Chemocline

A chemocline is a cline caused by a strong, vertical chemistry gradient within a body of water. A chemocline is analogous to a thermocline, the border at which warmer and cooler waters meet in an ocean, sea, lake, or other body of water. (In some cases, the thermocline and chemocline coincide.)

Chemoclines most commonly occur where local conditions favor the formations of anoxic bottom water — deep water deficient in oxygen, where only anaerobic forms of life can exist. The Black Sea is the classic example of such a body, though similar bodies of water (classified as meromictic lakes) exist across the globe. Aerobic life is restricted to the region above the chemocline, anaerobic below. Photosynthetic forms of anaerobic bacteria, like green phototrophic and purple sulfur bacteria, cluster at the chemocline, taking advantage of both the sunlight from above and the hydrogen sulfide (H₂S) produced by the anaerobic bacteria below.

In any body of water in which oxygen-rich surface waters are well-mixed (holomictic), no chemocline will exist. To cite the most obvious example, the Earth's global ocean has no chemocline.