What’s with all the algae this year?

What’s with all the algae this year?

Some small creeks in the Credit River Watershed looked a little green this spring. That’s because of algae! An overgrowth of algae, called an algae bloom can happen when we have a hot and dry spring. The last time there were algae blooms in the Credit River Watershed was in 2012. That spring, the temperature was 3.4 °C warmer than normal and we had about half as much rain. This spring, the temperature was 1.4 °C warmer than normal and we had about two-thirds the rain we normally get in spring. Algae is a normal part of the ecosystem but too much disrupts the balance in our waterways. Let’s take a closer look:

Algae bloom in Mississauga’s Cooksville Creek, May 2012.

Algae can reduce oxygen available to aquatic life

Aquatic life like fish and benthic macroinvertebrates (insects and other small organisms found on the stream bed) live in these creeks and they need oxygen dissolved in the water to survive. In Mississauga’s Sheridan Creek, for example, most aquatic life needs at least 4 milligrams per litre of dissolved oxygen to be healthy. If oxygen levels get too low for too long, fish and other animals in the water will either leave the area or die off.

Like all plants, algae make food for themselves through photosynthesis. They consume sunlight and carbon dioxide and produce oxygen. Overnight, plants use oxygen instead of producing it. Because of this, we see big variations in oxygen levels over the course of a day where levels are high during the day and low overnight. We pay close attention to low oxygen levels and our real-time monitoring stations help us to do that.

Algae bloom in Sheridan Creek in Rattray Marsh Conservation Area real-time water quality station on May 19, 2021.

Monitoring water quality in real-time

Our real-time water monitoring stations work around the clock collecting data on water quality across the watershed, including dissolved oxygen. Our station in Sheridan Creek at Rattray Marsh Conservation Area, for example, showed a big daily range in dissolved oxygen around May 20 when the algae bloom was at its peak. Fortunately, concentrations did not drop below the 4 milligram per litre threshold.

Dissolved oxygen and water temperature at the Sheridan Creek at Rattray Marsh real-time water quality station in May 2021. The algae bloom began around May 19th.

What’s next?

Our Water and Climate Change Science team is working to understand what causes algae blooms to occur. One way we are doing this is by measuring how much heat accumulates in water over time (growing degree days). In Cooksville Creek, we have been calculating growing degree days for the past 10 years. So far, the years with the highest growing degree days are also the years when algae blooms have happened. Understanding what causes these blooms may help us to mitigate their effects in the future through targeted restoration activities.

You can view live data from CVC’s 11 real-time water quality stations to explore water temperature in our local waterways. Learn how to access the data from real-time water quality stations.

By Lorna Murison, Coordinator, Water and Climate Change Science

6 Comment
  • Jane Pearson Sharpe says:

    Thanks for this. Really good info.
    Yesterday we noticed thousands of dead fish washing up on the Meadow Wood park beach. The water looked quite green.
    Are these fish a casualty of the algae lowering O2 levels in Lake Ontario?

    • Hi Jane, thanks for your question. Lake Ontario dissolved oxygen levels usually don’t go low enough to cause fish fatality. There may be other reasons, which could link to algae growth. Please report any sightings of dead fish to the Ministry of Natural Resources and Forestry at 1-800-667-1940 and any spills to the Spills Action Centre at 1-866-663-8477.

  • Bryan Smith says:

    What about the effects of runoff from fertilizer? Even if the monthly rain totals are low the greater number of brief heavy rain events (not so much this Spring) causes more runoff into the creeks with less rain soaking into the shoreline areas.

    • Hi Bryan, thanks for your question. Nutrients are needed for an algae bloom to occur, however, phosphorus is banned in urban fertilizers in Ontario and other nutrients are not the limiting factors for algal blooms. We don’t have enough data right now to say whether the levels this spring were unusual or how they relate to storm events.

  • Ann says:

    How does urban drainage contribute to these algae blooms? Does the CVC measure pollution levels and contaminants in the water?

    • Hi Ann, thanks for your question. There are number of contributors to algae growth and urbanization is one of them. CVC does monitor many contaminants of interest, however, at a sparse interval (once a month), so it is difficult to make the connection. Given the timing of these blooms the lack of spring freshet and warmer spring temperatures seem to be the main triggering factors.

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