By: Clint Calhoun
Lake turnover is a process that you usually don’t hear much about until the colder months, but the process leading up to it is something that occurs during warm weather and is underway right now. This process is called stratification and hopefully this article will describe a little bit about the process and what it means for the waters of Lake Lure.
Water is a very unique compound that has properties that no other substance on earth possesses. Those properties are what make it a valuable and life-giving resource. Those properties lend themselves to influence heat exchange and circulation within lakes and influence lake biology and chemistry more than any other factor. Lake water absorbs heat from the sun and the surrounding air; wind then moves across the surface of the water distributing the heated surface water evenly across the lake and “mixes” the water. Wave action from wind aids in this process as it continuously churns the surface.
As surface waters warm during the spring, the water’s density decreases and it becomes lighter than the dense cooler water at the lake bottom that never receives any heat from the sun. As the surface water continues to warm over the spring and summer and the density difference between the surface waters and bottom waters increases, the wind is no longer able to effectively mix the water and the lake becomes “stratified.”
Think of stratification as a layering process, where you have several layers of different temperatures from the surface all the way to the bottom. Within Lake Lure, the temperature gradient can vary as much as 20 degrees Celsius during the middle of summer (we record our temperature readings in Celsius). Within those layers, you have areas where temperatures tend to vary very little; for instance, at the surface to about 4 meters below the surface, the temperature is fairly uniform (less than a degree difference +/-). We call this area of the water column the epilimnion. These waters are well mixed from wind and boating activity. The cold, unmixed bottom water is called the hypolimnion. In Lake Lure the water temperature in the hypolimnion starts around the 14 meter mark and goes the rest of the way to the bottom. This water is very dense and does not mix with the epilimnion, at least not easily. The area in between these two strata is called the metalimnion. In the metalimnion, temperatures can change more than one degree Celsius for each meter of depth. This area of the water column prevents the epilimnion and hypolimnion from mixing. Another term you are probably more familiar with is thermocline. The thermocline is the depth within the metalimnion where the rate of temperature is greatest. In Lake Lure the thermocline is typically between 4 and 5 meters. In clear lakes, the thermocline is actually visible to scuba divers, appearing almost like a suspended layer of oil.
Maximum stratification typically occurs in August. Around this time, we typically see our warmest temperatures in the epilimnion. We also see depletion of dissolved oxygen in the hypolimnion (due to its isolation from the higher oxygen levels in the epilimnion).
Oxygen depletion in the hypolimnion can be an issue in lakes, particularly for fish. When temperatures get warm in the epilimnion during the summer months, fish want to go deeper to avoid the heat. If the oxygen is depleted in the colder waters, the available space those fish can take refuge becomes much smaller. This can cause stress for fish. Hypolimnetic anoxia can also lead to release of nutrients from bottom sediments such as phosphorus and nitrogen, a process known as internal loading. The release of these nutrients can promote algae production and more organic matter decomposition, further depleting oxygen levels.
As the days shorten and nighttime temperatures become considerably cooler, usually around mid-September, the epilimnion begins to cool and the temperature difference between the layers decreases, allowing mixing to occur. The mixing process continues as the surface water becomes cooler and more dense, allowing it to more easily mix with the waters in the metalimnion and upper hypolimnion. By late fall, usually late November or early December, mixing is complete and the lake has “destratified” or “turned over.” During this time, temperatures are much less variable throughout the water column and dissolved oxygen levels increase from top to bottom.
Lake Lure is rather interesting in that we don’t see complete mixing in the hypolimnion. This is partly due to the depth of the lake (more water to have to mix), but also due to the generally mild temperatures we see in the winter months. A cold winter and cool spring such as what we had this year definitely helps to increase the depth that mixing will occur within the hypolimnion, as mixing continues until the waters warm again.
While this information may seem insignificant or unimportant to most, this information gives us lake managers play-by-play information on what is happening with the lake and often serves as an investigative starting point should an issue arise. Significant changes to numbers within the water column can be indicators of environmental factors that could affect the overall health of the lake, so that’s why we pay close attention to these numbers. What is important for users of Lake Lure to know is that our lake has excellent water quality and this is due in part to the proactive efforts of the Town to protect its greatest asset. Until next time!
Clint Calhoun is biologist and Certified Lake Manager and has worked in the Hickory Nut Gorge area for over 25 years. He is currently the Environmental Management Officer for the Town of Lake Lure. Check out Clint’s blog at http://clintcalhounadventures.blogspot.com.