Environmental Engineering Reference
In-Depth Information
the potential to trigger a shift from natural suboxic to anthropogenic anoxic
conditions. This is because the stoichiometries of primary production (C:N =
6.6) and denitrification (C:N = 1.1) are such that new inputs of DIN into suboxic
waters get amplified by a factor of up to 6 [11]. That is, for each unit of DIN
added to the surface waters up to 6 units of NO
3
−
may be removed at depth
if the additional organic matter produced is oxidized by NO
3
−
. The potential
increase in denitrification rate (
>
1TgNy
−
1
) is of the same order as the rate of
denitrification over the shelf (see below).
5. IMPACT OF O
2
DEFICIENCY
5.1 Biological Impact
Phytoplankton Production .
While elevated PP is a key factor responsible
for the formation and sustenance of subsurface O
2
deficiency, the latter may,
in turn, affect the quality and quantity of PP in several ways. One of them is
by regulating the availability of both the macro and micro nutrients [especially
DIN and iron (Fe)]. While NO
3
−
is lost through denitrification, Fe is mobi-
lized from the suspended matter and sediments. The loss of NO
3
−
would lead
to an initial decrease in PP. However, once the system turns anoxic nitrogen
regenerated from organic matter would accumulate as NH
4
+
(the highest NH
4
+
concentration measured over the Indian shelf is about 21 µM), and its diffusion
may sustain moderate PP in the thin oxygenated layer overlying the anoxic
water. Within the O
2
deficient layer, which has sufficient nitrogenous nutrients
and generally extends to the euphotic zone, the ambient O
2
concentrations
may be too low for the “normal” phytoplankton to meet their metabolic re-
quirements, thereby inhibiting PP (although some carbon fixation may still be
carried out by anoxygenic photosynthetic organisms [45] about which nothing
is known from this region so far). Results of our observations at two stations
off Mangalore, presented in Figs. 7 and 8, provide rare evidence for such an
effect.
One of the stations (M1A) was located over the inner shelf (Lat. 13
o
08'N,
Long. 74
o
38'E; water depth 27 m; date of sampling 19/09/2001) and the other
(M8) was positioned close to the shelf edge (Lat. 12
o
54'N, Long. 74
o
11'E;
water depth 83 m; date of sampling 20/09/2001). At both locations, upwelled
water reached depths
10 m, but redox conditions in subsurface waters were
quite different. At M1A, denitrification had resulted in complete removal of
NO
3
−
, while H
2
S and NH
4
+
accumulated at depths
≤
≥
10 m (Fig. 7). Station M8
was on the verge of being suboxic, with O
2
levels at depths
≥
20 m being less
than 30 µM and NO
3
−
concentrations exceeding 20 µM (Fig. 8). In both cases,
very low rates of PP, as measured by the
14
C technique, were recorded in the O
2
deficient waters (at depths
>
10 m at the shallow station and
>
20 m at the deeper
one), in contrast with high values in the surface layer. Typically, PP peaked
