Environmental Engineering Reference
In-Depth Information
50
40
30
20
10
0
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
Figure 1.
Surfaces of hypoxic zones (dissolved O
2
<
= 2 mg.l
−
1
) on the North-western Black
Sea shelf in 1973-1990.
tones of fish, both commercial and not commercial species, adult specimens
and fry [20].
In autumn season, due to lowering in water temperature, disappearance of a
strong pycnocline and turbulent mixing, bottom waters are re-oxygenated and
life begins to return. Pelagic larvae of benthic organisms from the edge of the sea
and shelf elevations settle down and give rise to new generations of animals,
which develop until the next episode of hypoxia occurs next summer, or in
two-three years. That is why the populations of benthic invertebrates and fish,
except for the inhabitants of coastal marginal biotopes, are markedly young.
Now it is very difficult to find old specimens. Overall, the stocks of benthos
on the NWS have greatly decreased, including important commercial species -
mussels, oysters, shrimps, and other invertebrates, as well as vertebrates turbot,
flounder, gobies, sturgeons and other fish.
The following example illustrates the magnitude of the problem. At the be-
ginning of the twentieth century, the Odessa Gulf and adjacent NWS areas were
nicknamed “Kingdom of mussels” [19]. In the 1950s and 1960s, these mus-
