Environmental Engineering Reference
In-Depth Information
this line. The nearly linear trend of these values, completely unexpected from
the Redfield model, probably arises from mixing between two end members
representing a situation such as that encountered at Sta. M1a (Fig. 7).
Figure 11.
Plot of dissolved inorganic phosphorus (DIP) versus dissolved inorganic nitrogen
(DIN = NO
3
−
+NO
2
−
+NH
4
+
) for all samples taken from depths less than 200 m. Lines I and
II have been derived from linear regressions between the observed DIN and DIP data within the
oxic/hypoxic and suboxic waters with slopes (
∆
N:
∆
P) of 13.74 and -79.1, respectively, whereas
Line III has been drawn arbitrarily from the intercept of Line II on the DIP axis with the Redfield
(
∆
N:
∆
P) slope of 16 (modified from Naqvi et al. [38]).
The higher-than-expected DIP values in anoxic waters imply the involve-
ment of processes other than simple degradation of organic matter by SO
4
2
−
.
Under oxidizing conditions (Eh
>
200 mV), DIP is known to get adsorbed onto
iron hydroxide thereby forming complex iron-oxyhydroxo-phosphates (FeOP),
which settle to the seafloor; DIP is released back from sediments to the over-
lying water when FeOP complex dissolves under reducing conditions [50]. It
would appear that once the overlying waters turn anoxic over the Indian shelf
DIP is mobilized in large quantities from the sediments, accounting for the ob-
served departure of the DIN/DIP variations from the theoretical trend in anoxic
waters.
The large excess of DIP in coastal waters produced during anoxic conditions
in conjunction with adequate supply of Fe (both from land and through mobi-
lization in suboxic/anoxic waters) should prime the system for N-fixation. This
implies that N-fixation may be tightly coupled with denitrification in waters
over the western Indian continental shelf. It is therefore not surprising that
∼
