Environmental Engineering Reference
In-Depth Information
data. Repeat observations at several sites over the inner shelf off the central
west coast of India suggested an average NO
3
−
consumption rate of 0.83 µM
d
−
1
and applying this rate to an estimated volume of 1.2-3.6 x 10
12
m
3
and
a denitrification period of 3 months, the overall rate comes to 1.3-3.8 Tg N
y
−
1
[33]. This is 4-12% of the denitrification rate estimated for the perennial
suboxic zone of the open Arabian Sea. It may be pointed out that the rate of
N
2
production may substantially exceed that of NO
3
−
consumption because
of the involvement of anaerobic ammonium oxidation (ANAMMOX) which
under certain conditions (as observed over the continental shelf off Namibia)
can be the dominant pathway of N
2
production [29]. Although such conditions
(i.e. suboxia with high availability of NH
4
+
and NO
2
−
) also exist in the coastal
Arabian Sea, the quantitative significance of ANAMMOX in the region still
remains to be evaluated.
Through the removal of fixed nitrogen from the water column, denitrification
- both in water column and sediments (see below) - counters anthropogenic
nitrogen loading. In fact, as pointed out above, due to the much lower ratio of
carbon oxidation to NO
3
−
reduction than the Redfield C:N value, denitrification
has the potential to remove more nitrogen than the external inputs. This negative
feedback probably constrains PP in the eutrophied environments (as long as
they are not SO
4
2
−
reducing). However, while the loss of excessive nitrogen is
undoubtedly good for the environment, the problem, as discussed below, is that
a significant portion of the denitrified nitrogen might end up as N
2
O, a potent
greenhouse gas.
Sedimentary Respiration.
As with the water column, seasonal cycles of
PP and degree of oxygenation must also influence benthic respiration. As the
period of maximal production is also one with the least oxygenation in bottom
waters, one may expect a cycle wherein, on balance, organic matter accumulates
in sediments during the upwelling period and is degraded during the rest of the
year. Inorganic nutrients thus regenerated should then be expected to support
PP during the non-upwelling periods, thereby dampening the seasonality in
production to some extent. This aspect of benthic-pelagic coupling including
variability of benthic respiration rates is yet to be studied in detail. Limited
measurements of sedimentary denitrification, made mostly during the upwelling
period following the acetylene block technique, have yielded values ranging
from 0.27 to 1.45 pmol NO
3
−
cm
−
2
s
−
1
, which are generally within the range
of values from other areas [33, 34]. Scaling these rates to the area of the Indian
shelf provides an estimate of the annual nitrogen loss ranging between 0.21
and 1.15 Tg, which implies that sedimentary denitrification is an important but
not the dominant sink for fixed nitrogen in the region.
