Lake McGlashan, located in Val-Des-Monts, Quebec is a very beautiful clear, deep freshwater lake. It is more than thirty meters deep at its deepest point and connected to it’s neighboring water body Lake Girard through a small stream.
It is classified as a young, oligotrophic lake, meaning that the water is very clear, has sparse plant growth, and low nutrients.
Interestingly, at different depths the lake has distinct water chemistry. For example, the deeper you go the cooler the temperatures become because there is little to no sunlight to warm the water at great depths. However, at Lake McGlashan after going down 15 meters from the surface, the water temperature becomes constant at 4 degrees Celsius.
As shown on the graph above, the top 15 meters of the water changes depending on the season. In August and September (between 20-25 degrees Celsius), the temperatures in the top layer is quite warm, but in November it is around 6 degrees Celsius. This top layer is called Epilimnion and the 15 meter mark which acts as a boundary between the top and bottom layer is called Thermocline. The bottom layer where the temperature becomes constant across the board is called Hypolimnion. The formation of layers, called stratification often occurs in deep lakes. Temperature isn’t the only parameter that is different between layers. Check out the dissolved oxygen readings below.
You can see above the thermocline, dissolved oxygen is very high; between 8-13 mg/L between August to November. But after the thermocline 15 meter boundary, the water becomes anoxic meaning no oxygen is found at the bottom layer of the lake.
Why is there no oxygen at the bottom of the lake?
Organisms like fungi and bacteria, like to break down organic matter that accumulate at the bottom of the lake. They use up all the dissolved oxygen for microbial decomposition and there is no atmospheric oxygen or oxygen from photosynthesis there to replace it.
Lake McGlashan does not support large fish populations, but you can find Sunfish, Bass, and Minnows in the top layer of the water. You will not find fishes at the hypolimnion (bottom layer) because it does not have the oxygen fishes need in order to breathe.
What about conductivity?
The conductivity also changes drastically between the different layers. In the top Epilimnion layer, there are few ions (approximately 100 µS/cm), but at the bottom layer the conductivity spikes to about 900 µS/cm! The reason being the minerals and ions partly come from the lake bottom rocks and soil. This enriches the conductivity at the bottom of the lake. Although, spikes in conductivity reading can be sometimes be an indicator of a pollution event, in this case particles like salts, minerals, and metals accumulating at the bottom is natural for this lake.
Last but not least….pH
The graph shows that the lake becomes slightly acidic as the season becomes cooler. But in general, the pH does not change drastically because there are particles in the water called buffers that keep the pH relatively within range. You’ll notice at after the thermocline there is little variation in the pH at the lower depths. This is again due to lake stratification where there are distinct layers in the water that do not mix.
Lake stratification occurs because the variation in temperature in the different depths of the water have different densities. High density water at around 4 degrees Celsius separates from the low density water with high temperature. However, when the seasons change and the water becomes uniformly the same temperature, all the layers of the water can mix, which is called a ‘turnover’.
Think about what kind of chemistry your water body has the next time you are in the water.