Lakes Part 3: Why Are Some Lakes Tea-stained?

By Dylan Moesker

As we continue to learn more about the types of lakes found in Muskoka, tea-stained lakes still remain a misunderstood topic. These lakes are misunderstood since they are naturally darker in colour and often have a name like Tea, Black, Brandy or Dark Lake.

If you recall, we have been discussing the different trophic status of lakes and how phosphorus levels can affect this status. This week’s article will examine tea-stained lakes, what causes them, and how they are different.

Tea-stained lakes (also known as dystrophic lakes) are often shallow bodies of water with a large amount of wetland area in their watershed, which provides an increase in decaying matter. Much of the production in these lakes comes from bacteria.

Why are these lakes brown? Think of a wetland as a ‘tea bag’. Water running through the wetland washes out the tannins and the Dissolved Organic Carbon from the wetland plants.

Dissolved Organic Carbon is different than the previously discussed dissolved organic matter because it is broken down by a good type of bacteria rather than just through natural decomposition.

When organic carbon is abundant, it causes bacteria to become more phosphorus dependant. As this happens, bacteria will consume the available phosphorus before algae can develop often leading to a decreased accessibility of phosphorus to algae.

This is the reason why tea-coloured lakes are much less sensitive to phosphorus inputs than an oligotrophic lake. Higher levels of phosphorus may enter a tea-coloured lake and the bacteria will use it up before algae can, whereas if the same amount of phosphorus was put into an oligotrophic lake, the lack of bacteria would allow algae to feed on the phosphorus, creating the dreaded algal blooms.

Believe it or not, tea-coloured lakes are very similar to oligotrophic lakes in that they both have very low productivity (minimal plant growth), though visually, these two lakes are complete opposites. Oligotrohpic lakes are very deep and clear, having high oxygen levels and low organic matter whereas tea-coloured lakes are not nearly as deep or clear, having plenty of organic matter and a brownish tinge through the entire water column.

This is not to say that tea-coloured lakes are invincible to algae blooms. In the past, we have seen the green soupy mix of algae in some organic lakes, but they are generally much less sensitive and susceptible to algae blooms. Essentially, higher total phosphorus in a tea-coloured lake does not necessarily mean a higher occurrence of algal blooms.

Next week in the finale of this four part series, I will discuss the blue-green algae known as Gloeotrichia. This type of algae is becoming more common in Ontario, and seems to bend all the rules of how algal blooms are formed. For now, enjoy your summer and remember that summer isn’t half over, it has only half begun!

Past articles are available in this blog under the Watershed Notes Articles category or under Past Articles in the Resources section.