Environmental Engineering Reference
In-Depth Information
same time another small
Phyllophora
field, situated at 10-15 m depth in eastern
part of the NWS has continued its normal development.
Such a disastrous reduction of the Zernov's
Phyllophora
field means not only
loss of commercially valuable “agar weed” stocks, but the disappearance of a
large bottom biocoenosis with a specific red-colored fauna (
Phyllophora
fauna)
and the loss of an important source of oxygen. According to some data, these
seaweeds, in the period of their normal development, excreted in the bottom
layers up to 2 Mio. m
3
of oxygen per day.
4. HYPOXIA AND ANOXIA IN NEAR-BOTTOM
LAYERS OF WATER
At the beginning of the 1970s, the rate of sedimentation of dead planktonic
organisms (mainly phytoplankton) to the seabed was found to be 15-20 times
more than that observed in the 1950s. Decomposition of dead organic material
requires oxygen dissolved in the water. As a result, the amount of oxygen
consumed during the decomposition this organic fall-out increased and zones
of hypoxia and anoxia began to appear on the shelf, first of all, in areas with
pronounced stratification of water layers. This new environmental phenomenon
in the Black Sea was first observed during a cruise of Odessa Branch IBSS
RV “Miklukho-Maklay” in August-September 1973 [16]. Then, between the
Danube river delta and the Dnister river estuary, over a surface area of 3,500
km
2
at depths from 10 to 20 m, a mass mortality of benthic invertebrates and
fishes from hypoxia occurred. In more recent years, cases of mass mortality
of benthic organisms from hypoxia have become ordinary phenomena, with
scales depending on the meteorological, hydrological, chemical and biological
peculiarities of summer season (Fig. 1).
Among the most common victims of hypoxia are practically all species of
bottom invertebrates and fish. Among them there are mollusks (
Mytilus, Ceras-
toderma, Mya, Abra
), barnacles (
Balanus, Chtamalus
), shrimps (
Crangon,
Palaemon
), ghost shrimps (
Upogebia, Calianassa
), crabs (
Carcinus, Macropi-
pus, Pilumnus
), tunicates (
Botryllus
), and among fishes - species of gobies
(
Gobiidae
), flatfishes (
Soleidae, Pleuronectidae, Scopthalmidae
), sturgeons
(
Acipenseridae
). River damming was the additional factor for the decline in
migratory fish stocks such as e.g., sturgeon stocks. Only few species of in-
vertebrates: foraminiferans (
Ammonia tepida
), polychaets (
Nereis diversicolor,
Melinna palmata
), mollusks (
Scapharca inaequivalvis
), nematods (species of
Desmoscolex, Tricoma
and
Cobbionema
genera) can survive for longer periods
in the absence of dissolved oxygen.
The biological losses due to hypoxia and anoxia on the NWS are estimated
at 100 to 200 t of animals per km
2
of seabed. In some years, areas depleted in
oxygen reached 30,000-40,000 km
2
(Fig. 1). The total biological losses over 18
years, from 1973 to 1990, are estimated at 60 million tones, including 5 million
