Halokliini

Halokliini, or halocline in English, is the vertical region in a body of water where salinity changes rapidly with depth. It is a distinct layer within the water column that sits between layers of relatively uniform salinity and contributes to the overall vertical stratification of the ocean, seas, estuaries, or lakes. The halokliini is defined by a pronounced salinity gradient, typically measured as a sharp increase or decrease in salinity with increasing depth.

Formation and occurrence

Halokliini form where different water masses with contrasting salinities interact. Common causes include freshwater inputs from rivers or rainfall meeting saline seawater, evaporation in arid basins, and seasonal sea-ice processes in polar regions that couple with brine rejection and melt. In coastal and estuarine environments, the halokliini can be pronounced and highly variable with tides, river discharge, and wind-driven mixing. The depth and strength of the halocline vary widely by location, season, and hydrological conditions.

Measurement and ecological significance

Halokliini are detected by measuring salinity with depth, typically using CTD instruments (conductivity, temperature, depth) and identifying regions with large vertical salinity gradients. They interact with thermoclines (temperature gradients) and pycnoclines (density gradients); when halocline and pycnocline align, vertical mixing is strongly suppressed, influencing oxygen levels, nutrient distribution, and habitat use by marine organisms. In estuarine and coastal systems, the halokliini helps shape species distributions, larval transport, and ecosystem productivity.

Terminology

The term halokliini reflects the Greek-derived roots for salt (halo) and slope/gradient (cline). In English, halocline is the common term; halokliini is the equivalent term used in Finnish and some other languages.