Environmental Engineering Reference
In-Depth Information
for the physical and chemical exchange at the sediment/water interface and
at the interface between intermediate and bottom water masses. These mixing
processes at the interface between deep and bottom waters are particularly
important for hydrogen sulphide dynamics and its balance in the sea, because
about 30% of the Black Sea sulphide is concentrated in the layer below 1500
m.
There exist data indicating that the average concentrations of sulphide in
the upper 1000 m of the anoxic zone have increased by 0.6-0.8 % a year from
1985 to 1995 [41, 42]. The authors argued that the Black Sea anoxic zone is
not presently at a steady-state as it was in 1960-1970s. These data however
should be interpreted with a caution, since most chemical data on sulphide
concentrations obtained before 1980s using metal bottles are about 20% lower
than the data obtained using plastic bottles [66]. In addition, significant spatial
and temporal variability of physical and chemical parameters in the basin
(Table 1, particularly in the upper 500-1000 m of the anoxic layer, compare
variation coefficients of sulphide concentrations given in the first column) and
the absence of basin-wide monitoring in previous years, make comparative
assessments of sulphide concentrations over time for the entire basin difficult.
The possibility that dissolved sulphide concentration may be increasing in
the bottom waters of the Black Sea however cannot be ruled out. Mathematical
model of Ayzatullin et al. [5] suggests ongoing salination of the Black Sea bot-
tom waters with a rate of 0.0005‰ per yr, which may result in sulphide annual
increase of 0.003 µM. If such trend does exist it can be detected after 10-20 yrs
from now given the accuracy of routine dissolved sulphide measurements.
5. THE H
2
S SPATIAL DISTRIBUTION AND COASTAL
DYNAMICS
Close correlation between sulphide vertical distribution and density is also
reflected in the whole-basin hydrogen sulphide spatial distribution. Main cir-
culation structures of the Black Sea (Main Rim Current (MRC), Eastern and
Western Gyres, and anticyclonic eddies around Crimea and Batumi) are visible
on the H
2
S map. In the centers of cyclonic gyres, the location of the H
2
S upper
boundary decreases to 90-110 m, whereas at the periphery of the basin and in
the centers of anticyclonic gyres it can deepen to depths 160-240 m. The spatial
differences of the H
2
S topography recognizable at the oxic/anoxic interface can
be traced to depths below to 1000 m [61].
Recent advances in our understanding of the Black Sea coastal zone, and
in particular, of the Rim Current and transverse water transport have revealed
the crucial role of mesoscale eddies in the ventilation of the suboxic and the
upper anoxic zone [70, 73]. Oguz et al. [71] conclude that about nine mesoscale
nearshore anticyclonic eddies (NAE) propagate cyclonically around the basin
at a given time. Usually mesoscale cyclonic eddies (CE) occur at the inner
