Quick Definition
A layer where salinity changes rapidly with depth, creating a density gradient that resists vertical mixing.
📖 Full Definition
A halocline is a vertical zone in the ocean or estuary where salinity changes rapidly. High-salinity water is denser and sinks beneath lower-salinity water, creating a stable stratification. In estuaries, a strong halocline (or salt wedge) can form as dense, salty ocean water pushes beneath fresh river water. Many diadromous fish (salmon, eels) must cross the halocline during their migrations, requiring physiological adjustment (smoltification in salmon). In the Black Sea, the permanent halocline at ~150 m creates an anoxic lower layer.