Many of the Europe's largest rivers, including the Danube, the Dnister, and the Dnipro (also called Dnieper), dump freshwater into the sea. The sea's only source of salty water, on the other hand, is the narrow Bosporus Strait, which connects it to the Mediterranean Sea through the Sea of Marmara. The salty water is denser than the freshwater, and so it sinks to the bottom, leaving a layer of relatively freshwater on top. The density barrier between salt- and freshwater is great enough that the two layers do not mix. As a result, when freshwater enters the sea from rivers, it only mixes with the relatively fresh water in the top 150 m of the sea. This means that fertilizers and runoff carried in the river water remain concentrated at the top of the sea where they nourish the tiny plants (phytoplankton) that grow on or near the surface (see Figure 8).
In the spring of 2006, floods on the Danube River (see image captured by advanced spaceborne thermal emission and reflection radiometer (ASTER) on the Terra satellite: http://earthobservatory.nasa.gov/
NaturalHazards/natural_hazards_v2.php3?img_id=13521) swept over broad stretches of farmland. The floods likely washed sediment, fertilizers, and animal waste into the Danube and the Black Sea. The extra iron, phosphates, and nitrates in the flood debris may be supporting the extensive bloom seen here. Such blooms can be both beneficial, because they provide food for fish, and dangerous, because decaying plant matter saps oxygen out of the water. If enough phytoplankton from a large bloom die and decay, the water may become so oxygen poor that fish can no longer survive in it. The result is a dead zone where little can survive.
Dead zones normally happen near the mouths of large rivers where fertilizers and agricultural waste are concentrated in the ocean. The Black Sea is one of the world's largest dead zones, though its dead zone is related to its stratification as much as to fertilizer runoff. When plants and other organic matter sink to the floor ofthe sea, they decay in the salty layer of water. Since the denser saltwater does not mix with the fresher water at the surface, there is no way to replenish the oxygen used during the decay process. As a result, the lower layer of the Black Sea is totally oxygen free.
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