Environmental Engineering Reference
In-Depth Information
The disappearance of the oxidized nitrogen species (NO
3
−
,NO
2
−
and N
2
O)
coincides with the accumulation of both NH
4
+
and H
2
S (Fig. 3g, h). Anoxic
events are generally restricted to September and October, occasionally extend-
ing to November.
Distribution of chlorophyll
a
is relatively more patchy, in part because of
fewer and less regular observations (Fig. 3i). The lowest values, particularly
close to the surface, are recorded during the late NEM and late SI-early SWM
periods. The low phytoplankton biomass during the early SWM is contrary to
expectation and seems to reflect light limitation of PP due to high turbidity and
cloud cover. Peak chlorophyll
a
concentrations are expectedly associated with
the late SWM-early FI seasons.
The pattern of changes described above is also seen in cross-shelf sections
of properties during the period of peak O
2
deficiency as exemplified by obser-
vations off Goa in September 1999 (Fig. 4a) and September 2002 (Fig. 4b).
Upwelling is clearly indicated by the upsloping of isotherms on both occasions,
with the strong near-surface temperature and salinity gradients caused by the
above-mentioned low-salinity lens that caps the cold, saline upwelled water,
especially near the coast. Around the shelf break, near-bottom water has low
O
2
, but it is still oxidizing (inferred from the high NO
3
−
and undetectable NO
2
−
concentrations; Fig. 4a). But as this water ascends over the shelf and loses the
residual O
2
, denitrification occurs first over the mid-shelf (as evidenced by the
build-up of NO
2
−
and depletion of NO
3
−
) and SO
4
2
−
reduction follows over the
inner-shelf (indicated by the accumulation of H
2
S and NH
4
+
). Along both sec-
tions, intense N
2
O accumulation occurred at mid-depths, with the near-bottom
waters characterized by its depletion, especially in September 2002 when the
anoxic conditions were more severe.
Property distributions in the same region during January 1998 (Fig. 5a)
and February 2002 (Fig. 5b), representing the NEM, show the absence of
upwelling and the associated features described above. The water column was
well oxygenated and generally nutrient-depleted.
The sections presented above are typical of the region north of at least 12
o
N.
Although upwelling-driven O
2
depletion in the water column also occurs south
of this latitude, it does not always culminate in denitrification and SO
4
2
−
re-
duction. Nevertheless, as discussed in a following section, near-bottom O
2
concentrations have been known to fall below 0.25 mL L
−
1
as far south as
10
o
N), and on one occasion (in August 1998) denitrification did
occur over the inner shelf in this region in conjunction with a bloom of the di-
noflagellate
Noctiluca
[37]. Likewise, in September 2004 an incidence of fish
mortality accompanied by stench emanating from the sea that caused sickness
in children led to a public health alarm around Trivandum (Lat. 8.5
o
N). This
was probably due to a holococcolithophore bloom followed by O
2
depletion in
coastal waters [44]. The inter-annual changes, implied by these observations,
off Cochin (
∼
