A meromictic lake is a rare body of water where the deepest layers never fully mix with the surface water. Most lakes are classified as holomictic, meaning their water columns turn over completely at least once a year, driven by seasonal temperature changes that equalize water density. Meromictic lakes maintain permanent stratification, where a dense bottom layer remains isolated for years, decades, or even centuries. This persistent layering is maintained by a significant difference in water density, often caused by varying concentrations of dissolved salts or minerals between the layers. Only about one in a thousand lakes worldwide exhibit this distinctive behavior.
The Layered Structure of a Meromictic Lake
The permanent layering in a meromictic lake results in three distinct strata, each with its own chemical and biological profile. The upper layer is called the Mixolimnion. It behaves much like a normal lake, mixing seasonally due to wind and temperature changes, and remaining well-oxygenated. This surface layer supports the majority of the lake’s aquatic life, such as fish and plankton.
Below the circulating surface water lies the Monimolimnion, the deep, non-mixing layer. This bottom layer is denser than the Mixolimnion, typically due to a higher concentration of dissolved salts, and remains permanently cold and anoxic (lacking oxygen).
Separating these two major layers is the Chemocline, a thin, transitional zone where the concentration of dissolved substances and water density change rapidly. The Chemocline acts as a seal, preventing the denser Monimolimnion from rising and the lighter Mixolimnion from sinking. This density gradient, known as chemical stratification, is significantly stronger than the thermal stratification found in holomictic lakes, ensuring that seasonal turnover events cannot cause a full mix.
Mechanisms that Prevent Seasonal Mixing
Stable stratification is maintained by specific processes that introduce dense water to the bottom of the basin. The origin of this dense, non-mixing water classifies the lake’s meromixis into distinct types:
- Ectogenic meromixis occurs when an external event introduces a dense fluid, such as saltwater intrusion into a coastal freshwater lake or road salt runoff accumulating in the deepest parts of a lake.
- Crenogenic meromixis develops when submerged saline springs or groundwater seeps continuously feed dense, mineral-rich water into the deepest layer, maintaining the required density difference.
- Biogenic meromixis results from internal processes where the decomposition of sinking organic matter releases ions and gases, gradually increasing the salinity and density of the bottom water.
- Cryogenic meromixis occurs in cold regions when salts are “frozen out” during the formation of thick ice, causing the dense, super-saline water to sink and collect at the bottom.
The morphometry of the lake basin also plays a role in maintaining meromixis. Deep, steep-sided basins relative to their surface area are more likely to prevent wind-driven mixing from reaching the bottom.
Unique Chemistry and Specialized Life
The permanent isolation of the Monimolimnion creates a chemically extreme environment. Since the deep water is never exposed to the atmosphere, dissolved oxygen is rapidly consumed by the decomposition of sinking organic matter, leading to persistent anoxia (a complete lack of oxygen). This anaerobic environment allows for the accumulation of reduced compounds, such as high concentrations of hydrogen sulfide gas and methane, which are toxic to most aerobic life forms.
The chemocline and Monimolimnion support specialized microbial communities that thrive in the absence of oxygen. These organisms, including certain bacteria and archaea, perform chemosynthesis, using reduced compounds like hydrogen sulfide and methane as an energy source instead of sunlight. Layers of purple and green sulfur bacteria often form dense, colored plates right at the chemocline, utilizing the hydrogen sulfide that diffuses up from the bottom for anoxygenic photosynthesis.
The lack of physical mixing and low biological activity in the Monimolimnion is important for researchers studying the past. Sediment that settles to the bottom remains largely undisturbed, accumulating in fine, annual layers known as varves. These preserved layers act as a natural archive, allowing scientists to analyze pollen, microfossils, and chemical signatures to reconstruct centuries of local climate and ecological history.
Famous Meromictic Lakes Around the World
Meromictic lakes are found across the globe. The Black Sea, while technically a large marine basin, is the world’s largest meromictic body of water. Its deep waters below 50 meters remain permanently anoxic and rich in hydrogen sulfide due to the intrusion of dense, salty Mediterranean water.
In Africa, Lake Kivu, located between the Democratic Republic of the Congo and Rwanda, is a massive meromictic rift lake. It holds extremely high concentrations of dissolved methane and carbon dioxide in its deep layers, which is a significant geological concern.
Green Lake in New York State is a well-known North American example. Its meromixis is thought to be maintained by a deep, steep-sided basin and the accumulation of dissolved minerals.
Lake Vanda in Antarctica features a thick, permanent ice cover that seals the lake. The Monimolimnion is maintained by an extreme saline concentration, with temperatures at the lake bottom reaching an unusually warm 25 degrees Celsius.