Environmental Engineering Reference
In-Depth Information
Table 2.
Comparison of the mean and median O
2
concentrations (Winkler values in mL L
−
1
uncorrected for reagent blanks) for various temperature ranges off Karwar, central west coast
of India, during 1971-75 and 1997-2004. Data from stations shallower than 60 m and for the
upwelling period (August-October) only have been utilized (see also Fig. 6).
1971-1975
1997-2004
Temp Range
*
Mean
Median
n
Mean
Median
n
28-29
4.88
4.18
5
4.49
4.52
8
27-28
4.75
4.77
14
5.15
4.86
11
26-27
4.43
4.50
25
4.75
5.02
9
25-26
3.85
4.14
7
3.59
3.50
11
24-25
2.35
1.85
14
2.23
1.39
7
23-24
1.24
1.08
11
1.04
0.62
12
22-23
0.97
1.15
9
0.40
0.31
9
21.5-22
1.00
0.86
5
0.43
0.22
16
21-21.5
0.44
0.28
9
0.17
0.12
9
20.5-21
0.55
0.41
9
0.29
0.05
10
20-20.5
0.48
0.48
9
0.15
0.16
13
19-20
1.11
1.44
3
0.11
0.08
11
*
inclusive of the upper limit
certainty. The two obvious possibilities involve modifications of ventilation and
organic production, which in view of the precarious biogeochemical balance
already existing in the region could be quite subtle.
Changes in physical forcing have been invoked to explain the recent devel-
opment of hypoxia in the upwelling system off Oregon, on the west coast of the
United States [24]. However, a preliminary analysis indicates that a temperature
shift of comparable magnitude has not occurred over the Indian shelf. As a more
reasonable and persuasive alternative, increased supply of nutrients from land
has been suggested to have enhanced PP in the region, as in many other coastal
areas [38]. Indeed, the fertilizer consumption in the region has undergone a
7-fold increase since the early 1970s, and this is expected to have affected river
runoff. Based on the fertilizer consumption figure for 1990, Seitzinger et al.
[49] estimated that the riverine flux of dissolved inorganic nitrogen (DIN) to
the sea from South Asia could be as much as 4.2 Tg N y
−
1
(1 Tg = 10
12
g).
However, the measured concentrations of DIN in the Indian rivers near their
mouths are not very high (
<
20 µM), and consequently terrestrial DIN inputs to
the Arabian Sea via runoff probably do not exceed 0.1 Tg N y
−
1
[39]. The de-
position of DIN (NO
3
−
+NH
3
/NH
4
+
) from the atmosphere has been estimated
to be an order of magnitude larger (1.6 Tg N y
−
1
[2]), although such deposition
occurs over a larger area. If we assume that the Indian shelf receives 10% of the
estimated atmospheric deposition over the Arabian Sea, the total anthropogenic
nitrogen loading in the region would exceed 0.2 Tg N y
−
1
. This amount has
