Black Sea Ecosystem

• Category: Marine and Oceanic Biomes.
• Geographic Location: Southeastern Europe.
• Summary: The Black Sea is a great inland sea characterized by a low proportion of oxygen-rich water; it is not lacking in biological diversity at the surface and along its coastline habitats, so the sea has been under great environmental stress.

The Black Sea, located in southeastern Europe and western Asia, is a largely enclosed sea with a narrow link to the Mediterranean that permits two-way flow of waters. With a surface area of 178,000 square miles (461,000 square kilometers), the Black Sea is bound on the south by Turkey, the west by Bulgaria and Romania, the north by Ukraine, the northeast by Russia, and the east by Georgia. Its drainage basin draws upon 965,000 square miles (2.5 million square kilometers) in this Eurasian region. With its average depth of about 4,185 feet (1,275 meters), the Black Sea encompasses an average water volume of approximately 130 cubic miles (545 cubic kilometers).

Hydrology

Most of the freshwater flows into the Black Sea from the northern half. The Danube River, which drains a catchment of some 309,000 square miles (801,000 square kilometers) as it winds 1,775 miles (2,857 kilometers) across Europe, is the main inflow source of freshwater. Other major sources are the Dniestr, flowing from the northwest through Moldova; Dniepr to the north, the largest river in Ukraine; also to the north the Don, which flows into the Sea of Azov, the northeast lobe of the Black Sea; the Rioni and Chorokhi in Georgia; and several rivers in Turkey, among them the Yesil-Irmak, Filyos, and Sakarya.

The Black Sea receives inflow of 83 cubic miles (348 cubic kilometers) of freshwater annually, with by far the greatest flow from the Danube River, at some 48 cubic miles (200 cubic kilometers). About 1.8 billion cubic feet (52.2 million cubic meters) of sediment are deposited into the sea annually, of which well over half flows from the Danube. An undersea river of saline water flows in from the Mediterranean via the Bosporus Strait. The Black Sea surface-layer salinity is about half that of the average for oceans around the world, thus it is considered brackish.

Due in part to its particularly stable water column layering—there is a vast anoxic level in the depths below about 800 feet (245 meters), accounting for well over 80 percent of all the sea's water—the Black Sea harbors a relatively low number of species for its size. However, its surface waters and coastal realms support great diversity of species and abundance of biota. Still, with changing levels of pollution and nutrient runoff, species population shifts, and other factors that have interacted with its already low-oxygen regime and enclosed nature, the sea has at times sustained very high levels of eutrophication, hypoxia, and anoxia. Changing episodes of human impact have also altered the mix; it is scientifically notable that the collapse of the Soviet Union in 1991 and the bottoming-out of its intensive agriculture practices led directly to a profound decrease in the inorganic fertilizer-originated nutrient inflow, helping resuscitate the Black Sea from a period of oxygen-choking eutrophication.

Habitats and Vegetation

The Black Sea is ensconced in a wealth of coastal-area habitats that include continental shelf, coastal lagoons, intertidal zones, wetlands, estuaries, dunes, cliffs, grasslands, steppes, rivers, and forests. Among the most richly endowed is the Danube Delta, a large and critical habitat area encompassing some 1,740 square miles (4,500 square kilometers) and one of the largest estuary wetlands in Europe. Great breeding populations of birds nest here among estuarine forests and reeds, such as the white pelican (Pelecanus onocrotalus), glossy ibis (Plegadis falcinellus), pygmy cormorant (Phalacrocorax pygmeus), and marsh harrier (Circus aeruginosus).

Within the deeper waters of the Black Sea, populations of phytoplankton and zooplankton swell and contract in sway with seasonal changes to temperature, salinity, and nutrient inflow. These foundation species support the food web. In turn, oyster beds, marine mammals, and anadromous fish schools thrive or diminish; overflying and nesting bird flocks, too, have their own rhythms.

Some of the Black Sea habitat areas have undergone immense change. The northwestern shelf area, for example, was once dominated by the red seaweed Zernov's Phyllophora, but which, having been nearly eliminated by more aggressive macroalgae, now is sustained in the area mainly by carefully planned and protected human cultivation. Similarly, thickets of marine eelgrass (Zostera) in the same area have also been decimated.

Above the water, the mainly oak forests of the Strandzha region of Bulgaria, host to such types as white oak, Hungarian oak, and turkey oak (Quercus polycarpa, Q. frainetto, and Q. cerris), also feature Western Pontic beech (Fagus orientalis) and understory species such as Pontic rhododendron (Rhododendron ponticum) and twin-flowered daphne (Daphne pontica).

Fauna of the Region

The Black Sea has long supported significant fisheries. European anchovy (Engraulis encrasicolus) and sprat (Sprattus sprattus) are among the dominant commercial types currently, increasing in importance as stocks of predatory fish have been decimated; these have previously included bonito, horse mackerel, and bluefish.

Some thirty species of sturgeon (Acipenser spp.) were historically found in abundance here; favored feeding areas were located all around the coastal waters with the exception of much of the southern shoreline. However, overfishing and pollution have devastated their numbers. Aquaculture solutions have yielded mixed results, particularly in light of the long period of maturation (as much as twelve years) of sturgeon.

Among marine mammals, endemic (found only here) subspecies include the Black Sea bottlenose dolphin (Tursiops truncatus ponticus), Black Sea common dolphin (Delphinus delphis ponticus), and Black Sea harbor porpoise (Phocaena phocaena relicta).

Upwards of 300,000 seabirds and as many as 100,000 raptors rest here each autumn on their way south, part of the second-largest flyway in Europe. Threatened or endangered avian species include Dalmatian pelican (Pelecanus crispus), ferruginous duck (Aythia nyroca), and the slender-billed curlew (Numenius tenuirostris).

Human and Climate Issues

Along with urban, industrial, and agricultural pollution, the Black Sea biome has had to contend with overfishing, coastal habitat destruction, and invasive species. Key species that have been wiped out or severely depleted here by direct human activity include the endangered semi-aquatic European mink (Mustela lutreola) and the endemic Black Sea turbot (Scophtalmus maeotica). The Black Sea oyster (Ostrea edulis) has been a victim of the rapacious whelk (Rapana venosa), and also by an introduced jellyfish.

In the 1980s the warty comb jelly (Mnemiopsis leidyi), a species native to the Atlantic Ocean and with few predators, arrived in the Black Sea. The warty comb jelly gorged on zooplankton as well as the fish eggs and larvae of many species. Its population exploded, precipitating the collapse of anchovy, chub mackerel, and two dozen other fisheries, as well as engendering the rise of red algae blooms. Fortunately, quantities of pink comb jelly (Beroe ovata), a Mediterranean species that is a natural predator of the Mnemiopsis, arrived in the Black Sea in the late 1990s, coming there to feast upon its prey and to somewhat contain the warty population.

Climate change in some cases has augmented the effects put into action by humans and invasive species. Research has clearly shown, for example, that warmer average temperatures in several periods since the late 1980s likely contributed to the spread of the warty comb jelly. The warmer regime may also be contributing to increased anchovy and sprat catches, a habitat positive, but also results in increased blooms of toxic cyanobacteria, a habitat negative. Rising seawater temperatures have the potential to cause catastrophic damage in the Black Sea.

2023 data shows that warming has accelerated, with surface temperatures increasing at nearly twice the global ocean average. This rapid warming is particularly concerning for the Black Sea's unique layered ecosystem, as it affects oxygen levels and may destabilize the boundary between oxygenated and anoxic waters.

According to a 2021 study by the Institute of Marine Sciences of the Middle East Technical University, the temperature of the Black Sea increased by 3.6 degrees Fahrenheit (2 degrees Celsius) in the previous sixty years. If the temperature continues to rise, most of the marine life will perish.

Bibliography

Adams, Terry D., et al. Europe's Black Sea Dimension. Centre for European Policy Studies, 2002.

Bat, Levent, et al. "Biological Diversity of the Turkish Black Sea Coast." Turkish Journal of Fisheries and Aquatic Sciences, vol. 11, no. 4, 2011, pp. 683-92.

Commission on the Protection of the Black Sea Against Pollution. "Strategic Action Plan for the Environmental Protection and Rehabilitation of the Black Sea." BSC, 2009, www.blacksea-commission.org/‗bssap2009.asp. Accessed 9 Nov. 2024.

"Global Warming Threatens Marine Life in Black Sea." The Hurriyet Daily News, 1 Oct. 2021, www.hurriyetdailynews.com/global-warming-threatens-marine-life-in-black-sea-168275. Accessed 22 July 2022.

Kideys, Ahmet E. "Fall and Rise of the Black Sea Ecosystem." Science, vol. 297, no. 5586, 2002, pp. 1482-84.

Stanev, E. V., et al. "Recent Intensification of Black Sea Warming and Stratification: Analysis of Argo Float Observations." Progress in Oceanography, vol. 219, 2023, pp. 103039-48. Accessed 9 Nov. 2024.