The Science Behind the Seasons

It's official! Yesterday was the last day to have all ice shacks off the lake which only means one thing: spring is coming! Warmer temperatures mean melting ice and open water season! But did you know that the temperature above water has more of an impact on lakes than ice or no ice? There are a few things that the temperature of the season does to affect water chemistry and population dynamics of the ecosystem! 

First, we'll talk about water chemistry. As a part of our Water Quality Monitoring Program, the LSWC measures general water quality parameters like pH, temperature, conductivity, dissolved oxygen, total and dissolved nutrients, fecal coliforms and total suspended solids. This program helps gather important data about water chemistry throughout the open water season and on lakes during the winter months. There are a few parameters that are affected by the seasons, or more directly, the average air temperature. Dissolved oxygen is the measurement of how much oxygen is available in the water. This is important for all of the species that live in the waterbody or rely on it in some way. Colder water holds more oxygen than warmer water, which means that colder conditions can be better for many species! Some ways that oxygen can be mixed into the water include wave action and wind! But in the winter, when there is a layer of ice preventing air circulation, lakes can experience lower levels of oxygen as what is 'stored' from lake mixing gets used up during the ice cover months. 


Speaking of mixing, the change in water can also be attributed to something called lake stratification. Lake stratification is what happens, particularly in warmer weather, when a lake forms distinct layers within the water column (between the top and bottom of the lake). In warmer months especially, these layers become zones where different species tend to call home. Differences between fish species can have fish spending time at different layers in the water column, primarily because of their temperature needs. This mixing is initiated seasonally. Check out the brief diagram of what to expect for water temperatures through the year!

Lake mixing is triggered by the temperature of the season. Following the graphic, in the spring, the water temperature hovers around 4°C until the air temperature helps to warm the upper layer of water. The temperature of the land and air warm the temperature of the water and help to melt the layer of ice off the lake. From there, the water continues to warm. Depending on the average temperature of the summer months, duration of hot stretches and other environmental factors, the uppermost layer of the lake will be the warmest and the bottom will be colder. This will vary based on precipitation during the season, depth of the lake and other human factors like surrounding land use and thermal pollution on the waterbody. As the seasons shift to fall, the water temperature starts to shift back to 4°C in preparation for winter conditions. In winter months when the ice has developed, the warmest water is actually at the bottom of the lake! This will typically be 4°C, but varies depending on the depth of the lake and other environmental factors. In the winter, we tend to see a defined thermocline, which is a distinct temperature layer in the water column. 

Now that we know about lake stratification and the layers of the lake, we can better understand where different aquatic organisms typically spend their time! Depending on their temperature needs, or the temperature needs of their food source, that's where they'll be! 

By: Kate Lovsin