Seas are vast bodies of saltwater, not freshwater. This fundamental difference has significant implications for life on Earth. This article explores what defines freshwater, the characteristics of seawater, and the geological processes that make oceans salty.
What Defines Freshwater?
Freshwater is characterized by its very low concentration of dissolved salts and other total dissolved solids. It typically contains less than 0.05% dissolved salts, or fewer than 500 parts per million. This low salinity makes it suitable for direct human consumption, agricultural irrigation, and sustaining most terrestrial ecosystems.
Common sources include rivers, lakes, streams, and ponds. Significant amounts are also stored as groundwater beneath the Earth’s surface. Glaciers and polar ice caps hold a substantial portion of the planet’s freshwater in frozen form.
The Nature of Seawater
Seawater contains a high concentration of dissolved salts, predominantly sodium chloride, which is common table salt. On average, seawater has a salinity of about 3.5%, meaning every kilogram contains approximately 35 grams of dissolved salts. While sodium chloride is the most abundant, seawater also contains various other dissolved minerals and gases, including magnesium, sulfate, calcium, and potassium.
This high salt content gives seawater its distinct taste and influences its physical properties, such as density and freezing point. Its elevated salinity makes seawater unsuitable for direct human consumption without extensive desalination. Most agricultural crops cannot tolerate irrigation with untreated seawater due to its damaging effects on plant cells.
Why Seas Hold Salt
The salinity of oceans results from millions of years of geological and hydrological processes. Rivers continuously flow across land, eroding rocks and soils as they travel towards the sea. Water dissolves minerals and salts from the land, carrying these substances into the oceans. This constant input from rivers is a primary source of the ocean’s salt content.
Underwater volcanic activity and hydrothermal vents on the seafloor also contribute to oceanic salinity. These geological features release various minerals and gases directly into the ocean water.
Evaporation also plays a role in concentrating salt in seawater. When sunlight warms the ocean surface, water molecules evaporate and rise into the atmosphere, leaving dissolved salts behind. This water vapor condenses to form clouds and falls back to Earth as freshwater precipitation, completing the water cycle. Since only pure water evaporates, salts steadily accumulate in the oceans over geological timescales.
The Global Water Picture and Its Importance
Approximately 97% of all water on Earth is saltwater, found primarily in oceans and seas. This leaves only about 3% as freshwater.
Roughly two-thirds of this freshwater is locked away in glaciers, ice caps, and permanent snow cover, making it largely inaccessible. The remaining freshwater exists as groundwater, in lakes, rivers, and atmospheric moisture.
The scarcity of freshwater underscores its importance for human survival, agriculture, and supporting diverse ecosystems. Challenges related to freshwater access have led to technologies like desalination, which removes salt from seawater to produce potable water. Understanding global water distribution is important for managing and conserving this resource.