Environmental Engineering Reference
In-Depth Information
and on a monthly basis for the remaining period. When plotted against time,
these data reveal well-defined annual cycles of the measured variables, reflect-
ing the seasonality of hydrographic and biogeochemical processes, including
the evolution of O
2
deficiency (Fig. 3).
The temperature record (Fig. 3a) confirms the bimodal distribution pattern
previously reported from the region [3, 53]. The highest values occur during the
spring intermonsoon (SI; April-May) and late FI seasons; a minor minimum
occurs during the NEM, but the main minimum is found during the SWM
extending into the early FI. This is an apparent consequence of upwelling
which, at its peak in September, lowers the near-bottom temperature below
21
o
C.
The water column is vertically homogeneous during the NEM, but the salinity
changes considerably (Fig. 3b). As stated earlier, the WICC brings fresher
waters from the south, and this is reflected by lower salinities, particularly
in February. The highest salinity values (
36.000) are recorded during the
following SI season before the monsoon rainfall produces a low-salinity lens
that persists from June/July to October/November (Fig. 3b).
The duration of O
2
deficient conditions is somewhat longer than would be
expected from the temperature record (Fig. 3c) due to the occasional occurrence
of near-bottom O
2
depletion during the SI. We speculate that this might be
caused by the decay of
Trichodesmium
blooms. The lowest O
2
concentrations (
<
10µM) are, however, confined only to the period of upwelling (July-November).
The decreases in O
2
in subsurface waters are mirrored by increases in NO
3
−
(Fig. 3d). However, the NO
3
−
concentration begins to fall as denitrification
sets in by July/August, and in about one month all NO
3
−
is used up by this
process.
Traces of NO
2
−
occur in subsurface waters even when the environment is
not reducing (Fig. 3e), possibly produced through the assimilatory reduction of
NO
3
−
by phytoplankton, nitrification or denitrification in sediments. When the
environment becomes denitrifying, NO
2
−
accumulates in much higher concen-
trations with the peak values (averaging in excess of 6 µM) occurring slightly
above the seafloor during August-September, and declining rapidly thereafter
as subsurface waters become completely anoxic.
Low N
2
O concentrations prevail throughout the water column during the
NEM and SI seasons. However, a dramatic increase in N
2
O concentration is
noticed once the system becomes O
2
deficient (Fig. 3f). The highest concen-
trations (averaging in excess of 180 nM), observed after the subsurface waters
turn reducing, are coincident with the highest NO
2
−
. Toward the end of the
suboxic period, and coinciding with the onset of SO
4
2
−
reduction, N
2
O con-
centration declines sharply, but increases again to moderately high levels in
November.
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