Salinity refers to the concentration of salt in water. Usually expressed as a percentage or ppt (a few thousandths). Salinity is a very important index in oceanography, because it fully reflects the salinity of seawater. The salinity of pure water is 0, while that of fully saturated seawater is 35ppt. The change of salinity directly affects the survival and reproduction of marine life.

In addition to the salinity in seawater, many freshwater lakes and rivers also have a certain salinity. The salinity of these waters is usually much lower than that of seawater, usually 0.5-5ppt. The salinity of freshwater lakes and rivers is influenced by geographical location, climate, geological conditions and rock types around water bodies. Some seasonal droughts and evaporation aggravate the salt concentration in fresh water, leading to salinization of water bodies.

Salinity not only plays an important role in marine science, but also is widely used in other fields. For example, the salt index in agriculture can be used to evaluate the salt content in soil, which often affects the cultivation of crops. Plant growth requires a specific salinity range. If the salt in the soil is too high, it will have a negative impact on the growth and development of plants. In addition, in the field of environmental monitoring, salinity is an important parameter, which can reflect the pollution degree and salt content of water bodies.