What is Lake Mixing?
Lake mixing also referred to as turnover, occurs when the thermal stratification of a lake breaks down and the water column mixes due to density or wind-driven processes.
Thermal Stratification
Before discussing lake mixing, we need to go over thermal stratification. Thermal stratification occurs when a lake has different layers, or strata, of water with different temperatures. Thermal stratification occurs because of the unique properties of water. Unlike most other substances, water is most dense as a liquid at 4°C. When energy from the sun hits the water in our lakes, it is rapidly absorbed at the surface. As the water at the surface warms above 4°C, it will begin to float on the cooler water below. This process creates a positive feedback loop, whereby warm water at the surface floats above the cool water below, the surface water warms as it absorbs energy from the sun, becomes even less dense than the cool water below, and we end up with distinct layers of water in our lakes.
We refer to these layers as the epilimnion, metalimnion, and hypolimnion.
Epilimnion: The warm upper layer of the lake that is uniform in temperature. The epilimnion is the warm layer we enjoy swimming in during the summer.
Metalimnion: The layer below the metalimnion where temperatures rapidly cool down. The metalimnion shouldn't be confused with the thermocline, the specific depth where temperature changes the fastest.
Hypolimnion: The cold bottom layer of the lake that is uniform in temperature.
A cross-section diagram of a thermally stratified lake indicating the location of the stratified layers and thermocline.
Once a lake or pond is stratified, mixing the entire lake is difficult. The difference in density between the warm and cool layers resists being mixed, just as oil and water resist mixing due to differences in density. A windy day usually won't cause a thermally stratified lake to mix completely.
If a lake or pond is too deep for the sun's energy to reach the bottom, there is a good chance it will thermally stratify during the summer. Shallow lakes tend not to stratify because enough energy from the sun can penetrate the entire water column to warm the whole lake or pond.
Spring and Fall Mixing
When lakes stratify in the summer, they tend to go through periods of mixing in the spring and fall. Just as the unique properties of water lead to the thermal stratification of our lakes in the summer, these properties also contribute to mixing in the spring and fall.
Spring Mixing
In the spring, just as ice goes out on our lakes, the surface temperature will be at 0°C, and the bottom will be near 4°C. As the surface water warms from 0°C to 4°C, it becomes denser and settles through the water beneath it. Strong winds can help further this mixing process. The settling or sinking water from the surface brings oxygen-rich surface waters down to the lake's depths.
Once surface temperatures rise above 4°C, the lake or pond will begin to stratify thermally, significantly reducing mixing. As the waters at the surface continue to warm and the lake becomes strongly stratified, mixing will be confined to the epilimnion.
The period of spring mixing tends to be short because of the narrow temperature range over which the surface waters are becoming less dense (0-4°C).
Fall Mixing
Fall mixing occurs as the surface of our lakes cools from their warm summer temperatures (~28°C) down to 4°C. Similar to the process in spring, as temperatures move toward 4°C, water becomes more dense and settles through the less dense water beneath it. Because this occurs over a much larger temperature gradient in the fall, mixing can occur over weeks or months. Strong winds during this period can further aid lake mixing.
Just as in the spring, fall mixing brings oxygen down to the depths of our lakes. During summer stratification, the bottom waters, or hypolimnion, are isolated from the atmosphere and slowly lose oxygen while accumulating nutrients from the sediments below.
Fall mixing is essential for redistributing oxygen and nutrients throughout the lake's water column.
For further explanation and to see a demonstration of lake mixing and stratification, check out the video below from the North American Lake Management Society Student Video Series.
Lake Mixing Classification
Climate, size, and depth can all influence how lakes mix. In the Adirondack region, there are primarily two mixing regimes.
Dimictic Lakes: These are lakes that mix or turn over twice a year, once in the spring and once in the fall. Our moderate to large lakes with a maximum depth greater than 3-5m tend to be dimictic.
Polymictic Lakes: These lakes do not thermally stratify in the summer or only do so weakly. They will be warm from top to bottom in the summer and continually mix. Shallow lakes with a maximum depth of less than 3-5m tend to be polymictic.
These are the two most common mixing regimes for Adirondack lakes, but a small number of lakes don't mix thoroughly each year or only in the fall due to their size, depth, or road salt influences. Lakes that only mix once a year are called monomictic, and those that don't mix completely are called meromictic.
Why Lake Mixing is Important
The mixing regime for a lake influences everything from the lake's physics, chemistry, and biology. Mixing redistributes oxygen and nutrients throughout the water column. Small shallow lakes that continually mix have higher algal and plant growth than large deep lakes. Coldwater fish species such as lake trout need cold, well-oxygenated water throughout the year to survive. During the warm summer months, they can find those conditions deep in the water column of our larger lakes.
Changes to the mixing and stratification of a lake can have profound consequences on the lake ecosystem. Road salt reduced the spring mixing in Mirror Lake, limiting the amount of oxygen in the deeper waters. The oxygen reduction can limit habitat for coldwater fish, and in extreme cases, low oxygen can cause nutrients to be released from lake sediments through chemical reactions.
Climate change is prolonging the stratification period in lakes, extending the time between spring and fall mixing. With the time between mixing getting longer, oxygen concentrations in the deep waters of our lakes are declining. Low dissolved oxygen threatens coldwater fish like lake trout and increases the nutrients released from lake sediments into the water above. Elevated nutrient concentrations increase the likelihood of harmful algal blooms on a lake.
Prolonged periods of thermal stratification and the related loss of dissolved oxygen can cause nutrients to be released from lake sediments, this may be an important factor in the late-season harmful algal blooms occurring in the Adirondack Park.
AWI's monitoring and research programs are advancing our understanding of changes to the mixing and stratification patterns in Adirondack lakes. Our work on Mirror Lake, done in partnership with the Ausable River Association, documented road salt's impact on lake mixing and dissolved oxygen. This work has helped lake managers and researchers in other parts of the country understand the impact of road salt on their lakes. On Upper Saranac Lake, the Environmental Monitoring Platform is helping us understand how climate change is changing stratification patterns and the connection this has with harmful algal blooms documented on the lake. This work provides our communities with the information they need to protect our lakes and ponds.
