Haloclines in the ocean are either seasonal or permanent. In coastal inlets, lagoons, fjords, and poorly mixed estuaries, haloclines form where freshwater overlies seawater. A vertical salinity gradient in haloclines can reach 46 PSU m−1 (50-cm layer of Fjord Hunnbunn, Norway; Yakushev et al., 2013).
Because salinity (together with temperature) affects the density of seawater, it can play an important role in its vertical stratification. Pycnoclines (layers of rapid change of density with depth) are often coupled to haloclines and thermoclines (layers where the temperature gradient is greater than that of the warmer layer above and the colder layer below).
The formation of a pycnocline restricts vertical mixing and leads to oxygen depletion in deeper waters and the formation of anoxia. This process occurs, for example, in the Black Sea, many marine...