Lakes Part 1: What Kind of Lake do You Have?

By Dylan Moesker

In Muskoka, we are surrounded by pristine lakes that offer a huge variety of recreational opportunities. Some lakes are deep and clear while others are shallow and murky, and we classify these lakes into different trophic levels. The term “trophic” refers to the amount of nutrients or plant growth in a lake. This article is the first in a four part series where I will explain how and why lakes are classified, what can directly influence trophic states, and what it all means for our lakes.

To start, lakes are classified into different states by a simple measure of the total phosphorus (TP) in a lake. Phosphorus is the limiting nutrient in aquatic ecosystems and generally controls plant and algae growth in most lakes. It is a naturally occurring element that is released from rocks and sediment, and all living plants and animals. Left in a natural state each lake will find a healthy balance between nutrient input and life in the lake. Human inputs can upset that balance.

Things like septic system effluent, agricultural runoff, fertilizers, and cleaning agents also contain phosphorus and if not disposed of properly, can lead to higher levels of phosphorus in our lakes – changing the trophic status.

In Ontario, lakes with less than 10 µg/L (micrograms per litre or parts per billion) of total phosphorus are termed oligotrophic. This generally implies that the lake is very clear and deep with minimal aquatic plants and algal blooms, as well as high levels of dissolved oxygen. Lake Joseph and Skeleton Lake are good examples of oligotrophic lakes.

Lakes containing between 10 – 20 µg/L of total phosphorus are termed mesotrophic. These lakes have some aquatic vegetation and can support an array of fish species. Bass Lake in Gravenhurst and Halfway Lake in Bracebridge are good examples of mesotrophic lakes.

Lastly, lakes over 20 µg/L of total phosphorus are termed eutrophic. These lakes have large areas of aquatic vegetation and are often subject to algal blooms, thus having lower levels of oxygen.  Barron’s Lake in Georgian Bay is a good example of a eutrophic lake. This lake is surrounded by wetlands.

Different lakes offer different recreational opportunities. Oligotrophic lakes like Skeleton Lake offer beautiful boating, swimming, and snorkeling recreation. Mesotrophic lakes offer fantastic fishing opportunities as these lakes are able to support a wide variety of fish. Eutrophic lakes are often the home of a number of different waterfowl, creating a great environment for bird watching.

So why are we concerned with the state of each lake? Many lakes are very sensitive to phosphorus inputs, and as I stated before, our cultural activities are changing trophic states in many lakes. This is regarded as a direct cause for large algal blooms in some lakes. When large blooms die off and fall to the bottom of the lake, they begin to decompose. This decomposition uses up much of the oxygen in the lower half of the lake, leaving minimal oxygen for fish and other aquatic species. This lack of oxygen can be potentially fatal to a large number of fish.

It is important to realize that algae are a naturally occurring plant that is completely necessary in our ecosystems. They only become an issue when high levels of phosphorus feed and essentially fuel larger algal blooms.

In next week’s article I will discuss internal loading – a different form of phosphorus input into a lake. If you would like to know more about the lake you live on or near, take a look at the Muskoka Water Web website for the Lake Data Sheet for your lake. For now, the only thing I ask of you is to be conscious about what goes down your drain. I feel it’s safe to say that none of us would enjoy swimming in a soupy-green mixture of water and algae.

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