By Lizzy Gallagher 7 Lakes Alliance Assistant Lake Scientist
The Compass and Great-Great, 7 Lakes Alliance’s trusty water-quality sampling boats, shuttle team members to and from sampling sites on all seven ponds throughout the summer. But did you know we keep at it long after the swimming season ends?
Just as the beautiful fall colors signal a change in seasons in forests, lakes undergo seasonal changes, as well. One important change we look for during our fall monitoring is the shift from stratified to mixed waters. The lakes can become stratified during the summer, meaning they have distinct layers of water with different temperatures. If you plunged into a lake last summer, you’ll remember the top layers were warm. As you dove deeper, the water got cooler.
As these layers warm throughout the summer, they stay near the top because warmer water is less dense than cooler water. Once this pattern is established, it carries throughout the entire water column to the bottom of the lake, which can limit the transportation of dissolved oxygen. During periods of stratification, the bottom of the lake can become anoxic (without oxygen). That creates conditions that allow phosphorus to escape from the lakebed. Excess phosphorus in the water column is a problem we all want to avoid, as it can lead to algal blooms.
As top waters cool in the autumn, they eventually mix with the rest of the water column. The term for complete mixing is lake turnover. Once lakes turn over, dissolved oxygen is evenly distributed. Sampling late into the fall lets us capture
the timing of this change. This is vital for two reasons. First, because lake turnover means the layers of water are completely mixed, algal levels can increase during and after turnover because there is a release of phosphorus during the process. On some lakes, this can result in a fall algal bloom.
Additionally, it is important to know when the anoxia on the bottom of the lake has ended, as it will have implications for the next summer in terms of phosphorus levels.
In our deeper lakes, such as Great and Long ponds, Messalonskee and Salmon lakes, the timing of fall turnover varies. At some points in time, one part of the lake may have mixed while another has not. As of the end of October, this was the case for Great Pond and Messalonskee Lake. Long Pond continued to hold onto its stratified layers; that is, neither sampling site in Long Pond has fully mixed. Salmon Pond has mixed, and there is currently a resulting algal bloom.
We continue to monitor these sites for as long as the weather holds. At some point, ice on the lakes forces a temporary hiatus until it is safe to sample again. Although we measure many of the same parameters, winter sampling is an entirely different ballgame. By drilling through the ice to sample, we can monitor the many lake processes that occur beneath the ice. Winter sampling provides the unique opportunity to study these processes while the lake is buffered from the atmosphere.
7 Lakes Alliance is proud to have a robust sampling program that captures seasonal conditions throughout the year.