Turnover, or mixing, is an important physical process that many of our area lakes undergo twice a year. The process is driven by the unique density characteristics of water, whereby water is most dense at 4°C. This is what leads to thermal stratification in the summer, and ice forming on the surface of our lakes in the winter. This natural process of turnover redistributes oxygen and nutrients throughout the water column. Without this process, oxygen concentrations in the cold, deep-waters of a lake can become too low to support cold-water fish species and certain benthic organisms. It can also lead to a depletion of nutrients in the surface waters, potentially disrupting the aquatic food web.
Diagram of the process of turnover typical for a lake in the Ausable River watershed.
Mirror Lake did not mix in the spring of 2017. Our work has shown that the higher salt concentrations at the lake bottom, due to road salt runoff, are an important contributing factor to the lack of turnover. As a result, the lake has had persistently higher chloride concentrations in the bottom waters throughout the spring, summer, and fall. The oxygen concentrations in the bottom waters have also been low throughout this period of time.
Elevated chloride concentrations at the lake bottom built up over last winter and persisted throughout the spring, summer, and fall. There was partial mixing in the spring, but the lake did not completely turnover. The lake completely turned over on or just before November 20th, bringing the top and bottom chloride concentrations into equilibrium.
As a result of the lack of spring turnover, the oxygen was never replenished in the bottom waters. Concentrations remained low throughout the spring, summer, and fall. Lake Trout require oxygen concentrations above 7 mg/L to remain healthy, they become severely stressed at 4 mg/L, and will not survive below 2 mg/L. The turnover that occured at the end of November replenished the oxygen throughout the entire water column.
As we approached the period of fall turnover we increased our monitoring frequency to weekly visits. This allows us to closely track the progression of fall mixing in the lake and document when turnover occurs. On our November 20th, 2017 monitoring visit, in the middle of the first major snowfall of the year, we observed that the chloride and oxygen concentrations were uniform throughout the water column. This is an indicator that the lake has mixed. This is great news for Mirror Lake. The interruption of spring turnover is a serious concern for the health of the lake. If the fall turnover were interrupted as well, the severity of this problem would be much worse.
We will continue to monitor the lake on a bi-weekly basis until it freezes, at which point we will resume our monitoring once the ice is safe to walk on. This work is critical to our understanding of how road salt is impacting Mirror Lake. You can help reduce the use of road salt by following these five watershed friendly snow removal tips.