Turquoise Mystery Lake - Part 1 of 2
Today we’re off to investigate a pair of very odd lakes in Quebec. They’re right next to each other, but while one looks pretty nondescript, the other is positively mediterranean. Where does its strange colour come from, why only one of the two lakes, and why does it seem like it comes and goes with the seasons? Those ate questions that will take us back to the ice age, and through some unexpected chemistry.
EPISODE NOTES
This episode represents the closing of a very large loop. One of the first shoots I ever did for this project was at this very lake, two and a half years ago. Back then, these portrait-mode videos were serving more as little snippets meant to entice people to the forthcoming main event. Over time, they evolved to be the main event themselves as it was clear they were getting the most attention.
But that meant one of the first Canadianimals videos I posted to Instagram and TikTok was ‘Turquoise Mystery Lake’ in September of 2022, promising a longer and more in-depth look at why this lake looked the way it did. Well… two and a half years (and several more shoots at that lake) later, I can finally say I fulfilled that promise.
TRANSCRIPT
Deep in the forests of rural Quebec are a very weird pair of lakes.
One of them looks pretty unassuming. But its neighbour can almost glow with a mediterranean turquoise.
So they sit in pretty stark contrast, even though they’re only separated by this thin strip of land.
In fact they make such a strange sight from the air that pilots use them as a way marker.
And what makes the more colourful of the two even stranger… that colour seems like it can switch on and off. Depending on the weather and the season, it can sometimes look like any other lake.
So: what is it about the lake that gives it that striking colour? And: why only this lake?
A big clue appears as soon as you break the surface. The bottom of the lake is almost uniformly covered by this whitish-grey substrate.
That substrate is marl - a chalky, clay-like blend of primarily calcium carbonate with whatever local conditions add to the mix.
And in these quantities, it has a few dramatic effects on lakes.
For one, it makes the water intensely alkaline.
By covering everything, it also makes the lake less productive - which in turn makes the water clearer.
And it absolutely suffuses that water with calcium carbonate.
It’s those two effects - clear water, filled with calcium carbonate - that start to explain the wild colour - and why it only happens at certain times.
How? Because of ‘retrograde solubility’, of course!
So calcium carbonate has a bit of a backwards relationship with temperature.
When the water’s cold - below around 12 degrees celsius - the calcium carbonate stays dissolved in the water.
But as the temperature rises, unlike most substances, calcium carbonate actually becomes less soluble, and starts to precipitate. Suddenly the lake is full of tiny particles suspended in the water.
And all those tiny particles reflect and refract incoming sunlight. This has the effect of reinforcing blue wavelengths.
Blue are the shortest wavelengths in the visible spectrum. and that makes them more likely to be reflected by particles - same reason the sky is blue.
Add to that the fact that water already preferentially absorbs longer wavelengths and the clear water of the lake allowing more light to penetrate deeper, and you get this. But - only when it’s warm enough.
In fact something as simple as cloud cover can affect the intensity as well. If the clouds are already softening and scattering the light, you’ll get less of it directly into the water to reflect.
OK, so that’s one of our questions answered: why does this lake look mediterranean? But that leaves: where did this marl come from, and why, of our two lakes, does only one seem to be affected?
The most likely source of marl in this region is, in fact… glaciers.
As they make their way south across North America in the last ice age, they might encounter deposits of limestone along the way. Being a relatively soft rock, the limestone eventually gets ground down by the action of the glacier - from boulders to rocks to pebbles to powder.
It becomes part of the glacial till pushed ahead of the ice until it gets deposited in a convenient depression, like a lake bed. And it’s possible that lake bed itself could have been carved out by the same glacial action.
That would make this ‘glacial marl’. It’s also possible it could be ‘Lacustrine Marl’ from existing limestone deposits on the lakebed.
But either way it’s only half an answer. When we look under the surface of the lake we see, for example, these trees covered with a thick crust of marl. And we can be pretty sure they fell in the lake after the last ice age. So, glaciers didn’t deposit any of this marl on them. There must be an ongoing process here.
Actually, more than one. As we’ve learned, temperature lets the marl precipitate in and out of the water, and when it precipitates out it can settle and build up.
But we have the lake’s plants to thank too. The act of photosynthesis actually produces more marl.
As the plants take carbon dioxide out of the water, they disrupt the carbonate equilibrium. This also precipitates calcium carbonate out of the water solution, further building it up - even on the plants themselves.
And it’s a feedback loop. Every surface being covered in marl restricts the productivity of the lake. But for the plant species that can deal with the extreme environment, that makes the water clearer, letting in more sunlight, meaning more photosynthesis, meaning more marl… and so on and so on.
That helps us answer why you can have two lakes right next to each other, with only one of them having this colour phenomenon. It could start with something as simple as the glacial till or initial marl deposit being in one basin but not the other. Direction of water flow also helps, in this case flowing into the marl lake so the other one doesn’t get any marl outflow.
But as we’ve seen, once the process starts, it’s self-reinforcing. So a lake with this tendency will become more extreme over time. And a lake that starts with more productivity, less marl, or both, won’t ever have that snowball effect.
And that’s how you end up with our bizarre pair of lakes. And next time we’ll find out what that means for both the animals and plants that brave this extreme environment - and us.