Environmental Engineering Reference
In-Depth Information
S
2
O
3
2
−
,SO
4
2
−
) species. Microbial processes were only formally taken into
account in most of the models developed [61].
Most of the processes occurring in the redox zone are mediated by microor-
ganisms of different physiological groups. However, the information about
the physiology, diversity and biogeochemical role of bacteria inhabiting the
oxic/anoxic interface in the Black Sea is still limited. Microorganisms in the
interface zone serve as a “biogeochemical filter” between oxic and anoxic wa-
ters. They directly or indirectly mediate all oxidation-reduction processes that
occur there, particularly those related to carbon, sulfur, nitrogen and metal cy-
cling. Furthermore, bacteria of the redox zone represent an important source of
chemosynthetically-produced organic matter that feeds microbial communities
in the anoxic zone and sediments. Bacteria associated with protozoa link the
two distinct food chains of the oxic and anoxic zones. Here we summarize
the existing data about different physiological groups of microorganisms in the
interface zone between oxic and anoxic waters of the Black Sea and discuss
the specific activities and biogeochemical roles these microorganisms play in
carbon, sulfur, nitrogen, and manganese (iron) cycling.
2. GENERAL STRUCTURE OF THE REDOX ZONE
Depth distributions of hydrochemical parameters in the Black Sea water
column correlate strongly with density, which allows using density distributions
instead of depth distributions to make the Black Sea water column spatial data
comparable [27, 59].
The following layers in the redox zone are distinguished based on the distri-
bution of major nutrients [28, 62]:
1. The upper part of the redox (suboxic) zone is located in the density
interval σ
t
=15.5-15.7 (70-100 m depth in the central Black Sea). The
oxygen content in this zone decreases to 15-20 µM. Dissolved oxygen
and nitrate are the main electron acceptors.
2. In the middle part of the redox zone (σ
t
=15.9-16.0; 100-120 m) dissolved
oxygen and nitrate disappear and reductants such as ammonium, soluble
manganese and iron, elemental sulfur, thiosulfate, and methane appear.
3. Hydrogen sulfide and particulate Mn and Fe are present in the lower redox
zone (σ
t
=16.1-16.5; 120-160 m). The upper H
2
S boundary corresponds
to σ
t
=16.15-16.25.
Recent more accurate measurements have shown that dissolved oxygen con-
centrations are in the range from 0 to 5 µM at the hydrogen sulfide boundary
[Murray and Yakushev, this volume].
