The Black Sea is a unique body of water defined by its low salinity. While it contains dissolved salts, classifying it as saline, its structure differs drastically from a typical ocean basin. This characteristic makes the Black Sea a brackish environment, meaning its salinity level falls between that of freshwater and full-strength seawater. This balance of fresh and salt water creates a deeply stratified system with profound environmental consequences.
Defining Black Sea Salinity
Salinity is measured in parts per thousand (ppt), representing the grams of dissolved salt per kilogram of water. The average salinity of the world’s oceans hovers around 35 ppt. In contrast, the surface waters of the open Black Sea exhibit a much lower average salinity, typically ranging from 17 to 18 ppt—roughly half that of the Atlantic or Pacific Oceans.
The salinity does not remain uniform throughout the water column; instead, it increases with depth. Below the surface layer, the salt content stabilizes in the deep water mass, reaching levels between 22 and 30 ppt in the deepest parts of the basin. This vertical gradient in salt concentration is a defining physical feature of the sea.
Geographic Factors Shaping Salinity
The Black Sea’s distinct salinity results primarily from two geographic mechanisms: massive freshwater influx and restricted exchange with the Mediterranean Sea. The basin receives a tremendous volume of fresh water annually from Europe’s largest rivers, including the Danube, Dnieper, and Dniester. This constant input of over 400 cubic kilometers of river water significantly dilutes the surface layer, maintaining its low salt concentration.
The primary point of water exchange is the Turkish Straits System, a narrow and shallow passage composed of the Bosporus and the Dardanelles. This choke point prevents the rapid mixing and circulation typical of open ocean basins. The movement through the Bosporus is a complex, two-layer flow driven by the density difference between the two seas.
Lighter, brackish surface water flows out from the Black Sea over the top of the sill and into the Sea of Marmara. Simultaneously, denser, saltier Mediterranean water (around 38 ppt) flows inward along the bottom. Because river input exceeds evaporation, the Black Sea has a positive water balance, meaning the outflow of less-saline water is greater than the inflow of saltier water. This restricted exchange maintains the sea’s overall brackish nature and stratified structure.
The Permanent Anoxic Layer
The persistent density difference between the less-saline surface water and the saltier, heavier deep water creates a permanent barrier known as a halocline. This density discontinuity acts as a physical lid, preventing the vertical mixing of the upper and lower water masses. The halocline is typically found between 100 and 200 meters below the surface, effectively dividing the Black Sea into two distinct layers.
This stratification impacts the deep-sea environment significantly. Oxygen from the atmosphere and from photosynthesis in the surface layer cannot penetrate the deep water below the halocline. As organic matter sinks, its decomposition by bacteria consumes the available oxygen, resulting in a condition called anoxia.
Below this oxygenated surface layer (which accounts for only about 13% of the sea’s volume), the vast majority of the deep water is devoid of oxygen. The deep anoxic zone is characterized by dissolved hydrogen sulfide gas, a compound toxic to most marine life. This chemical environment limits complex organisms to the upper waters, but it also preserves organic material, including ancient shipwrecks, which are protected from biological decay.