Monimolimnion

Monimolimnion is the permanently stratified bottom layer of a meromictic lake, lying below the chemocline and not participating in the seasonal overturn that mixes the upper waters. It remains chemically distinct from the overlying mixolimnion, creating a persistent two-layer water column.

In typical meromictic systems, the monimolimnion is characterized by anoxic or reducing conditions. Oxygen is scarce or absent, and reduced compounds such as hydrogen sulfide, methane, and iron can accumulate. The layer often contains high dissolved solids and elevated salinity or mineral content, contributing to its greater density relative to the upper layers. Because it is effectively isolated from the surface, the monimolimnion changes only slowly over time, recording the lake’s long-term chemical history.

Formation and persistence of a monimolimnion depend on stable density stratification that prevents mixing. This stratification can arise from salinity differences, temperature gradients, or a combination of factors that create a stable density gradient. The boundary separating the monimolimnion from the overlying mixolimnion is the chemocline. Disturbances such as strong storms or hydrological changes can alter stratification, but complete mixing of the monimolimnion with the upper waters is uncommon in permanently stratified systems.

Ecologically, the monimolimnion hosts specialized microbial communities adapted to anoxic conditions. The chemistry of this layer drives unique biogeochemical processes, including sulfate reduction and methanogenesis, and influences gas production and nutrient cycling within the lake. Overall, the monimolimnion represents a key feature of meromictic lakes, shaping their physical structure, chemistry, and microbial ecology.