Environmental Engineering Reference
In-Depth Information
faster than in the interior. This appears to be especially important in the SW
region influenced by oxygen injected by the Bosporus Plume.
The distribution and cycling of Fe is similar to Mn but the concentrations are
lower. Dissolved Fe increases to a maximum of 200-300 nM between 130 to
200 m then decreases to 14 - 40 nM in the deep water. The oxidative removal
is also bacterially mediated. The deep concentrations appear to be controlled
by solubility with FeS (mackinawite) or Fe
3
S
4
(greigite), even though FeS
2
(pyrite) is more insoluble and is present in the water column [53]. In general
the “iron interface” is slightly deeper than the “Mn interface”, though both are
shallower than the first appearance of sulfide [31].
4.7 Phototrophic Reactions
Most of the redox reactions in the suboxic zone are mediated by bacteria
resulting in
in situ
consumption of CO
2
[5]. Chemosynthetic bacteria grow
on carbon dioxide and water and get their energy from reduced compounds
like H
2
S, NH
4
, Mn (II), Fe (II) and CH
4
. High rates of chemosynthesis have
been measured (Yilmaz et al., in press). In addition, the discovery by Repeta
et al. [54] of high concentrations of bacteriochlorophyll BChl e in the suboxic
zone suggests that anoxygenic photosynthesis occurs [22]. This is surprising
because the light availability at these depths (
<
4 µEinst m
−
2
s
−
1
) is equal to
between 0.0005% to 0.00005% of the surface irradiance [50]. The BChl e
is associated with brown phototrophic
Chlorobium
bacteria. These bacteria
are obligate phototrophs and compete with other bacteria under conditions of
severe light limitation. They require light and sulfur. Their growth rates are
extremely slow and their calculated doubling times are on the order of 2.8
yrs but somehow they maintain their existence. The role these bacteria play in
elemental cycling is still unclear.
5. SPATIAL VARIABILITY
At any given time there are not any significant differences in the thickness
of the suboxic zone in most of the Black Sea. Representative profiles from the
central gyre, NW margin (both from Knorr 2001) and NE margin (by SBSIO)
are shown in Fig. 12. These profiles were all collected about the same time of
year. These data show that the density values of the upper and lower boundaries
of the suboxic zone vary only slightly from the western central gyre to the NW
and NE regions. Variations in the density of the lower boundary of the suboxic
zone (the first appearance of sulfide) are typically small. The changes that are
seen are at the upper boundary of the suboxic zone where small changes in the
shape of the oxygen profile have a big impact. In any given region the vertical
profiles of oxygen are very similar but they vary slightly on a sub-basin scale.
The thickness of the oxic surface layer and the Cold Intermediate Layer (CIL) is
