Most Finnish lakes have brownish water. The colour is a result of various substances contained in the water. This does not mean that the water is dirty. However, the water of a brown-water lake warms up easily during the summer months. In addition, these kinds of lakes contain a lot of nutrients, which are substances needed by plants to grow. This results in rapid algae and plant growth. These kinds of lakes are called eutrophic lakes.

The water of a clear lake contains fewer nutrients. It also warms up more slowly during the summer months. Because of this, there is less algae and aquatic plant growth. Clear lakes are called oligotrophic lakes.