Selected Examples

Table 2 provides an extensive list of examples of meromixis from many countries. The list is incomplete but gives a rich indication of this unusual lake type around the world. Other examples may be reported in the literature and, of course, many more probably remain unrecognized. Obviously, we need far more information and data but, based on current knowledge, it seems safe to say that less than 1 out of 1000 water bodies are known to have meromictic characteristics.

However, even among the hundreds of meromictic lakes known, there are still a few that are different or noteworthy (the unusual among the unusual?) enough to merit special attention. It may be something about their size, morphometry, chemical stratification, educational utility, or other features that makes them worthy of special interest. The following 9 examples, in no special order of importance, are meant to demonstrate the history, diversity, and distribution of meromictic lakes that have fascinated limnologists for many years.

1. Lake Tanganyika (>1450 m deep) and

2. Lake Malawi/Nyasa (>700 m deep), tectonic lakes in Africa, are the second- and third-deepest freshwater lakes in the world, after Lake Baikal, and the two deepest meromictic lakes in the world. Below roughly 150 m in Lake Tanganyika and 200 m in Malawi, the waters are continuously anoxic. Interestingly, huge numbers of fish species (including endemic ones) have evolved over time within the upper oxygenated 'mixolimnetic' waters. Indeed, Lake Malawi is regarded as having more species of fish than any other lake in the world! The fish in both these ancient lakes are under increasing pressure from a growing proximate human population and overfishing. The biotic resources of these huge lakes, like resources anywhere, are not endless and sustainability appears doubtful. The Nyanza program (sponsored by the US National Science Foundation and the Tanganyika Biodiversity Program, and carried out at the town of Kigoma on the northeastern shore of Lake Tanganyika in Tanzania) provides some limnological training and research opportunities for students from Africa and other parts of the world.

3. Skua Lake, on Ross Island, Antarctica, is the shallowest (0.75 m deep) known meromictic lake. Its very small area (170 m2) is probably less significant for the maintenance of meromixis than its salinity gradient.

4. Hot Lake, in northern Washington State, USA, is also small (13 000m2) and shallow (3.2 m deep), but has a strong salinity gradient that fosters the 'trapping' of solar radiation into a saline lens where the summer water temperature may exceed 50 °C and, even under the winter ice, the water is warmer than 25 °C. These are so-called heliothermal lakes. Hot Lake (with Lake Vanda, below) has the warmest known 'under ice' temperatures of any lake in the world. Solar Lake, a small coastal pond by the Gulf of Aqaba near Elat, Israel, also has a marked salinity gradient, and bottom-water temperatures exceeding 50 °C, but it is monomictic and does not develop an ice cover. Interestingly, nearly a century ago, limited studies were done on a small wind-protected water body (Medveto Lake) in Hungary and temperatures up to 56 °C, attributed exclusively to solar radiation, were found in a saline stratum not far below the water surface. Unfortunately, the 'mictic' status of this latter small lake, and information about ice cover, are unknown.

5. The volcanic crater lakes Lake Monoun (95 m deep) and Lake Nyos (210 m deep), in Cameroon, Africa, have a history of explosive degassing of carbon dioxide (Monoun in 1984; Nyos in 1986). Some 1800 people and hundreds of cattle and other animals, some 25 km away, were killed by gases resulting from these explosions. Efforts to artificially degas these 'killer lakes' are underway, by reducing the build-up of monimolimnetic gases.

6. Lake Mary (~21 m deep) and Green Lake (Fayettville) (^52 m deep), in Wisconsin and New York, respectively, USA, are of interest for their educational utility. For decades hundreds of students from several universities and colleges have learned about and visited these lakes on field trips. Consequently, these lakes have provided first hand knowledge about meromixis for generations of students.

7. Lake Vanda (66 m deep), in Victoria Land, Antarctica, appears to be a crenogenic meromictic lake. The lake surface is covered by ice year-round, and even in the Antarctic summers of 1962-1963 the ice was 3.5-4.3 m thick. The waters of Lake Vanda are exceptionally clear, and comparable to the clearest ocean waters. The lake also maintains a monimolimnetic temperature, beneath thick ice, of more than 25 °C. The latter lower water temperatures are thought to be caused by geothermal heating from below, rather than by solar heating from above.

8. Lake Kauhako is a small (^3500 m2), deep (248 m) water body located in a formerly drained volcanic conduit, on the Kalaupapa Peninsula of the Island of Molokai, one of the Hawaiian islands, in the Pacific Ocean. Because of its small surface area and relatively great depth, Lake Kauhako has the greatest 'relative depth' (Zr = 371%) of any known natural (not human-made) water body in the world! Initial studies by some Hawaiian scientists suggest that only the top 4-4.5 m of water show substantial stratification of salinity, temperature, and dissolved oxygen. Hydrogen sulfide becomes prominent within five meters of the surface and extends to the bottom. If so, most of Lake Kauhako resembles a giant anoxic (and stinky?) tube of sea water. Other than the top few stratified meters, most of the water column below about five meters of depth is relatively homogeneous in temperature and salinity. Kauhako (248 m) is the deepest natural meromictic lake in the USA and the sixth deepest lake in the United States. Some comparative depths for other deep, but nonmeromictic, lakes of the USA are Lakes Crater (608 m), Tahoe (501m), Chelan (458 m), Superior (407 m, border shared with Canada), and Michigan (282 m).

9. Soap Lake (27 m deep) is located in the Lower Grand Coulee area of the central part of Washington State, USA. The lake probably originated from fresh water flowing over a saline lake that today remains as the monimolimnion (salinity 146 g L-1). In winters when the surface freezes, the upper waters (mixolimnion) behave as a typical dimictic lake. However, in milder winters, mixing probably persists and the mixolimnion behaves as a monomictic lake. In that sense, Soap Lake may be reflective of a variety of meromitic lakes. Indeed, one way of viewing meromixis is to consider that there is an irregular seasonal stratification within the mixolimnion that effectively generates a holomictic lake on top of more dense or saline monimolimnetic waters.

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