Meromictic lakes
Introduction
Meromictic lakes are stratified lakes that consists of two layers. The top layer, the mixolimnion, is a portion of the lake, usually ending a couple of meters under the surface of the lake, that is exposed to the atmosphere. Being so, it is allowed to mix freely. The mixing of the mixolimnion usually occurs from fall to spring assuming that there is not a layer of ice over the top of the lake during the winter. The bottom layer, the monimolimnion, is cut off from the atmosphere by the mixolimnion. These two layers are separated by either a thermocline or a chemocline (CWTA). A thermocline exists when there is a temperature difference between the surface and bottom of a lake. This temperature difference causes a difference in water density with the heavier cold water near the sediments. This temperature difference holds as long as the temperature of the lake does not cool off. Therefore, meromictic lakes that is stratified by a thermocline are typically found near the tropics where the surface of lakes never freeze. This is not to say that a thermocline cannot separate the mixolimnion and the monimolimnion in temperate climates. However, if this is the case, the lake needs to have a small surface area compared to its depth. This also means that the sides of the lake need to be steeply sloped. The small surface area compared to a lake's depth helps prevent the wind from being able to fully mix the lake when the temerature of the lake is uniform from late fall through early spring. After all, the larger the fetch, the length of a lake exposed to a gust of wind, is , the more likely the lake will turn completely over. This is especially true for shallow lakes with large fetches. Other than a thermocline, meromictic lakes may also be stratified due to a chemocline. A chemocline, in the context of meromictic lakes, is a gradient of salinity. The saltier, and more dense, water would settle to the bottom of the lake therefore forming the monimolimnion. The mixolimnion would have water with significantly less salt dissolved in it. These lakes are somewhat common near coastal areas where fresh and salt water interact with each other.
The size of meromictic lakes vary greatly with the largest being the Black Sea (wikipedia). The Black Sea is about 436,400 square kilometers and has a maximum depth of 2,212 meters. This great depth and the fact that the bottom water is more saline than the top is what keeps the Black Sea from fully mixing. Regardless of its size,a meromictic lake is home to a very diverse set of species, especially when it comes to microbial communities. Although individual species do vary depending on which lake is being studied, multiple studies have indicated that there are a few aerobic species at the top of the mixolimnion. Populations and the diversity of the bacteria tend to increase as the thermocline or the chemocline is reached just a couple of meters under the surface of the lake. This barrier between the two pools of water seems to be the place that has the most amount of microbial species and therefore the highest biodiversity. After all, both aerobes and anaerobes can potentially be within a few meters of each other. Also, light can reach this area, so both autotrophs and heterotrophs can potentially exist in this region. Past the chemocline or thermocline is the monimolimnion which is depleted of oxygen. Therefore, only anaerobic bacteria and archea can function in this zone. Since the monimolimnion hardly ever, if at all, mixes, it is a very anaerobic place where sulfur redox reactions dominate. Methanogenesis also occurs by some archea when there is not any more elemental sulfur to reduce.
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The Environment
The Physical Environment
Temperature
Since most meromictic lakes are deep and have a thermocline, temperature tends to be important. Temperature varies in the different layers of a meromictic lake. In the summer, the surface of a meromictic lake is the warmest. In fact, the mixolimnion may be stratified as in the picture to the left by "source." In this depiction, there is an epilimnion, a metalimnion, and a hypolimnion in the mixolimnion. This stratification is caused by varying temperatures with the warmest water being in the epilimnion and the coldest in the hypolimnion. This separation will affect the way the lake mixes during the summer, and more than likely only the epilimnion and the top of the metalimnion will be able to mix well. However, once the colder fall temperatures arrive, this stratification ends up breaking apart, and the whole mixolimnion is allowed to mix entirely again.
Density
Density is another important physical property in meromictic lakes. Temperature plays a key role in determining the density of the water in the lake. This is why the epilimnion, having the warmer, less dense water, is on top of the mixolimnion, and why the hypolimnion, containing the colder, more dense water, is on the bottom of the mixolimnion during the summer months. Another important factor that affects the density of water is the amount of solutes dissolved in the water, or in this case, the salinity. The more saline the water is, the more dense it is. Therefore, very saline water will stay right above the sediments of meromictic lakes. The gradient in salinity is the reason for the chemocline, and is why the hypolimnion in the mixolimnion is not a part of the monimolimnion even though they are roughly the same temperature. This varying in density throughout the meromictic lakes, especially in summer, is what controls most of the chemical processes within the lake.
The Chemical Environment
Dissolved Oxygen
Dissolved oxygen is the amount of oxygen that is held, dissolved, in water. Using the meromictic lake model from ((((CWTA)))), one can assume that the amount of oxygen that is dissolved in the water is greatest in the epilimnion of the mixolimnion and is lowest in the monimolimnion. The epilimnion has the greatest amount of contact with the atmosphere and can be thoroughly mixed. This makes it very easy for the oxygen in the atmosphere to dissolve into the epilimnion. Some of this oxygen is able to get mixed in with the top of the metalimnion, but nothing beyond that. Also, since the epilimnion has a warmer temperature than the layers below it, it can hold less oxygen. However, the continuous mixing during the summer keeps this layer oxygenated. Another way that a lake is able to obtain oxygen is through the photosythesis of plants and phototrophic microbes. Plants are limited to the shorelines, and both plants and phototrophic microbes are limited to the ammount of light they recieve. Therefore,plants and microbes can only photosynthesize in the photic zone (unk. Source). The lower layer of the photic zone is where 99 percent of the light from the sun is absorbed by the water. Not all organisms in water contribute to the dissolved oxygen concentration. Some, like fish, heterotrophic bacteria, and benthic invertebrates, deplete the oxygen concentration. However, the epilimnion is usually robust enough to deal with these oxygen utilizing organisms.
The metalimnion is the portion of the meromictic lake that starts to show an oxygen deficiency, especially the lower portion of the metalimnion. Even though the metalimnion has a cooler temperature than the epilimnion, and can therefore hold more oxygen, it will have a lower dissolved oxygen concentration due to the fact that it is not in direct contact with the atmosphere and cannot be mixed. To add to this lack of oxygen, the aerobic microbes and fish that live in this region of the lake will eventually deplete as much oxygen as they can before they have to move to a different location or die. This is even more true for the hypolimnion and the monimolimnion. In fact, since the monimolimnion rarely, if ever, gets mixed and it does not have organisms that photosynthsize, it usually only has a dissolved oxygen concentration of less than 1 ppm (CWTA). This is far below the amount of oxygen that most fish need to survive.
Microbial communities
What kind of microbes do we typically find in this environment? Or associated with important processes in this environment? Describe key groups of microbes that we find in this environment, and any special adaptations they may have evolved to survive in this environment. List examples of specific microbes that represent key groups or are associated with important processes found in this environment. Link to other MicrobeWiki pages where possible.
Are there important biological interactions that are important in this environment? Do these interactions influence microbial populations and their activities? How do these interactions influence other organisms? Describe biological interactions that might take place in this environment, using as many sections/subsections as you require. Look at other topics available in MicrobeWiki. Create links where relevant.
Subsection 1
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Subsection 1b
Subsection 2
Microbial processes
What microbial processes define this environment? Describe microbial processes that are important in this habitat, adding sections/subsections as needed. Look at other topics in MicrobeWiki. Are some of these processes already described? Create links where relevant.
Subsection 1
Subsection 1a
Subsection 1b
Subsection 2
Current Research
Enter summaries of recent research here--at least three required
References
Edited by <your name>, a student of Angela Kent at the University of Illinois at Urbana-Champaign.
