page_title: Black Sea
section_title: Hydrology
hierarchical_section_title: Black Sea / Hydrology
caption_attribution_description: English: This cloud-free Sea-viewing Wide Field-of-view Sensor (SeaWiFS) view of the Black Sea reveals the colourful interplay of currents on the lake’s surface. The green crescent shaped pattern in the north-western corner of the Black Sea is due to the nutrient-rich, sediment-laden discharge of the Danube River stimulating biological activity. The brighter, more turquoise patterns on the lake to the south and east are probably blooms of another type of phytoplankton.
context_page_description: The Black Sea is a body of water and marginal sea of the Atlantic Ocean between Eastern Europe, the Caucasus, and Western Asia. It is supplied by a number of major rivers, including the Danube, Dnieper, Southern Bug, Dniester, Don, and the Rioni. The watersheds of many countries drain into the Black Sea beyond the six that immediately border it.
  The Black Sea has an area of 436,400 km², a maximum depth of 2,212 m, and a volume of 547,000 km³. It is constrained by the Pontic Mountains to the south, Caucasus Mountains to the east, Crimean Mountains to the north, Strandzha to the southwest, Balkan Mountains to the west, Dobrogea Plateau to the northwest, and features a wide shelf to the northwest.
  The longest east–west extent is about 1,175 km. Important cities along the coast include Odessa, Sevastopol, Samsun, and Istanbul.
  The Black Sea is bordered by Ukraine, Romania, Bulgaria, Turkey, Georgia, and Russia. It has a positive water balance with an annual net outflow of 300 km³ per year through the Bosporus and the Dardanelles into the Aegean Sea.
context_section_description: The Black Sea is a marginal sea and is the world's largest body of water with a meromictic basin. The deep waters do not mix with the upper layers of water that receive oxygen from the atmosphere. As a result, over 90% of the deeper Black Sea volume is anoxic water. The Black Sea's circulation patterns are primarily controlled by basin topography and fluvial inputs, which result in a strongly stratified vertical structure. Because of the extreme stratification, it is classified as a salt wedge estuary.
  The Black Sea only experiences water transfer with the Mediterranean Sea, so all inflow and outflow occurs in the Bosporus and Dardanelles. Inflow from the Mediterranean has a higher salinity and density than the outflow, creating the classical estuarine circulation. This means that the inflow of dense water from the Mediterranean occurs at the bottom of the basin while the outflow of fresher Black Sea surface-water into the Marmara Sea occurs near the surface. The outflow is 16 000 m³/s (around 500 km³/year) and the inflow is 11 000 m³/s (around 350 km³/year), acc to Gregg (2002).
  The following water budget can be estimated:
  Water IN:
  Total river discharge : 370 km³/year
  Precipitation : 180 km3/year
  Inflow via Bosporus : 350 km³/year
  Water OUT:
  Evaporation : 400 km³/year (reducing since the 1970s)
  Outflow via Bosporus : 500 km³/year
  The southern sill of the Bosporus is located at -36.5 m below present sea level (deepest spot of the shallowest cross-section in the Bosporus, located in front of Dolmabahçe Palace) and has a wet section of around 38 000 m2. Inflow and outflow current speeds are averaged around 0.3 to 0.4 m/s, but much higher speeds are found locally, inducing significant turbulence and vertical shear. This allows for turbulent mixing of the two layers. Surface water leaves the Black Sea with a salinity of 17 Practical salinity unit (PSU) and reaches the Mediterranean with a salinity of 34 PSU. Likewise, an inflow of the Mediterranean with salinity 38.5 PSU experiences a decrease to about 34 psu.
  Mean surface circulation is cyclonic and waters around the perimeter of the Black Sea circulate in a basin-wide shelfbreak gyre known as the Rim Current. The Rim Current has a maximum velocity of about 50–100  cm/s. Within this feature, two smaller cyclonic gyres operate, occupying the eastern and western sectors of the basin. The Eastern and Western Gyres are well-organized systems in the winter but dissipate into a series of interconnected eddies in the summer and autumn. Mesoscale activity in the peripheral flow becomes more pronounced during these warmer seasons and is subject to interannual variability.
  Outside of the Rim Current, numerous quasi-permanent coastal eddies are formed as a result of upwelling around the coastal apron and "wind curl" mechanisms. The intra-annual strength of these features is controlled by seasonal atmospheric and fluvial variations. During the spring, the Batumi eddy forms in the southeastern corner of the sea.
  Beneath the surface waters—from about 50–100 meters—there exists a halocline that stops at the Cold Intermediate Layer (CIL). This layer is composed of cool, salty surface waters, which are the result of localized atmospheric cooling and decreased fluvial input during the winter months. It is the remnant of the winter surface mixed layer. The base of the CIL is marked by a major pycnocline at about 100–200 metres (330–660 ft) and this density disparity is the major mechanism for isolation of the deep water.
  Below the pycnocline is the Deep Water mass, where salinity increases to 22.3 PSU and temperatures rise to around 8.9 °C. The hydrochemical environment shifts from oxygenated to anoxic, as bacterial decomposition of sunken biomass utilizes all of the free oxygen. Weak geothermal heating and long residence time create a very thick convective bottom layer.
  The Black Sea undersea river is a current of particularly saline water flowing through the Bosporus Strait and along the seabed of the Black Sea. The discovery of the river announced on August 1, 2010, was made by scientists
caption: This SeaWiFS view reveals the colorful interplay of currents on the sea's surface
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