SUBJECT: North-western part of the Black Sea (NWBS)
AUTHOR: Yuvenaly ZAITSEV
DATE: 15th April 2003

Really, the Mediterranean should be the best-studied basin in the world (Roberto Donovano). The Black Sea, which is a large low-salinity area of the Mediterranean basin, is also proof of this. Especially the shallow north-western part of the Black Sea (NWBS), including the Gulf of Odessa (GO). This area resembles the Gulf of Trieste (described by Dr Serena Umani): it is controlled by riverine discharge and open Black Sea water advections, winds etc. From the late 1950s until today, continuous plankton and benthos investigations have been carried out especially by the Odessa Branch of the Institute of Biology of Southern Seas (Black Sea Biological Diversity. Ukraine, 1998). Since the late 1960s - early 1970s, the NWBS was subjected to strong man-made influences: eutrophication, transformation of coastal and shelf habitats (constructon of new harbors, dredging, enlargement of beaches, coast-protection, urban concentration etc.), accidental and intentional introduction of exotic species and other kinds of anthropogenic impact.Because plankton and benthic communities in the area were well defined, we have proof of serious change in some ecological compartments (in reply to the question 3 of Prof. Ferdinando Boero). Normal (natural) changes and man-induced changes in the Black Sea ecosystem can be well discriminated and relevance of the later on the biota was investigated.

Anthropogenic eutrophication has an influence on the phytoplankton species diversity stimulating the development of dinoflagellates at the expense of diatoms with corresponding change in biomass. So, in the 1950s and 1960s the dinoflagellates in the NWBS accounted for 18.8% of the phytoplankton biomass, by the 1970s it accounted for 55% (Zaitsev & Mamaev, 1997). Eutrophication directly affects the species composition of bottom-living macroalgae by reducing the number of species with a low level specific surface (S/V), even leading to their disappearance (as with the brown alga Cystoseira barbata along the north-western coast), and the growth of the number of filamentous species with a high S/V values (Minicheva, 1993). Since Cystoseira barbata is a key species of the specific community (Cystoseira biocoenose), its disappearance entails serious consequences: disappearance of a coastal community, composed by 50-60 species of invertebtates and fishes. This community is important for the biology and ecology of coatal waters (Zaitsev, 1993). Blooming of phytoplankton in eutrophicated areas has an indirect effect on bottom-living algae, such as reduced water transparency. As a result, only algae adapted to dim sunlight survive at depths over 15-20 m, thus substantially limiting the species diversity of macrophytes on the NWBS. The case of the famous Zernov's Phyllophora field is very significant in this sense. In the 1950s, this field was still the largest aggregation of red agar-bearing algae of the Phyllophora genus in the world, occuping an area of 11,000 km2 in the central part of the NWBS with a total biomass estimated at 7-10 million tons. By the early 1990s the field was a mere 500 km2 and its biomass did not exceed 300,000 tons (Zaitsev, 1992). Moreover, Phyllophora was the key species of a community known as the Phyllophora biocoenosis, which included about 110 species of invertebrates and 40 species of fish. A second Phyllophora field, so-called "small field", located in the same area (NWBS), but at a depth of 8-10 m still exists (Zaitsev & Mamaev, 1997).

Among other consequences of eutrophication in the NWBS are the replacement of large-size species of zooplankton (e.g. copepods), that dominated in the NWBS until the 1960s by small-size species, blooming of gelatinous species like Noctiluca scintillans and Aurelia aurita, appearance since 1973 of seasonal (summer-autumn) of hypoxic zones on the NWBS shelf (a consequence of sedimentation of large amount of phytodetritus) and mass mortality of bottom invertebrates and fish (Zaitsev, 1993). A series of consequences and impacts on biological diversity is connected with the accidental introduction into the Black Sea of exotic (alien) species (Zaitsev & Ozturk, 2001).

Due to a relatively low specific diversity in comparison with the Mediterranean (as a result of low salinity, hydrogen sulphide contamination of deep waters, many relic (Pontian relics) and endemic species) the Black Sea has a low "bilogical immunity". A consequence of this is the naturalization of many exotic species, which in the absence of its antagonistic species are free from an practically unlimited growth of population with an adequate impact on native species (Zaitsev & Ozturk, 2001). Such is the case of ctenophore Mnemiopsis and gastropod Rapana in the Black Sea, whose behaviour in new habitats is quite different than in original waters and whose impact on comercial fisheries is very important. Existing data on habitat changes in the NWBS are crucial for changes in biological diversity and habitat conservation should be a priority in the marine environment conservation (answer to the question of Prof. John S. Gray). Particularly it concerns "hot-spots" of high species richness.Especially when in the same habitats an accumulation of pollutants occurs.This is the case of contour (marginal) biotopes of the sea situated in its interfaces with atmosphere, shores, bottom and river waters (Zaitsev, 1986). These habitats provides favourable conditions for marine organisms but recently contour biotopes turn out to be the most contaminated marine habitats by chemical and radioactive substances. That is why the contour-living organisms are now among the most threatened species. More than 75% of species included into Black Sea Red Data Book (1998) are living in contour biotopes during all their life-cycle or during a certain stage of onthogenetic development (Zaitsev, 1971, 1986, Zaitsev & Mamaev, 1997).

References:

Black Sea Biological Diversity. Ukraine. (Compiled by Yu.P. Zaitsev and B.G. Alexandrov). United Nations Publications, New York, 1998, 351 pp.
Black Sea Red Data Book (Edited by H.J. Dumont, Scientific Coordinator Y.P. Zaitsev), Publ. by the United Nations Office for Project Services, 1999, 413 pp.
Minicheva, G.G. Structural-functional peculiarities of the development of marine benthic algal communities. Algologya, 1993, 3, ? 4, P. 33-12 (in Russian).
Zaitsev, Yu.P. Marine Neustonology. Israel Program for Scientific Translation, 1971, 207 pp.
Zaitsev, Yu.P. Contourobionts in Ocean Monitoring. Environmental Monitoring and Assessment (by D. Reidel Publ. Comp.) 7, 1986, P. 31-38.
Zaitsev, Yu.P. Recent changes in the trophic structure of the Black Sea. Fisheries Oceanography, 1992, 1, 2, P. 180-189.
Zaitsev, Yu.P. Impacts of eutrophication on the Black Sea fauna. Genera Fisheries Council for the Mediterranean. Studies and Reviews, Rome, FAO, 1993, P. 63-86.
Zaitsev Yu. and Mamaev, V. Marine Biological Diversity in the Black Sea. A Study of Change and Decline. United Nations Publications, New York, 1997, 208 pp.
Zaitsev Yu. and Ozturk B. (Eds) Exotic Species in the Aegean, Marmara, Black, Azov and Caspian Seas. Publ. by Turkish Marine Research Foundation, Istanbul, Turkey, 2001, 267 pp.

A contribution by:

Yuvenaly ZAITSEV
Odessa Branch
Institute of Biology of SouthernSeas,
National Academy of Sciencies of Ukraine