When someone mentions 'salinity' and 'water' in the same sentence, most people think of the ocean, the Gulf or coastal bays. However, simply put, 'salinity' refers to the amount of salts dissolved in water, any water, and that includes "freshwater". Generally speaking, the salts that constitute salinity in oceans are the same as those in lakes and streams; the amount (concentration) of those salts is, of course, much higher in oceans.
The salinity of any waterbody is extremely important to all creatures living in and around it. For example, a slight change in salinity can cause fish, frog or shrimp eggs to float too much (high salinity) or not enough (low salinity), thus reducing or eliminating their chances of development into adults. Salinity is also important for water uses on land by people and wildlife. The saltier the water, the more difficult and expensive it is to prepare for drinking, and the more dangerous it is to apply to crops.
How does water become more or less salty?
The most important sources of salts, and therefore salinity, in all Earth's waters are: (1) washing (dissolving) of salts from the soil and rock of the Earth's crust; (2) precipitation (dust, rain and snow falling into the water) and (3) the evaporation and precipitation cycle. Let's look at a simple example.
Positively and negatively charged elements or "ions" create the salinity of all waters. These ions are found in the soil and rock of Earth's crust, and some, like sodium chloride (NaCl) are incredibly abundant. Often referred to as "common salt" or "common table salt", sodium chloride is comprised of two ions, a positively-charged sodium (Na) cation, and a negatively charged chloride (Cl) anion. When water saturates soil and washes over rock, NaCl is dissolved in the water (that is, the ions separate and bond with the ions of water) and the water becomes more salty; that is, its salinity increases.
As freshwater (streams and springs, for example) reaches marine waters, it mixes and reduces the salinity. These freshwater sources also provide nutrients, oxygen and food for the plants and animals there. The result is an amazing ecosystem, the "estuary," where a tremendous variety and abundance of life can flourish. Over 90% of all of the animal species that dwell within miles of land use the estuaries as their mating or spawning grounds; that is why estuaries are referred to as the nurseries of the oceans.
Protecting the flow of freshwater to estuaries is critically important. Organizations such as the Southwest Florida Water Management District are working to determine the amount of freshwater flows that are necessary to ensure rivers continue to maintain historical salinity patterns in estuaries. Salty water can also enter surface waters through groundwater systems. The process is known as saltwater intrusion, and it occurs as groundwater is pumped for use as public water supply, which allows saltwater to migrate inland. To prevent this problem from continuing, groundwater resources must be conserved. Water conservation on land is critical to protecting life in the Gulf!