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 17‰ (gram salt in 1 liter of seawater),
two times less than that of the Ocean (average 35‰).
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 ‰
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 21‰ - 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.
