Salinity and Ionic Composition of Inland Waters

Salinity refers to the mass of dissolved inorganic solids found in water and is important both for its influence on water density, and therefore water movements, and for the individual constituents the term encompasses. Owing to the power of water as a solvent, salinity comprises numerous ions and molecules, but for most limnological purposes, salinity can be considered the mass fraction of the major cations (Na+, Mg2+, Ca2+, K+), anions (Cl−, SO42−), and carbonate species (HCO3−, CO32−). The salinity of inland waters is highly variable (almost pure water to over 400 g kg−1) and depends upon drainage and exchange from the surrounding land and groundwater, atmospheric sources, and human activity. In lakes, more so than in rivers, evaporation, biotic and abiotic losses from the water column, and exchange with sediments also regulate salinity. Environmental and anthropogenic influences on inland water salinity are predicted to increase in the future due to climate change and increasing demands for freshwater resources. All major ions are essential to sustaining life in inland waters, and salinity is of major importance in the osmotic regulation of metabolism and in the distribution of biota.