reflects the state of water quality in the Great Lakes based on total phosphorus concentrations. These concentrations do not show the effects of spills or other transient events, unless these are frequent or long-lasting.

Comparing this indicator with similar indicators for rivers requires a degree of caution.

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The indicator excludes nearshore phosphorus levels because there are currently no nearshore water quality objectives.

Offshore data from the United States are not included in this indicator; however, data collected by Environment and Climate Change Canada in United States waters have been included for all lakes.

In lakes, suspended particles tend to settle out. Water quality for each Great Lake is determined by comparing average, spring-time offshore total phosphorus concentrations to the lake's water quality objective. This differs from assessing water quality for a river system, where total phosphorus concentrations are influenced by suspended particles in the water that increase during high-flow events. It is still reasonable to compare lake and river systems as long as the methods to determine the classifications are clear.

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