Black Sea

(geography) A large inland sea, area 163,400 square miles (423,000 square kilometers), bounded on the north and east by the Commonwealth of Independent States (former U.S.S.R.) on the south and southwest by Turkey, and on the west by Bulgaria and Rumania.

A semienclosed marginal sea with an area of 420,000 km² (160,000 mi²) bounded by Turkey to the south, Georgia to the east, Russia and Ukraine to the north, and Romania and Bulgaria to the west. The physical and chemical structure of the Black Sea is critically dependent on its hydrological balance. As a result, it is the world's largest anoxic basin. It has recently experienced numerous types of environmental stress.

The Black Sea consists of a large basin with a depth of about 2200 m (7200 ft). The continental shelf is mostly narrow except for the broad shelf in the northwest region. Fresh-water input from rivers, especially the Danube, Dniester, and Don, and precipitation exceeds evaporation. Low-salinity surface waters are transported to the Mediterranean as a surface outflow. High-salinity seawater from the Mediterranean enters the Black Sea as a subsurface inflow through the Bosporus. This estuarine circulation (seawater inflow at depth and fresh-water outflow at the surface) results in an unusually strong vertical density gradient determined mainly by the salinity. Thus the Black Sea has a two-layered structure with a lower-salinity surface layer and a higher-salinity deep layer.

The vertical stratification has a strong effect on the chemistry of the sea. Respiration of particulate organic carbon sinking into the deep water has used up all the dissolved oxygen. Thus, conditions favor bacterial sulfate reduction and high sulfide concentrations. As a result, the Black Sea is the world's largest anoxic basin and is commonly used as a modern analog of an environment favoring the formation of organic-rich black shales observed in the geological sedimentary record.

Before the 1970s the Black Sea had a highly diverse and healthy biological population. Its species composition was similar to that of the Mediterranean but with less quantity. The phytoplankton community was characterized by a large diatom bloom in May-June followed by a smaller dinoflagellate bloom. The primary zooplankton were copepods, and there were 170 species of fish, including large commercial populations of mackerel, bonito, anchovies, herring, carp, and sturgeon.

Since about 1970 there have been dramatic changes in the food web due to anthropogenic effects and invasions of new species. It is now characterized as a nonequilibrium, low-diversity, eutrophic state. The large increase in input of nitrogen due to eutrophication and decrease in silicate due to dam construction have increased the frequency of noxious algal blooms and resulted in dramatic shifts in phytoplankton from diatoms (siliceous) to coccolithophores and flagellates (nonsiliceous). The most dramatic changes have been observed in the northwestern shelf and the western coastal regions, which have the largest anthropogenic effects. The water overlying the sediments in these shallow areas frequently go anoxic due to this eutrophication. In the early 1980s the grazer community experienced major increases of previously minor indigenous species such as the omnivorous dinoflagellate Noctilluca scintillans and the medusa Aurelia aurita. The ctenophore Mnemopsis leidyi was imported at the end of the 1980s from the east coast of the United States as ballast water in tankers and experienced an explosive unregulated growth. These changes plus overfishing resulted in a collapse of commercial fish stocks during the 1990s.