It used to be that swimming in Muskoka in late summer meant a dip in cooler water. But things are changing, and the lakes are getting warmer and staying warm longer than they used to. While that’s good news for swimmers, it may be harming fish and opening the doors for some nasty bacteria.
As with many impacts of climate change, it’s hard to see the incremental change unless you look closely. We only see the big, dramatic impacts when things suddenly shift for the worse — the massive wildfires, the complete disappearance of ice shelves and glaciers, the loss of a signature species.
In Muskoka, many well-informed people are looking extremely closely at the incremental changes, and they are alarmed at what they see.
A freshwater scientist we know cottages on a small lake in Muskoka and was concerned by the high temperatures on his lake. In mid-August the surface temperature of his lake was typically around 25 degrees; this year, the temperature in the third week of August was 31.5 degrees! He had to go down more than 1.25 metres before he could find water below 25 degrees.
So why is that a problem? After all, can’t species that need cold water just go deeper?
It’s not that simple. The problem is lack of oxygen.
Oxygen in water
As we all know, water is H2O — two hydrogen atoms and one oxygen atom. But there is also dissolved oxygen in the water, and it’s that dissolved oxygen that fish and other animals need.
In a typical summer, oxygen levels at the bottom of the lake drop through the season as the available oxygen is depleted by the gradual decay of organic matter. By August in our friend’s lake, at the bottom it’s typically down around 2 mg/l. That’s much too low for warm water fish like perch and bass, which need oxygen levels of 5 mg/l or more. So they hang out in the middle and upper layers of the lake.
The problem is that warm water naturally holds much less oxygen than cold water. So as the water at the surface heats up, the amount of available oxygen there also declines. The result is that the lake develops “dead zones” — areas where there is just not enough oxygen at any depth to support life.
Other creatures aren’t as sensitive to changes in oxygen levels but are heavily affected by changes in temperature. One of the most significant is daphnia, tiny freshwater shrimp that are found in water bodies throughout Muskoka.
These amazing creatures feed on algae, which they get by filtering the water through their bodies. It’s estimated that the entire volume of Lake Muskoka is filtered by daphnia every 10 to 14 days! Without daphnia, our lakes would be thick soups of algae.
But daphnia are temperature sensitive. If the water gets too warm, they become less active and eventually start to die off.
More algae and bacteria
Even without the loss of daphnia, warm water is a fantastic place for algae to grow. There are millions of species of algae, and many of them do nothing worse than make it unpleasant to swim.
But there are a handful of species — most notably blue-green algae, or cyanobacteria — that pose a serious risk to human health. As cyanobacteria decay, they release toxins into the water. When the concentrations get high enough, it can be deadly to anyone — including wildlife who drink it.
Prior to 2005, blue-green algae was unknown in Muskoka. That summer, Three Mile Lake experienced the first blue-green algae bloom the region had ever seen. Since then, it’s become much more common. Between 2019 and 2021, at least 19 blue-green algal blooms were recorded, some of them even on larger lakes that should have been immune.
Even other species of algae can be harmful to the lakes if there’s too much of it. As algae dies off, it sinks to the bottom of the lake where it decays. And what does decaying vegetation consume? Yep, oxygen. Too much decaying vegetation, and oxygen levels get even lower.
What can be done?
This is a local part of a global issue, but even as individual property owners there are things we can do to help.
- Keep excess nutrients out of the water. Fertilizer and faulty septic tanks are two big sources of excess nutrients. As nutrients run into the lake, they encourage the growth of plants including algae.
- Naturalize the shoreline. A buffer strip along the water’s edge will absorb nutrients, and also shade the water and help to cool it.
- Support groups that are working on solutions. The Friends of the Muskoka Watershed, the Muskoka Conservancy, and the Muskoka Watershed Council are local groups that are actively working on solutions to help watershed health. They need our support.
- Fight climate change. It’s a big, complicated problem, but doing nothing is no longer an option.
Photo by Jonathan Larson