Environmental Engineering Reference
In-Depth Information
for example [6], we are able to consider the 1950s as a period of “ecological
norm”.
Until the middle 1960s, the most productive area of the Black Sea, its North-
western shelf (NWS), in terms of biological diversity, biomass and biological
productivity, was considered the principal “nursery” of the sea and the area
of the most important stocks of commercial species of algae, invertebrates
and fish and traditional fisheries [18]. In the manuscript I present an overview
of the ecosystem changes on the NWS due to increased eutrophication such
as water transparency, harmful algal blooms, the extent of hypoxic zone, and
discuss structural changes in pelagic and benthic communities. The analysis
showed deterioration of many ecosystem parameters and components during
1960s-1980s. In the late 1990s some signs of recovery of the NWS habitats
were observed. Continuing monitoring of chemical parameters and biodiversity
studies of the NWS are particularly important since the area is the key zone of
the Black Sea in terms of its biological resources and their reproduction.
2. MAN-MADE EUTROPHICATION
First considerable changes in the Black Sea ecosystem, associated with man-
made or cultural eutrophication, were noted in the NWS area in the late 1960s
and early 1970s. Now it is obvious, that the main reason of these changes was
the river runoff, because the largest Black Sea Rivers - Danube, Dnister and
Dniper - are discharging just in this area.
It was established, that during the 1970s and 1980s, the influx of river-borne
nutrients into the NWS area was considerably increased [3]. For example, the
average influx of ammonium nitrogen in 1950-1960 was 52.2 thousand metric
tones per year, and in 1986-1989, 206.4 thousand metric tones per year. Nitrite
influx was 2.8 and 15.4 thousand tones per annum respectively, phosphates
14.6 and 69.8 (same units) and organic phosphorus 7.8 and 35.8 (same units).
As a result of these trends, the NWS area of the Black Sea became the largest
heavily eutrophic (hypertrophic) marine zone in the whole Mediterranean basin
[17].
It is generally accepted, that changes in phytoplankton communities, species
diversity, number and biomass, are directly related to changes in the composi-
tion and concentration of the nutrient supply.
As a general rule, the most favored species in eutrophic conditions are
the small-size planktonic algae such as
Dinoflagellates, Coccolithophores, Eu-
glenoids
and some other. The average biomass in the NWS area of the Black
Sea in the 1950s, 1960s, 1970s and 1980s was 670 mg.m
−
3
, 1,030 mg.m
−
3
,
18,690 mg.m
−
3
and 30,000 mg.m
−
3
respectively [7, 9, 11].
During major algal blooms, the phytoplankton community can become dom-
inated by a single species to the near exclusion of others. These “red”, “green”
