A chemocline is a type of

cline Cline may refer to: Science * Cline (biology), a measurable gradient in a single trait in a species across its geographical range * Cline (hydrology), a fluid layer with a property that varies * Cline (mathematics) or generalised circle, a circl ...

, a layer of fluid with different properties, characterized by a strong, vertical chemistry gradient within a body of

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

. 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 Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

, where only

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

forms of life can exist. Common anaerobic organisms that live in these conditions include phototrophic

purple sulfur bacteria The purple sulfur bacteria (PSB) are part of a group of Pseudomonadota capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in stratified water environments including ...

and

green sulfur bacteria The green sulfur bacteria are a phylum of obligately anaerobic photoautotrophic bacteria that metabolize sulfur. Green sulfur bacteria are nonmotile (except ''Chloroherpeton thalassium'', which may glide) and capable of anoxygenic photosynthe ...

. The

Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...

is an example of a body of water with a prominent chemocline, though similar bodies (classified as

meromictic A meromictic lake is a lake which has layers of water that do not intermix. In ordinary, holomictic lakes, at least once each year, there is a physical mixing of the surface and the deep waters. The term ''meromictic'' was coined by the Austr ...

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 Stratification may refer to: Mathematics * Stratification (mathematics), any consistent assignment of numbers to predicate symbols * Data stratification in statistics Earth sciences * Stable and unstable stratification * Stratification, or st ...

. In any body of water in which oxygen-rich surface waters are well-mixed (

holomictic Holomictic lakes are lakes that have a uniform temperature and density from surface to bottom at a specific time during the year, which allows the lake waters to mix in the absence of stratification. Details Holomictic lakes mix at least occasi ...

), 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 Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Ther ...

Chemocline structure

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 Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

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. A lack of gas exchange between the monimolimnion layer and the atmosphere causes an increase in oxygen consumption over oxygen production. This creates a negative redox potential along with anoxic and

euxinia Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified, have an oxic, highly ...

conditions. Chemocline instability is characterized by vertical mixing events. These can be triggered by an increase in H₂S concentrations higher than 1 mmol/kg in the sulfide-rich deep monolimnion layer. The euxinic deep water would then upwell into the mixolimnion near the surface and hydrogen sulfide would be expelled into the atmosphere. This can also be triggered by other gases such as carbon dioxide. In many lakes, chemocline instability is typical. Lake stratification can be upset due to mixing events that occur 1, 2, or more times per year. These mixing events occur in

monomictic Monomictic lakes are holomictic lakes that mix from top to bottom during one mixing period each year. Monomictic lakes may be subdivided into cold and warm types. Cold monomictic lakes Cold monomictic lakes are lakes that are covered by ice thro ...

, dimictic, or

polymictic Polymictic lakes are holomictic lakes that are too shallow to develop thermal stratification; thus, their waters can mix from top to bottom throughout the ice-free period. Polymictic lakes can be divided into cold polymictic lakes (i.e., those tha ...

lakes. However, in meromictic lakes, stratification is permanent. These lakes, with a stable chemocline, are typically narrow and deep with low surface to volume ratios, low wind disturbance, and ongoing

eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...

Life and chemoclines

As a result of the differences between the upper and lower layers,

aerobic Aerobic means "requiring air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Aerobic respiration, the aerobic process of cel ...

life is restricted to the region above the chemocline, while anaerobic species able to live in anoxic conditions reside below the cline. Additionally, above the chemocline, photosynthetic processes can occur due to the presence of light, but below, sufficient light is not present for photosynthetic bacteria to thrive. In the mixolimnion, above the chemocline, examples of phototrophic species include cyanobacteria, while the monolimnion contains sulfate reducers and sulfide oxidizers. At the chemocline itself, photosynthetic forms of anaerobic

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...

, like green phototrophic and

, cluster and take advantage of both the sunlight from above and the hydrogen sulfide (H₂S) produced by the anaerobic bacteria below. Due to the gradient of conditions, the chemocline layer may contain an abundance of phototrophic bacteria and high concentrations of thiosulfate and elemental sulfur. Methanotrophic bacteria have also been found in the anoxic gradient of some chemoclines. A study conducted in Ace Lake, located in Antarctica, investigated the process of anoxygenic photosynthesis done by green sulfur bacteria in the lake and found that they were located exclusively in the chemocline of the lake due to the presence of light and sulfide. Furthermore, microbial processes can be responsible for the presence of chemical differences in a chemocline. Processes like carbon dioxide fixation, sulfur cycling, and exoenzyme activities occur at heightened rates in the cline compared to the surrounding body of water. Because of the various chemical properties of a chemocline, it can often support a diverse array of lifeforms in a small layer. However, chemocline instability can upset the balance of bacterial species found in each layer. Euxinic deep water that upwells into the photic zone can introduce sulfides and cause a bloom of sulfur oxidizing bacteria in the upper mixolimnion.

References

Aquatic ecology Limnology Physical oceanography {{ecology-stub