Environmental Engineering Reference
In-Depth Information
biological uptake. It starts to increase at 40 m and reaches a maximum at 65m
(
σ
θ
= 15.5) (approximately where O
2
decreases to 0). Nitrate then decreases to
zero at 75m (
σ
θ
= 15.95) which is well above the first appearance of sulfide
at 90 m or
σ
θ
= 16.15. After oxygen has decreased to zero, nitrification of
ammonium released from organic matter no longer occurs, yet ammonium
still does not accumulate. There are often two NO
2
−
maxima located near the
upper and lower boundaries of the NO
3
−
maximum corresponding to zones
of nitrification (shallow maximum) and denitrification (deep maximum) [73].
NO
2
−
is an intermediate in both reactions. Ammonium starts to increase at
σ
θ
= 15.95 and increases progressively into the deep water. The disappearance of
NO
3
and NH
4
at the same depth is consistent with a downward flux of NO
3
and an upward flux of NH
4
that are consumed over a narrow depth interval
by the anammox reaction (NO
2
−
+NH
4
+
=N
2
+2H
2
O). Anammox stands
for
an
aerobic
amm
onium
ox
idation. Note that the anammox reaction reduces
NO
2
−
, not NO
3
−
, so there must also be some heterotrophic denitrification
that occurs that reduces NO
3
−
to NO
2
−
or nitrification that makes NO
2
−
from
NH
4
+
(from PON) in order for anammox to occur. The fact that there is a
NO
2
−
maximum implies that the rate of denitrification may be faster than
anammox. So far these rates are unknown. The lack of ammonium accumulation
between 15.5 and 15.95 suggests that anammox may be occuring throughout the
suboxic zone. An additional sink for ammonium is consumption by microbial
chemosynthesis, but this rate is hard to estimate.
N
2
has been measured in the Black Sea as the N
2
/Ar ratio [15]. The N
2
/Ar
ratios in the center of the western gyre are shown versus density for three
different cruises in 2000, 2001 and 2003 in Fig. 8. The vertical dashed line
indicates the ratio expected when the water is at atmospheric equilibrium. The
horizontal dashed line shows the density of the CIL, the deepest layer ventilated
directly from the surface. Most of the water column is supersaturated with N
2
with a maximum centered at
σ
θ
≈
16.0. This is the density where NO
3
−
and
NH
4
+
decrease to zero. These data from three different years suggest that
N
2
concentrations vary significantly on interannual scales suggesting that its
production is sensitive to small perturbations in the vertical gradients and fluxes
of NO
2
−
and NH
4
+
driven by variability in ventilation. The maximum in N
2
/Ar
was largest in 2000 which was a period when the CIL was relatively warm and
less ventilated.
Kuypers et al. [28] used 16S RNA gene sequences, RNA probes,
15
N label
experiments and ladderane membrane lipids to show that anammox bacteria are
indeed present at this level in the Black Sea. This reaction where ammonium
is oxidized anaerobically to N
2
is important in the nitrogen cycle of the Black
Sea [15] and results in a prominent maximum in N
2
gas concentration (Fig.
8). The anammox reaction had long been inferred from chemical distributions
[5, 55] and has now been confirmed. The relative importance of anammox
