Amixis - a lack of mixing: some lakes never circulate. These amictic lakes are usually ice covered throughout the year. Well, they're not around here, you might say, and you are right, thankfully. These lakes are under the polar ice caps (at the North and South Poles of our globe) or high mountain lakes where the temperature is mostly below freezing. Since these are not our usual lakes, we won't say anymore about them.

Holomixis - Entire mixing - a more typical lake situation where the winds mix the "whole" lake once or more annually. We will discuss this category in much more detail since it includes most of the lakes we know.

Meromixis - partial or incomplete mixing - some lakes have one or more periods of annual mixing, but only part of the lake is involved for some notable reasons. There are some of these lakes around so we'll discuss the meromictic lakes later.

Water - An Amazing Substance -
Before we discuss this lake mixing business we should talk about how lakes are mixed. First we need to know something about the temperature and density relationship of water. Water is really an amazing substance. It is a rare substance where the solid state is lighter than the liquid state, but it is true for water. Ice floats, luckily for us. As can be seen in the figure, water is most dense (heaviest) at a temperature of 4C or about 39F. So, if we start with ice at 0C, we have a lattice type of structure that is lighter and less dense than water, and the ice floats. When we heat the ice, like the sun does as spring approaches, it melts, first to water at 0C. This really takes a lot of heat energy. The water is more dense than the ice and it continues to absorb heat and to get warmer. As it gets warmer it gets heavier until it reaches its maximum density at 4C. With additional warming above 4C water becomes lighter (less dense) in the manner illustrated in the figure. At 4C, under normal pressure, water reaches its greatest density. In the winter, under the ice, the water commonly has a temperature that increases with depth, from 0C just under the ice to 4C, at the bottom of the lake. This increase to water's greatest density at 4C occurs within a meter of two of depth (3-5 feet), and then the water temperature stays much the same to the bottom. This is true for most of our inland lakes.
In the spring, the surface water warms until it reaches its maximum density of 4C and the entire lake is about the same temperature (about 4C) -and density- from top to bottom. When it reaches that time of uniform density, there is no density resistance to mixing. Think about the density difference of oil floating on water. You can blow on it and even tilt the container back and forth and the oil and water doesn't mix much - because of the difference in their densities, but a container of water mixes easily. When we eventually reach the uniform density stage in the spring in a lake, there is no density resistance to mixing of the water by the wind. Remember, we usually get some good wind in the spring - that's when we fly kites. The force of the wind mixes the water of the lake. Once uniform density is reached, strong winds can mix the lake from top to bottom in most of our lakes, unless they are unusually deep. This period of mixing is often call the "over-turn" and it is an interesting phenomenon. Nutrients from the lower water are brought to the surface and oxygen from the surface is mixed to the lower depths. After the overturn, for a week or two, we have uniform temperature, oxygen, nutrients etc. from top to bottom. If we wish to take a measure of the spring phosphorus concentration of a lake, it is imperative that we wait until the lake mixes after it reaches a constant temperature from top to bottom. It is better to be a week late in collecting water for phosphorus determination than a day too early. During this mixing period the water will warm and may reach a temperature of 5-7C.
Back to the unique character of water. As water warms above 4C, it becomes less dense again. Remember, water is most dense at 4C, and at temperatures either above or below it becomes less dense. So as the sun continues to warm the surface of the water and the winds begin to lessen in intensity, most of heat is absorbed in the first few centimeters, the surface water temperature increases and becomes lighter (less dense) and "floats" on the more dense water below. This heating of the surface water continues and eventually forms an upper layer of warm water. It acts as a layer of insulation, absorbing the heat from the sun and preventing the lower water from getting any heat. This upper layer is eventually warmed and mixed to a depth of about 5-6 meters (15 to 20 feet). We call this layer of water the epilimnion (epi - on top, limnion - layer). Below the epilimnion, the temperature of the water drops rapidly for about 2 meters (6-7 feet) and then remains about the same to the bottom. So we have a bottom layer of water that is cold, usually about 4-7C, and a middle transition layer.
Remember, the upper layer has been warmed to a temperature of 25-30C (depending on how warm the summer happens to be or where our lake happens to be). By early summer, our lake is layered - or stratified. Our epilimnion 5-6 meters deep, is warm (25-30C) and much less dense than the lower waters.
Our middle layer is called the metalimnion (meta - middle, limnion - layer) and it is about two meters thick with rapidly decreasing temperature and increasing density. The epilimnion continues to mix when we get strong winds in the summer, but, the increase in water density in the metalimnion prevents this summer mixing from going any lower.
Below the metalimnion we have the cold, much more dense hypolimnion (hypo - below, limnio - layer). In a deep lake, the hypolimnion can be a very large volume, a large percent of the lake, and the reverse is true in a shallow lake. Given a top layer of 5-6 meters and a middle layer of 2-3 meters, if our lake is only 6 meters deep (about 19-20 feet) we would not have any middle or bottom layer. Thus, some of our shallow lakes can mix entirely during the summer whenever we get strong winds. These lakes behave quite differently from those lakes that are stratified into three layers.
But, we're off here of a bit on a tangent. We need to get back to this classification based on mixing. So, when the water is uniform temperature ( and thus density) from top to bottom and we add wind, we usually get mixing. Lets look at holomictic lakes, those lakes that mix entirely at least during one period a year.

We have four types of holomictic lakes

1. Oligomictic lakes. These lakes are usually located in the tropics and have poor (oligo) mixing. The mixing is irregular, or sporadic and usually of short duration. These lakes are usually warm throughout, but the surface waters are even warmer, creating some stratification. Only occasional, and maybe rare, cooling of the surface waters allow any chance of mixing.
2. Polymictic lakes. Poly means many and the lakes have many mixing periods even to the extent that they are mixed nearly continuously throughout the year. These lakes are often small, shallow and in tropical or at least warmer climates or at higher altitudes. The temperature changes are often influenced more by cooling of the surface at night and warming during the day rather than by distinctive seasonal changes. In some of these lakes circulation occurs mostly at night by convection currents rather than by the wind. We'll talk a little more about convection currents later.
3. Monomictic lakes Mono means one, so these lakes have one regular period of mixing during the year. To complicate the situation, conditions that allow mixing in monomictic lakes can occur either under cold or warm climates. So we have cold monomixis and warm monomixis. The cold monomixis lakes are usually near the polar areas. They freeze over during the long winter months and are ice-free during the summer. However, the summer temperatures never are warm enough to heat the water much above 4C. So these lakes could mix often during a relatively short summer. In some years they may not be ice-free and then would be amictic for that year. Warm monomictic lakes are usually those sub-tropical lakes that have a long summer and a short winter. They are stratified most of the year and then for a short period in the winter the stratification breaks down and the lakes can mix. When the Great Lakes don't freeze over, they commonly mix throughout the winter and have only one extended mixing period from Fall to Spring and can be classified as warm monomictic lakes.
4. Dimictic lakes. Di means two. Finally we come to our average temperate zone lake that is the type we have (mostly) here in Michigan. These lakes normally have two periods of mixing, one in the spring and one in the fall. We have already discussed how the dimictic lake warms in the spring until the entire water mass is uniform both in temperature and density; and how summer stratification develops. Now we can discuss how the summer stratification breaks down and the fall overturn occurs. As summer wanes in September (in Michigan) we begin to have cooler nights (and shorter days) and less solar heating during the day. The surface water of the lake cools and becomes more dense (heavier) than the warmer water below. Thus, the surface water sinks. The sinking of more dense water develops slight vertical currents that we call convection currents. This process continues, the days and nights get cooler, and the surface water continues cooling and sinking. In September and October when night air is cooler than the lake water we often see morning fog over the lake. The warmer surface water is evaporating into the cool air and condensing, forming the fog. This process continues until the lake once again reaches the same temperature and density from surface to bottom. We have already had some mixing by the convection currents. While the lake can turn over in the Fall without wind, strong winds can accelerate this mixing of the water. This Fall "overturn", which usually happens in Michigan in October, re-mixes dissolved oxygen and nutrients throughout the lake. As the daily temperatures continue to cool below 4C, we eventually reach freezing, usually at night, and we awaken one morning to find a skim of ice over the shallower parts of the lake. Finally, the skim of ice covers the lake and any further mixing is stopped and we enter the winter stagnation period.
   So the typical dimictic lake has two periods of mixing each year under normal conditions. Let's talk about those situations when the conditions aren't normal or typical. We mentioned earlier about the shallow lakes less than 6-7 meters (about 20 feet) deep that don't stratify very well and thus might mix anytime during the summer when there are strong winds. There are also times when there is not enough wind to force the overturn. thus, it is possible to have a spring, or time, when the water density is uniform from top to bottom, but there is no overturn because there is no wind. And we can have all degrees of climate conditions from no wind to very strong wind. Mild or moderate winds can cause incomplete or partial overturns. Consider a very deep lake, especially with a relatively small surface area. The force of the wind doesn't have much distance to work on and the deep water needs more force to drive currents all the way to the bottom. So, only the upper waters are mixed. Likewise, some of our lakes have steep shorelines with the surface of the water being sheltered by the land and/or by tall trees. These barriers prevent the winds from exerting their full force on the surface of the water, especially when the high land or trees are on the windward side of the lake. The result can be no overturn or a partial mixing. So this classification is like most others and is generally, but not always, accurate.
   There is another factor that can cause density differences in water besides temperature differences, and that leads to our final category.
   
5. Meromixus lakes. Meromictic lakes in our area would probably be typical dimictic lakes; however, their periods of mixing usually are incomplete. These lakes, over time, have developed a deep layer of water that has a much greater amount of material in solution than does the upper waters. These solutes, substances in solution, cause these lower waters to have a greater density that resists mixing the same as density differences caused by temperature. Thus when temperature of the water is uniform from top to bottom, we still have a density gradient that limits the normal mixing only to the depth where the currents encounter this dense lower layer. The concentration of substances in the lower depths accumulate usually over an extended time. The meromictic situation usually develops in deeper waters. This allows the substances that settle or precipitate to the bottom annually to accumulate along with substances that come into solution from the bottom due to bacterial breakdown of organic debris. Thus, this layer of increased density due to substances in solution becomes thicker over the years, eventually occupying a significant volume of the deep waters of the lake. There are not a lot of meromictic lakes in Michigan, but perhaps there are a few more than we realize. A common cause of meromixis in Michigan is associated with the runoff of road salt. There are not enough lake surveys to allow us to have a good idea of the number or percentage of our lakes that may be meromictic.

• Amictic - never mixing, usually ice covered year around, in the polar regions.
• Meromictic - lakes that may mix once or more annually, but do not mix completely.
• Holomictic - mixing of the entire lake.
• Oligomictic - mixing is unusual, irregular and of short duration, these lakes are relative few in number and are mostly tropical.
• Polymictic - these lakes have many periods of mixing annually, even ap- proaching continuous mixing and are influenced more by daily temperature changes than seasonal.
• Monomictic - one period of mixing annually, the cold monomictic lakes are ice covered most of the year and mix during a brief summer, while the warm monomictic lakes are the opposite with a brief ice-free winter period of mixing.
• Dimictic - lakes that usually mix twice annually in the spring and in the fall. This category covers the lakes in the temperate zones of our globe and includes the majority of our lakes.

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Editors Note: This is the second part of a two part series of articles written by the ML&SA Science Advisory Committee on "Lake Systems". The February 1996 issue of The Riparian carried the first of the series. If you would like a copy of the February issue, contact the Riparian Office, either by phone (616) 273-8200 or write to P.O. Box 249, Three Rivers, MI 49093-0249
or click here to see it -> "Lake Classifications - Part 1"

Dr. Niles Kevern, chairman of ML&SA Science Advisory Committee, has a Ph.D. Degree in Limnology and Pollution Biology from Michigan State University. He has authored and co-authored more than three dozen articles dealing with algae, zooplankton, and other lakes and streams matters.

Dr. Darrell King also has a Ph.D. Degree from Michigan State University, and has authored or co-authored 49 articles for professional journals on subjects such as carbon and oxygen cycles and carbon and nitrogen cycles in inland lakes, waste pollution management and how lakes and streams are affected, etc.

Dr. Robert King has a Ph.D. Degree in Aquatic Entomology, Stream Ecology and Limnology from Michigan State University. He has worked for the United States Army Corps of Engineers in the study of stream flow and the role of macrophytes on aquatic ecosystems. He has co-authored more than 25 professional studies of stream and rivers in Michigan and Mississippi.

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