Chemocline

A chemocline is a type of cline, a layer of fluid with different properties, characterized by a strong, vertical chemistry gradient within a body of water. In bodies of water where chemoclines occur, the cline separates the upper and lower layers, resulting in different properties for those layers. The lower layer shows a change in the concentration of dissolved gases and solids compared to the upper layer. Chemoclines most commonly occur where local conditions favor the formation of anoxic bottom water — deep water deficient in oxygen, where only anaerobic forms of life can exist. Common anaerobic organisms that live in these conditions include phototrophic purple sulfur bacteria and green sulfur bacteria. The Black Sea is an example of a body of water with a prominent chemocline, though similar bodies (classified as meromictic lakes) exist across the globe. Meromictic lakes are the result of meromixis, which is a circumstance where a body of water does not fully mix and circulate, causing stratification. In any body of water in which oxygen-rich surface waters are well-mixed (holomictic), no chemocline will exist, as there is no stratification of layers. Chemoclines can become unstable when dissolved gases become supersaturated, such as H2S, due to mixing associated with bubbling or boiling (ebullition). Containing the largest chemical gradient, the chemocline is a thin boundary layer that separates a meromictic lake into two parts: the upper mixolimnion and the lower monimolimnion. The mixolimnion is a region that is accessed by the wind where the water can be fully mixed and circulated. However, the monimolimnion is dense and cannot interact with the wind in the same manner, preventing mixing. Furthermore, the chemocline's variability in density determines the degree to which the body of water will experience mixing and circulation. Since the chemocline acts as a barrier between the mixed and non-mixed layers, the deeper monimolimnion layer is often anoxic.