4.1.11 (HL)—Stratification

A thermocline is a depth zone of rapid temperature change; the density contrast separates a mixed surface layer from deeper water.

Surface water often gains oxygen from air and photosynthesis, while sinking organic matter decomposes below, releasing nutrients and consuming oxygen. Seasonal wind and productivity can change the profile.

A strong thermocline can leave deep nutrients below the lighted surface, limiting phytoplankton until mixing occurs.

Below a strong thermocline, decomposition releases mineral nutrients and consumes oxygen, while the density contrast restricts oxygen resupply from the surface.

A thermocline is a gradient, not a permanent sharp wall; depth and strength vary with season and circulation.