Temporal Patterns

How is water quality in the Western Durham nearshore changing over time?

This is a work in progress! The province is monitoring certain nearshore regions approximately every 5 years to try to assess the water quality. Our results fill in the gaps between the years the province monitors the area. However, there is not a long enough record to determine whether conditions are improving or worsening with time, yet.

Since there have not been a lot of management changes to the watersheds in the timeframe of this program, any differences we see are likely due to climatic or biological (e.g. mussel/goby fish) influences. However, there were some milestones at the Duffin Pollution Control Plant (e.g. 2010: Initiation of Stage 3; 2012: Stage 3 complete, Stage 1 and 2 phosphorus treatment upgrades complete).  Preliminary water quality analyses suggest that there might be responses observed in nearshore water quality.

The TRCA monitoring program has measured water quality from 2007 to the present. During the timeframe of the program, climate has been variable. In some years, precipitation has been greater than the 30 year average (“wet” year), while in other years, precipitation has been lower than the 30 year average (“dry” year). On the graph below, the first bar is the 30 year average between 1971 and 2000. The red line across the graph is just showing the height of that bar so it’s easy to compare with the years of the nearshore monitoring program.

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 1: Annual precipitation in comparison to the 30 year average. Data from Environment Canada – Atmospheric Environment Service.

If we consider only the lake sampling locations that were consistent from 2007 to 2012, we can compare the changes that occur annual and seasonally, and try to determine whether any relationships are observed with “wet” and “dry” years. Since all ice-free seasons were not sampled in 2010 and 2011, we have excluded them from the comparison.

Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 2: A map illustrating the location of sites that are consistent within the entire monitoring program. “Map Source: Data provided by Ontario Ministry of Natural Resources © Copyright: 2005 First Base Solutions Inc. All Rights Reserved”.

The graphs below show the range in nutrients observed over the years, and within each season (spring, summer, fall). A diagram in the Nutrient Sources section (Figure 3) defines what all the lines on the graphs below mean.

Annual Variation

Total Phosphorus:

  • Concentration ranges are variable from year to year.
  • Median total phosphorus concentrations tend to be lower in dry years.
  • Median total phosphorus concentrations tend to be higher in wet years.
  • Statistical differences exist between the median concentrations in the dry years versus the wet years.
Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 3: Annual total phosphorus concentration differences. The water quality objective recommended by the International Joint Commission for open waters (10 µg/L) is illustrated. The maximum detection limit (DL) is 6 mg/L due to sampling in 2007, however, the detection limit improved to 3 mg/L in 2008. Years that are wet compared to the 30 year annual average have a (W) under the year. Similarly, if the precipitation is dry or average compared to the 30 year annual average, then the years will have a (D) or an (A) under them.

Soluble Reactive Phosphorus:

  • Concentration ranges are variable from year to year.
  • Majority of samples are below the 2 mg/L detection limit.
  • No statistical differences exist between the median concentrations in the dry years versus the wet years.
Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 4: Annual soluble reactive phosphorus concentration differences. The maximum detection limit (DL) is 4 mg/L due to sampling in 2007, however, the detection limit improved to 2 mg/L in 2008. Years that are wet compared to the 30 year annual average have a (W) under the year. Similarly, if the precipitation is dry or average compared to the 30 year annual average, then the years will have a (D) or an (A) under them.

Nitrate + Nitrite:

  • Concentration ranges are variable from year to year.
  • Median concentrations are not statistically different from year to year.
  • No statistical differences exist between the median concentrations in the dry years versus the wet years.
Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 5: Annual nitrate + nitrite concentration differences. Years that are wet compared to the 30 year annual average have a (W) under the year. Similarly, if the precipitation is dry or average compared to the 30 year annual average, then the years will have a (D) or an (A) under them.

Seasonal Patterns

There are a number of seasonal patterns that emerge when looking at the water quality of overlapping sites. In general, between seasons there appears to be greater variability in spring and fall. Some of the large ranges observed in the summer are actually only due to one or two summer samples with high concentrations.

Total Phosphorus:

  • Summer and fall median concentrations appear to be lower in dry years.
  • Median concentrations in the wet years are statistically different than the other years with the exception of spring 2007.
Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 6: Seasonal concentration ranges for total phosphorus. The red dashed line represents the International Joint Commission water quality objective for open waters (10 mg/L). The solid black line represents the maximum detection limit (6 mg/L in 2007; 3 mg/L from 2008-2012). Years that are wet compared to the 30 year annual average have a (W) beside the year. Similarly, if the precipitation is dry or average compared to the 30 year annual average, then the years will have a (D) or an (A) beside them.

Soluble reactive phosphorus:

  • More variation is observed in spring and fall.
  • Almost all samples are below detection; all samples from the summers of 2007 and 2011, and the spring of 2010 are below detection.
  • No statistical difference between yearly spring and summer data.
Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 7: Seasonal concentration ranges for soluble reactive phosphorus. The solid black line represents the maximum detection limit (4 mg/L in 2007; 2 mg/L from 2008-2012). Years that are wet compared to the 30 year annual average have a (W) beside the year. Similarly, if the precipitation is dry or average compared to the 30 year annual average, then the years will have a (D) or an (A) beside them.

Nitrate + Nitrite:

  • Summer and fall median concentrations appear to be statistically higher in wet years.
  • Highest concentrations observed in spring and fall (this is when discharge from the creeks is greatest).
Lake Ontario Waterfront nearshore monitoring water quality patterns
Figure 8: Seasonal concentration ranges for nitrate + nitrite. Years that are wet compared to the 30 year annual average have a (W) beside the year. Similarly, if the precipitation is dry or average compared to the 30 year annual average, then the years will have a (D) or an (A) beside them.

Although there are not enough years of data to say that a trend exists, we do see some similarities and differences between wet and dry years. There also appears to be seasonal differences in concentrations.

Take Home Messages

  1. Concentration ranges differ year to year, no apparent relationship in variability and wet/dry years.
  2. Higher nutrient concentrations are observed in spring and fall.
  3. Although there are not enough years monitored to say there is a trend, it appears that concentrations might be lower in “dry” years than in “wet” years.