The presence of water defines Earth, but not all water is the same. The planet’s two dominant aquatic environments, freshwater and saltwater, support distinct ecosystems and possess fundamentally different physical characteristics. Freshwater, found in rivers and lakes, contains minimal dissolved salts, making it the primary source for terrestrial life. Saltwater, concentrated in the world’s oceans and seas, is defined by its high mineral content. This difference in dissolved solids creates profound chemical, physical, and biological consequences across the globe.
The Fundamental Difference: Salinity and Chemical Composition
The primary distinction between the two water types is the concentration of dissolved inorganic solids, known as salinity. Salinity is typically measured in parts per thousand (ppt), representing the mass of dissolved material found in one kilogram of water. Seawater has an average salinity of approximately 35 ppt, characterized by a high concentration of dissolved ions. Water is considered freshwater when its salinity is less than 1 ppt, often falling below 0.35 ppt.
The composition of saltwater is dominated by sodium chloride (NaCl), which accounts for over 85% of the dissolved solids. The six most abundant ions—chloride, sodium, sulfate, magnesium, calcium, and potassium—maintain constant proportions throughout the world’s oceans. Freshwater contains much lower total concentrations of dissolved solids, but its composition is more variable. It often features a higher relative proportion of minerals like calcium and magnesium bicarbonate, leached from local geological formations.
This difference in dissolved content also alters the physical properties of the water. Dissolved salts increase the density of saltwater, making it heavier than freshwater at the same temperature and pressure, which drives large-scale ocean circulation patterns. The addition of salt lowers the freezing point of water; pure freshwater freezes at 0°C, while average seawater remains liquid until it reaches approximately -1.8°C.
Global Distribution and Water Volume
The sheer volume of water on Earth is immense, yet it is highly unevenly distributed between the two types. Saltwater constitutes about 97.5% of all water on the planet, almost entirely contained within the oceans and seas. This massive reservoir forms a continuous body that covers approximately 71% of the Earth’s surface.
The remaining 2.5% of the global water supply is freshwater, but most of this is not readily available for human or ecological use. Nearly 70% of all freshwater is locked away in the form of glaciers, ice caps, and permanent snow, primarily in Antarctica and Greenland. A significant portion of the rest, roughly 30%, exists as groundwater stored beneath the Earth’s surface.
The freshwater that is most accessible—in rivers, lakes, and the atmosphere—accounts for less than 1% of the total global freshwater budget. This scarcity highlights a geographical paradox: while water covers most of the planet, the usable, low-salinity resource is a limited commodity. The small amount of surface water is continuously renewed through the hydrological cycle, which involves precipitation and runoff.
Biological Consequences: Life Adaptations
The osmotic gradient between freshwater and saltwater environments forces organisms to employ drastically different physiological mechanisms for survival. This process, known as osmoregulation, is the active maintenance of salt and water balance within an organism’s tissues and fluids. Without these specialized adaptations, cellular processes would fail due to uncontrolled water movement.
Freshwater fish are hypertonic, meaning their internal salt concentration is higher than the surrounding water. This osmotic imbalance causes water to constantly flow inward, while salts diffuse outward, risking cellular swelling and salt depletion. To counteract this, freshwater fish actively excrete large volumes of highly dilute urine to eliminate excess water. They also use specialized cells in their gills to actively transport salts back into their bloodstream.
Marine fish face the opposite challenge, as their body fluids are hypotonic to the high-salinity seawater. This causes a constant tendency for water to leave their bodies and for salts to diffuse inward. These organisms must continually drink seawater to replace lost water and possess highly efficient kidneys that produce a small amount of concentrated urine. They also use chloride cells in their gills to actively pump excessive salt ions out of their bodies and back into the environment.