The Black Sea has a characteristic feature of much lower salinity than that of the Mediterranean Sea or the ocean because of its positive water balance (vapors are less than the water that flows in). This makes the fertile waters suitable for development of many species, including Mytilus (common mussel). At the same time the basin is almost in isolation from the neighboring seas, having only the Bosporus strait. The difference in salinity and the relatively low vertical water circulation are the reason why it can support life in only 13% of its volume - top 200 meters layer.
Micro- and macro-algae are the main sources of oxygen for the World Ocean. A major problem of the second half of 20th century is eutrophication (overfeeding) and pollution of rivers that flow into the sea. The eutrophication is caused by the increased use of fertilizers in agriculture and industrial wastewater. These contaminants result in faster algal blooms (red tides), which cannot proceed up along the food chain due to their excessive amount. The amount which has not been consumed dies and starts decomposing at the bottom of the basin. In the process of biological decay further deprives the water from oxygen. Hence starts a gradual degradation of ecosystems, as its exponential disappearance of species leads to gaps in the biological diversity, thus interrupting or tipping the balance of the food chains - from the lower organisms to larger marine life.
Another problem which contributes to the poor state of the biodiversity in the Black Sea is the so called "trawling" - a fishing method, which plows the seabed in strips spanning 10-12 km. Thus, raises the precipitate which causes death of many creatures. In 1990's species diversity has shortens by up to 71%, in population numbers by 60%, and in biomass by 95% in areas affected by bottom trawling.