How is freshwater distinguished from saline inland water by salt concentration?

In the study of inland water ecosystems, freshwater and saline water are distinguished primarily by their total concentration of dissolved salts, known as salinity.1

• Freshwater: Inland waters are conventionally classified as fresh when their total salt concentration is less than 3 grams per litre (equivalent to 3 parts per thousand or 3 grams per kilogram). Most freshwater systems typically exhibit even lower salinities, often measuring less than 0.5 grams per litre.1
• Saline Inland Water: Waters are defined as saline when their salinity exceeds 3 grams per litre. These environments can reach significantly higher concentrations, with maximum values in some salt lakes reaching approximately 350 grams per litre, depending on the dominant ions present.1

This distinction is fundamental to the biology of these ecosystems, as organisms must possess specific physiological adaptations—such as osmoregulatory abilities—to survive in either dilute freshwater or more concentrated, stressful saline environments.2