Black Sea is an inland sea connected to the small Sea of Marmora (= Marmara Sea ) by the narrow (750m) and shallow (min. depth 32m) Bosporus Strait; Strait of Dardanelles further connects Sea of Marmora (= Marmara Sea ) to Mediterranean Sea.
The Catchment Area - Black Sea Drainage Basin
An important feature of the Black Sea is an unusually high river discharge into the relatively small semi-enclosed Sea. The Black Sea drainage basin covers almost third part of Europe; the largest river is Danube flowing through 10 European countries, other big rivers are Dnieper, Bug, Dniester, Don, Kuban, Rioni. The average annual river runoff into Black Sea is 350 km3, whereas the Black Sea volume is 550000 km3. This water head creates a current from the Black Sea - through the Straits and the Sea of Marmora (= Marmara Sea ) - to the Mediterranean.
Rivers dilute Black Sea water - its surface layer salinity is 170 (gram salt in 1 liter of seawater), two times less than that of the Ocean (average 350). The reduced salinity is the most important environmental factor influencing marine biodiversity in the Black Sea: most marine animals and plants can't survive there. Two to five times fewer species in various taxa of benthic animals (i.e. bottom-dwelling - worms, mollusks, crabs etc.) live in the Black Sea as compared to the neighboring Mediterranean. Macroalgal variety is twice smaller then in the Mediterranean, and planktonic biodiversity is decreased about 1.5 times. There are no corals, no octopuses and squids; no seastars, sea urchins in the Black Sea - of all echinoderms only several small ophiuran and holothurian species adapted to local conditions. Probably we should not regret the absence of spiny sea urchins, as well as the fact that of all sharks we can only see a harmless spiny shark - dogfish Squalus acanthias and 2 species of skates in the Black Sea. There are very little dangerous marine creatures in Black Sea - no deadly jellyfish or stingy sea anemones. Despite the fact that the Black Sea biodiversity is reduced due to a lower salinity level, the marine life of Black Sea is full of wonders - scientists have been studying it for hundreds of years already, and still discover new species; Black Sea is really unique among all seas on our planet
Map of the Black Sea; Biodiversity, Hydrology, Geography data
This SeaWiFS satellite image made on April 9th of 2003 shows concentration of chlorophyll a - and correspondingly, of phytoplankton - in surface layer of the sea. Blue color denotes a small amount of chlorophyll, whereas green and yellow point to rich phytoplankton, and the red color to algal bloom. Chlorophyll concentration, phytoplankton density, and therefore marine ecosystem production are higher in the Black Sea then in the Mediterranean. The red color in the western part of the Sea - phytoplankton bloom in nutrient enriched waters brought by the Danube. Shallow Sea of Azov off North-East was also in bloom due to the fertilizing discharge of Don and Kuban rivers.
Eastern Mediterranean coasts are arid, particularly as compared to the Black Sea coast: there are too little rivers bringing mineral nutrients. As a result there is less phytoplankton in the sea - the blue color on the picture. Even the huge Nile can not supply enough nutrients - only around its estuary higher chlorophyll concentration can be seen.
Many rivers falling into the Black Sea bring not only fresh water diluting the sea; they also provide more then sufficient concentration of mineral nutrients necessary for the growth of marine plants such as phosphates and nitrates - those needed also by ordinary plants growing on soil. Many rivers bringing along many nutrients guarantee the fast growth of marine phytoplankton and dense underwater macroalgal vegetation off rocky shores of the Black Sea.
A uniquely high river discharge into an inland sea has at least two major consequences for the Black Sea marine life: rivers dilute the Black Sea water reducing its biodiversity - and at the same time they supply sea vegetation with unusually high amount of nutrients, providing its fast proliferation and high biomass. In other words, marine life in the Black Sea is not diverse, whereas its growth rate (ecosystem production) and biomass are high.
The prolific nature of the Black Sea ecosystem was first discovered by Ancient Greeks - we know that from Herodotus (V century BC), Ptolemaios (II century BC), Strabo (I century BC), and earlier (VII BC) explorers who left us accounts of innumerable Black Sea stocks of sturgeon, mullet, salmon, tuna. They compared the Black Sea with their native and less abundant (at the time!) Eastern Mediterranean. Based on their pioneer's reports, the Hellenes had made decision of enormous significance for the future of the Black Sea region: they started founding colonies on Black Sea shores, as early as 7th Century BC. These colonies are regarded as the first ones in the history of European civilization. Most of modern cities here are successors of Ancient Greek colonies such as Sevastopol, Theodosia, Kerch, Anapa, Novorossiysk, Sokhumi, Sinop, and many others. In some places ancient artifacts and sometimes the whole colonies dug out by archeologists are open for the public - as excavations of Chersoneses in Sevastopol, or Gorghippia in Anapa.
Of course, there were people living there before the Greek settlers: first Scythian, then Sarmat nomads played the major role in the early history of the Northern Black Sea area; the famous warlike women tribe of Amazons roamed the steppe between the Don and Kuban rivers. Lingual historians regard the Northern Black Sea plains as the focus of origin of the Proto-Indo-European language - the source of Indo-European languages. With the Hellenic arrival a new, European history of the Black Sea began.
It was the fish that attracted the Hellenes to the Black Sea shores, and fishery became the main industry of the early colonies: salted and smoked Black Sea fish was shipped to Greece. Later the colonists developed agriculture on fertile soils around settlements - and for a long time Black Sea colonies (later states independent from metropolises) were the main suppliers of wheat and wine for Ancient Greek states. Especially successful and important was the Bosporan Kingdom prospering on both sides of the Kerch strait. Modern vineyards of Taman peninsula are now growing where there used to be Bosporan vineyards over 2000 years ago. Following the Greeks, the Byzantine Empire dominated Black Sea cities and states. After the devastating forays of Eastern nomad hordes during the Great Resettlement of Nations, the Genoese and Venetians had built fortresses on the same shores. Greeks, Georgians, Bulgars, Italians, Adygeis, Tartars, Turks, Russians - history of these coastal settlements has continued for almost 3000 years already. Thus Black Sea gives us a remarkable example of how unusual marine ecosystem features which were brought about by the unique combination of natural conditions led to an importance of the region in social history.
Unfortunately, and due to the efforts of the two preceding generations of our ancestors - the situation now with the Black Sea fish stock is quite different from that described by Herodotus. It is exhausted by overfishing: salmon, sturgeon and bonito became rare species. The Black Sea turbot Psetta maxima maeotica population is on the decline for the same reasons. Hamsa, or khamsi, Black Sea anchovy, Engraulis encrasicholus ponticus population - once the main species for industrial fisheries and incredibly abundant some 40 years ago - declined to the level of by-catch in 1980-90s due to overfishing, plaktivorous ctenophore Mnemiopsis leidyi invasion, and natural reasons (Adriatic anchovy population had depressed at the same time). Black Sea anchovy stock has recovered, and khamsi fishery (almost ecxclusively Turkish industry by now) accounts for over 90% of fish catches in the Black Sea. There is no more tuna and bonito coming to the Black Sea for summer foraging because the fish is unable to pass the Bosporus Strait over-polluted by Istanbul wastes. Traditional fishermen communities has survived only at Southern coast of Black Sea.
Seawater features
Powerful inflow of nutrients brought by rivers result in proliferous growth of phytoplankton, because of which coastal waters of the Black Sea usually have greenish color. Not all phytoplankton can be grazed by zooplankton, and the Black Sea marine ecosystem cannot immediately digest all produced organic material; because of it Black Sea water, particularly during warm months, contains substantial amount of organic suspension - detritus. Living plankton, detritus, and clay particles brought by rivers make coastal Black Sea waters relatively turbid, e.g. compared to the Mediterranean Sea. Underwater visibility in the Black Sea rarely exceeds 7 meters; an exception is the South Crimean coast where visibility reaches 20 meters even in summer, because the Crimean peninsula protrudes into the central part of the Sea, has no high mountains (a small drainage basin) and very few rivers. And the water in the central part of the Black Sea is more blue. What exactly determines the color of seawater?
Seawater absorbs red and yellow quanta of sunlight more than the others: blue and green quanta of light remain less absorbed because of which they prevail in the light that is reflected by seawater into our eyes. Blue-green - aquamarine - is the color of pure seawater.
Chlorophyll of planktonic algae (depicted by dots) absorbs red light quanta, and algae use their energy for photosynthesis. Therefore, seawater becomes more green. There are also many marine phytoplankton algae that have high concentration of accessory photosynthetic pigments, carotenoids, absorbing blue-green light; the color of those algae varies from yellow to deep brown and even red. They are mainly diatoms and dinophlagellates - the two major groups of marine macro-phytoplankton ("macro" refers to "big" cells - over 30 um). When phytoplankton of that sort reaches high concentration, seawater turns into the color of the algae and we can speak of red tide - algal bloom.
Not only does algae render seawater its color. After heavy rains at the Caucasian coast the sea becomes yellow to the horizon line - it is a color of a suspension of clay particles brought by swollen rivers. On the coasts formed of limestone, when the sun is bright, the sea seems turquoise, and waves coming on the shore bring along enchanting white luminescence: tiny white limestone particles are acting as finest mirrors. This can be seen in many places of the Mediterranean coast, and sometimes at the South-Western Crimean coast of the Black Sea.
Clouds strongly affect seacolor. In sunny days we see their deep-purple shadows moving on the seasurface. When the sky is overcast the sea becomes dark - sometimes, especially during a storm, it is so menacingly dark that one may call it black; it is partly a psychological effect. One of the most popular hypotheses on the etymology of the Black Sea name refers to that effect. The name Black Sea is of Turkic origin - Kara Denghis, Karadeniz in modern Turkish. The translation of the Black Sea in all languages is actually a translation of that Turkic name: Chyornoe More - in Russian, Cherno More - in Bulgarian, Marea Neagra - in Rumanian, etc. It is hypothesized that Turkic nomads when they first came to the Black Sea shores from the sunnier Middle Asia were shocked by rough weather conditions (particularly possible in winter). Indeed, the history might have repeated itself: much earlier, when Ancient Greeks first entered the Black Sea, the most appropriate name they found for the Sea was Pontos Axenos i.e. literally inhospitable, hostile sea. Albeit after a while, getting to know the Sea better, they changed the name for Pontos Euxinos, i.e. kindly, hospitable sea.
Another popular hypothesis on the origin of the Black Sea's name refers to the phenomenon known to seamen since ancient times: any metallic object submerged deep - over hundred meters - to the Black Sea waters becomes absolutely black. For example, sounding lead used to measure depth on ancient ships turned black here. This happens because the deep waters of the Black Sea contain sulfuric hydride H2S - chemical substance that form salts of metals - sulfides - of black color (e.g. FeS - iron sulfide). In marine sediments - silt, sand - H2S can be found within millimeters of bottom surface. We can find signs of it on any beach: black seashells. These shells spent some time buried in the bottom where they interacted with sulfuric hydride and thus blackened; stormy waves could then dig the shell out of sand and bring it to the beach.
Finding H2S in bottom sediments, or presence of deep water anoxic zone in smaller lakes and enclosed bays is not unusual; but the whole stratified sea with anoxic water mass accounting to 87% of the Sea water is exceptional: there is only one such sea on our Planet.
It is one of the most well-known Black Sea properties: deeper than 200 meters, Black Sea water does not contain oxygen, and contains dissolved sulfuric hydride. How that becomes possible?
Oxygen comes into seawater from air, and is being constantly produced by phytoplankton in the upper layer of the sea, photic zone, ca. 0-100m depth. Both processes are functions of the sea surface.
Water mixing (driven by currents and waves) is needed for the oxygen captured from air and generated by algae at sea surface reached lower, aphotic layers of the sea; Black Sea is a unique case of an extremely low vertical water mixing - it is the world's largest stratified water body.
Vertical circulation in Black Sea is weak and enough to compensate for respiratory demand only in the upper 150 meters of water column, all oxygen being exhausted there, and can not be detected below the 200 meters mark. The causes of this situation are:
" Two layers of water with different salinity exist in the Black Sea: upper layer with 17 0 salinity as the result of constant influx of fresh water from rivers, and lower layer of higher (20 to 30‰ near the Black Sea bottom) salinity originating from the Marmora Sea water leaking along the bottom of the Bosporus Strait and then submerging to the Sea deeper layers. Surface-to-bottom change in salinity is not gradual: fast salinity change to 210 - halocline (and corresponding change of seawater density - picnocline) - occurs at 50 to150 meters depths. Temperature in the sea changes in a similar way: whereas at seasurface it is determined by air temperature, it is 8-9oC all year round in the depth; most of the change in water temperature takes place at 50-100m layer, it is the Black Sea thermocline depth. Thermocline depth range in the Black Sea is called cold intermediate layer because it is colder than deeper waters: cooling to 5-6oC during winter, this water layer does not warm up in summer.
" relatively low surface-to-watermass ratio in the Black Sea (the Sea is small, but deep); narrow (<2.5 km) continental shelf at 2/3 of the Black Sea coastline (steep underwater slopes of the Sea);
" high marine ecosystem production in Black Sea (and biomass per surface area) generated by high nutrient supply by rivers.
Due to the lack of oxygen, there are no animals and plants below 150 meters depth - all the way down to the bottom of Black sea at 2000 meters. Only saprothrophic bacteria inhabit anoxic deeper Black Sea; they recycle organic remains falling from photic zone into inorganic compounds. Among other products of bacterial disintegration of proteins is sulfuric hydride H2S, originating from sulfur-containing amino-acids. H2S is a substance toxic for aerobic organisms - it blocks mitochondrial breathing, thus making the deeper Black Sea twice unsuitable for aerobic life. Another source of H2S are bacteria using sulfate SO42- of seawater for oxidation of organic material. One of the consequences of this situation is a complete absence of deep-sea fauna in the Black Sea.
Map of the Black Sea
Bottom relief of the Black Sea
Black Sea is deep; central area of its bottom is an abyssal plain at 2000m depth, covered by silt sediments, it is an accumulation area of the basin. Maximal depth of the Black Sea is 2210m.
Black Sea shelf is a low gradient underwater slope to 100-150m depth; the shelf narrow (1-2.5km) at the mountainous coasts of the Black Sea (Caucasus, Crimea, Anatolia). The shelf is terminated by the abrupt (up to 20-30о) basin slope to the basin apron area with depths over 1000m. An exclusion is the shallow Nor-Western part of Black Sea all belonging to the shelf zone; actually it is not a part of the Black Sea hollow.
Bottom Sediments of the Black Sea
Nearshore shallow bottom is an underwater continuation of the beach: various types of sand or big rocks. Starting at 30-50 meters depth slow shelf slope is covered by sand, different size gravel, and molluscan shells. As the depth is growing, mussel Mytilus galloprovincialis shells are changed to Modiolus phaseolinus shell fragments; even deeper broken molluscan shells form fine silt covering the rest of Black Sea shelf.
Thickness of sediment layer of Black Sea abyssal plain varies from from 8 to 16 km, thicker in the Western part of the Sea, bordered by the central Black Sea meridional rise. Thickness of the recent sediments layer, accumulated during the last 3000 years of the modern Black Sea history is 20 to 80cm depending on the bottom region.
Sediment layer rests on the 5-10km thick basalt plate that covers the mantle. Intermediate granite layer between sediment and basalt layers is absent in the Black Sea (some fragments of granite layer were found in the Eastern part of the abyssal plain). Thus the Black Sea bottom exhibits features typical for the Ocean bottom.
Black Sea Main Rim Current is directed counter-clockwise forming the two rings over basin apron in western and eastern parts of the Sea ('Knipovich spectacles' - after the name of one of the Russian oceanographers who described the phenomenon). The major driving force for the Black Sea Rim current is the Coriolis force provided by the Earth rotation. Due to the relatively small area of the Sea winds strongly effect the Main Rim current, as a result it is very variable; sometimes it becomes barely discernible, sometimes it is well expressed with the mainstream velocity up to 100 сm s-1. Most of Black Sea Main Rim current flow is limited to the upper 100-200m layer of Black Sea.
Formation of anticyclonic eddies is characteristic for the Black Sea coastal waters; they are more pronounced and stable at the Caucasian and Anatolian coasts.
Tidal oscillations of the Black Sea level do not exceed 10cm because Mediterranean tidal waves extinguish in the Straits.
Most noticeable fast sea level changes in the Black Sea are the results of the strong on-shore and off-shore winds: Persistent North-East wind driven negative water setup at Caucasian coast of the Black Sea can reach 30 cm in 24 hours.
