Meromictic lakes

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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. 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 seems to be the place that has the most amount of microbial species and also has the most amount of biodiversity.

Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.

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Legend/credit: Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.
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Physical environment

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References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Edited by <your name>, a student of Angela Kent at the University of Illinois at Urbana-Champaign.