of the Black Sea
During the past ages, Black Sea -
or a water body that existed on its place - several times
turned to lake and then back to sea again; that is the
connection to the Ocean opened and closed again. 250-40
mln years ago it was a marginal sea of the great Tethys
Ocean that existed where is now most part of modern
Europe and Asia. 5-7 mln years ago the Tethys Ocean disintegrated
as a result of new mountain ranges formation. Then a large,
enclosed Sarmatian Sea-lake formed - modern Black,
Caspian, and Aral Seas originate from its parts. The Sarmatian
Sea existed for 2-5 million years, and during that time
an endemic freshwater life was formed there. Crimea and
Caucasus were islands in the Sarmatian Sea at the time.
2-3 mln years ago connection of Sarmatian
Sea to the Ocean had been restored resulting in the ancient
Sea called the Meotic Sea. It was not a freshwater lake
anymore, and due to the drastic environmental change, the marine biota changed almost completely. Meotic marine life
was typically oceanic, and even large whales lived there
- paleontologists have been lucky to find their bones here.
The last 18-20 000 years the Black Sea existed
as a brackish lake, and only ca. 6000-7000 years ago the connection
to the Mediterranean basin via the Bosporus Strait was formed.
The Bosporus area was and still remains an area of high
seismic activity. Many geologists share the view that the Bosporus breach occurred very quickly, which resulted in
a catastrophe on a global scale; this hypothesis is called
the Black Sea deluge hypothesis. This theory argues that the
water level in the Black Sea (a lake at that time) was about
50 meters lower than that in the Mediterranean and Marmora
Seas. Therefore, the Bosporus formation became a debacle
of a huge natural dam - water from one sea poured into the
other from 50 m height, and great waves flooded all shores
of the Black Sea immediately. All coastal settlements (people,
cattle, houses) were covered by flooding, and large areas
of the former coastal ground remained underwater - there
are archaeological evidences to that.
There is another opinion concerning those
remote events: geologists are finding evidence to a
gradual formation of the modern Bosporus Strait and correspondingly
to a gradual last change of salinity in the Black Sea.
of these geological hypotheses is more correct the consequences
of the ensued environmental changes for the aquatic biota
were well established: freshwater living community gave
place to the marine life in Black
Sea this last time.
The current evolution of the Black Sea is
most obviously illustrated by its rising water level, average
20 mm per decade in 20th century; although this secular
trend is masked by more pronounced (up to 20 cm) interannual
variability of the sea level caused by the changing river
discharge. Recent studies of satellite altimetry data have
shown sea level elevation secular trend of 200mm/decade
for the central part of Black Sea; conservative esimates
is 3-4cm/decade current sea level rise. The current trend
of the Black sea level rise was shown to be irresponsive
even to the influence of North Atlantic Oscillator, the
major factor affecting climate in the region. Manifestations
of the Black Sea's rising water level can be seen almost
at any beach: submerged old piers; concrete foundations
of beach guards' towers (established some 30 years ago and
now covered by waves), sand beaches (once crowded as seen
on 20-year-old photographs) having disappeared completely
where stone embankments had been constructed.
sturgeon Acipenser guldenstadti colchicus - Pontic relict species, old resident of
the Black Sea
A unique endemic biota formed in the lake preceding
modern Black Sea during millions years of its isolation
from the Ocean. Most of the species inhabiting freshwater Sarmatian Sea-lake are extinct now, whereas few of
them managed to adapt to the changing environment
and have survived to this date - mostly those that
managed to escape the rise in the salinity level in
river estuaries. The latter are called Sarmatian or
Pontic relicts. The famous example of such species
is the Black Sea sturgeon
Pontic relicts contribute less then 5% species to
the modern Black Sea biota, whereas most of the species
- about 85% - came from the Mediterranean after the
formation of the Bosporus Strait. And since the Black
Sea is young, they still keep arriving. Barnacles,
small white houses of cirripedean crustacean Balanus
are so ordinary - they are everywhere: on coastal
rocks, on piers, on any piece of wood or bottles washed
ashore, and they have settled here only in the middle
of the 19th century. No doubt, barnacles entered the Black Sea earlier, and on numerous occasions - on
the bottoms of ships. However it took them thousands
of years to adapt to the Black Sea conditions - reduced salinity,
cold winters. Each year we are finding the
species of planktonic microalgae which are new to
the Black Sea.
- shell-houses of cirripedean crab Balanus improvisus.
About 10% of marine organisms inhabiting the
Sea came from the Atlantic during the period of retreating
glaciers - it is supposed that there was a water connection
between the North Sea and the Black Sea - across the Europe
- at the time. Black Sea spiny shark, dogfish
Squalus acanthias is thought
to be among those North Sea invaders.
Alien Marine Species in Black
Sea: Recent Invasions
Young Black Sea ecosystem is still far from
equilibrium: new alien
invasive species keep coming, old ones becoming
extinct as a result of an interaction with invaders. During
the last two centuries, many alien species invasions
have been facilitated by human activities.
Rapana venosa eating
mussel Mytilus galloprovincialis
- Black Sea Caucasian Coast
Sea - Mollusca - details on
Rapana venosa in the Black Sea
Fierce predator - bivalve-eating
Pacific gastropod, veined rapa whelk Rapana
venosa came here in 1947 from the Chinese
coast. Most probably the snail was brought around
half of the globe on a ship on its way from Far East
to the Black Sea port of Novorossiysk, where it was
first discovered. Rapana rapidly dispersed
around the whole coastline of the Sea, conquered all
the Black Sea benthic habitats, proliferated immensely,
because there were no predators in the Black Sea to
eat this marine snail when it grew over 3-4cm. Starfish
that are normal predators of Rapana venosa
cannot survive the low salinity of Black
Sea. During the past 50 years, Rapana venosa have
eaten to extinction half of the bivalvian species
that were living in Black Sea before the invasion.
Black Sea populations of edible oyster Ostrea edulis
and Black Sea scallop Flexopecten ponticus
are now on the brink of extinction.
In Black Sea,
Rapana eats decapod crabs too:
holsatus crab was killed
and digested by Rapana venosa
using this hole drilled in the carapace.
Sand bottom near Tuapse.
After its Black Sea conquest, Rapana venosa
entered Mediterranean Sea, and settled (1974) in the brackish
parts of the upper Adriatic Sea (Rapana
page of Mediterranean Science Commission). We were finding
Rapana along the Adriatic coast of Montenegro in 2008.
Rapana venosa invaded Chesapeake Bay at Atlantic US coast in 1990s (local oysters were not threatened by the invader, since they all gone due to overharvesting) and Rio de la Plata Gulf in Uruguay. Recently drilling whelks were found at Brittany coast and in Northern Sea - Dutch coast, and wider Thames estuary. Chesapeake Bay resembles Black Sea by its low salinity; just like in Black Sea there are no natural predators to eat adult Rapana.
In 2007 turkish zoologists reported finding of Asterias rubens seastars (an echinoderm species usual in North-Atlantic) in higher saline near-Bosporus area. In case this invasion (or introduction) becomes successful, and red benthic predators spread along Black Sea coasts, the consequences of that might be quite negative since starfish’s main local food most probably might become bivalves, mussels first of all.
Similarity between Black Sea and Chesapeake
Bay conditions probably became an important prerequisite
for the earlier successful invasion of ctenophore
Mnemiopsis leidyi from Chesapeake Bay to Black Sea -
an alien species invasion that resulted in catastrophic
consequences for the whole Black Sea ecosystem.
Planktivorous comb jelly - ctenophore Mnemiopsis
leidyi was found from time to time in the
Black Sea since the early 1980s. Most probably, it
was brought here on ships' ballast waters from the
US East Coast. However, it was only in the summer
of 1988 when having adapted to the Black Sea, a portion
of Mnemiopsis population gave an outburst - and comb
jellies virtually filled the upper layer of the Black
Sea. The Sea resembled a white jelly in August 1988
- it looked like there were more ctenophores than
water. To grow to that extent ctenophores ate most
of Black Sea plankton - including fish and invertebrate
eggs and larvae. Planktivorous fishes were left without
food. The effect relayed through the food web of the
Black Sea was that numbers of all studied marine species
reduced - including macroalgae and benthic shellfish.
The consequences for fisheries expressed in lost catch
were estimated as 300-400 millions US dollars a year
in the late 1980s; on the other hand fishery itself
was one of the causes of the Black Sea fish stock
leidyi - ctenophore that changed marine
life in the Black Sea in 1980s
- ctenophore that helped out the Black Sea biota
Black Sea marine ecosystem was helped
by another ctenophore invasion: carnivorous Beroe
ovata came here from Mediterranean in the
1990s. Beroe eats Mnemiopsis - only
Mnemiopsis, swallowing it as a whole with a
wide mouth-slot. It is a rare case of a very narrow
Mnemiopsis population decreased,
and during the last years, one could observe the following
annual chain of events in the Black Sea ctenophore
life: Mnemiopsis of new generation and overwintered
ones first appear in cold April water; it is a right
time for them, since it is when spring plankton rise.
Mnemiopsis population reaches its peak in the
middle of summer - water column sometimes is filled
by transparent ctenophores. Usually it is at this
moment when first Beroe jellycombs can be found;
they start grazing on Mnemiopsis. It is very
interesting to watch how Beroe gulps Mnemiopsis
- just snorkel nearshore in August, and you see it.
To the end of September there are no Mnemiopsis
in the water, whereas Beroe can be seen everywhere.
Having eaten all their food, they fill water column
with white threads of their eggs and remains of disintegrated
bodies, and completely disappear in late autumn.
After the Black Sea invasion,
reached the Caspian Sea - apparently having been carried
there in ballast waters of ships as earlier; now the living
community of this sea-lake undergoes changes similar to
those that occurred in the Black Sea.
The ctenophores invasion was an inevitable
way of an ecosystem evolution accelerated by the involvement
of man. And jellycombs are very beautiful: transparent creatures,
hovering in midwater, shimmering with hundreds tiny rainbows
running along their bodies - it is their thinnest rowing
plates refracting sunlight when moving in matched modes.
If you touch them in dark night water, they flash with green
light; Mnemiopsis and Beroe are the largest
luminescent plankters in the Black
Anthropogenic Evolution: What We
did to the Black Sea Ecosystem
During the past 50 years evolution of the
Black Sea ecosystem was mostly determined by human activities,
including alien marine species introduction. The Black Sea
that once fed the Ancient South-European world, in 20th
century became a sink for 175 millions Europeans - with
the resulting over-nutrification - or eutrophication,
and ensuing phytoplankton
blooms and mortality of marine organisms.
eutrophication problem became most acute in 1970-80s,
and it coincided with the peak of overfishing in
the Black Sea.
Officially reported total fish catch in the
Black Sea increased from 300000-400000
ton per year in 1970s to
700000-800000 ton in 1980s. Most of it
was represented by planktivourous fish, since larger predatory
fish species were fished out earlier.
Total nitrogen concentration in the Danube estuary
increased from 1.4 mg/l
in late 1950s to 7.2 mg/l
in 1990; corresponding phosphorus concentrations were
0.10 and 0.33 mg/l.
The increase of nutrient load mostly refers to the more
intensive use of fertilizers in the regional agriculture in 1970-80s.
Eutrophication was the cause of the
phytoplankton blooms, particularly in the Western part of the
Black Sea. In some cases it caused marine fauna dye-offs;
increased phytoplankton and detritus concentration were reducing
water transparency and consequently the amount of sunlight
available for macroalgae. Unique benthic habitat of the
shallow North-Western part of the Black Sea, the Zernov's
Phyllophora field almost disappeared due to this reason.
Brown macroalgae Cystoseira barbata belt of the Black
Sea rocky shores has reduced
During the period of progressive
eutrophication the phytoplankton primary production increased
up to three-fold across the whole Sea
(up to ten-fold in Western part of the Sea), whereas the
planktivourous fish stock was exhausted
by overfishing; pelagic community responded by the increase
in gelatinous zooplankton biomass. First, the Scyphozoan
Aurelia aurita population grew, its biomass exceeding 1000 g/m2 sea
surface; later the Ctenophore Mnemiopsis leidyi intervened.
Changes in the Black
Sea pelagic foodweb due to overfishing and Mnemiopsis
The situation was further aggravated by the
removal of bonito Sarda sarda from the Black Sea
foodweb, due to the overfishing and because of banning its
coming to Black Sea for summer foraging through the
overpolluted Bosporus Strait; bonito was the major predator of
gelatinous plankton in the Black Sea (Dr. Yu.Sorokin hypothesis).
The combined effect of eutrophication and
overfishing in the Black Sea was that in 1970s,
the main stream of matter and energy within the Black Sea
ecosystem was shortcut from the phytoplankton primary producers to
bacterioplankton reducers via gelatinous zooplankton.
The Black Sea ecosystem is recovering since
early 1990s, when eutrophication slackened and Beroe ovata
ctenophore was introduced into Black Sea. The data on the
current Danube nutrient loads vary between different sources,
however there are no more harmful algal blooms like in 1980s
(with the exception of the shallow, brackish, over-nutrified
coastal waters of North Western and Western parts of the
Sea). Ukrainian biologists report the onset of Zernov's
Phyllophora field revival.
Many data show that Black Sea ecosystem absorbed
the impact of the Mnemiopsis invasion: scyphozoan
jellyfish biomass portion has visibly grown after the Beroe
involvement. According to P.P. Shirshov Institute of Oceanology
researchers' data, the crustacean zooplankton abundance
in the Eastern part of the Sea recovered to the values of
the pre-Mnemiopsis years (in corresponding periods
Total fish catches
in Black Sea, according to UN FAO data, tons.
UN FAO reports show that catches of the Black
Sea anchovy hamsi Engraulis encrasicholus ponticus
recovered to the level of late 19970s, the khamsi fishery
is almost exclusively Turkish industry now. The Black Sea
horse mackerel Trachurus mediterraneus ponticus
population is recovering at a slower rate, and still tere
is no mackerel in the Black Sea. Most remarkable was the
growth of Mugil sojui mullet (Far-Eastern species
introduced to the Black Sea in 1980s) population in the
Still the total portion of gelatinous plankton
in the Black Sea remains very high, constituting about 90%
of total zooplankton biomass in summer.
Black Sea keeps changing in front of our
eyes; its rich history of both natural and man-made
catastrophes suggests that making predictions on its future
Excellent books about
the Black Sea in English:
Ascherson, N. Black Sea. New York,
1998. - it is both a precise account of the social history
of the Black Sea region, and
a gripping reading.
Sorokin Yu.I. Black Sea Ecology and Oceanography.
(2002) Amsterdam, "Backhuys Publishers". - A most comprehensive
reference book on 20th century natural scientific data on the Black Sea.
Zaitsev Yu., Mamaev V. Biological Diversity
in the Black Sea: A study of change and decline. New