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072992B.CHM
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Country: India
From: Indian National Institute of Oceanography
Annual Report 1990-1991
p. 40-50. Some topics extracted from original
section on Chemical Oceanography for transmittal
later as .ENV (Environmental) message. This message
includes purely chemical programs.
CHEMICAL
OCEANOGRAPHY
4.2.1
CHEMICAL AND Environmental STUDIES IN THE SEAS AROUND INDIA (R. Sen Gupta)
(a) Denitrification and nitrous oxide in the Arabian Sea (S. W. A. Naqvi,
M. S. Shailaja and P.J. Noronha)
The distribution of nitrite at the secondary nitrite maximum was studied to
delineate the boundaries of the denitrification zone and to understand the
processes that control the extent of denitrification in the Arabian Sea.
Nitrite distribution showed little correspondence with primary productivity.
The three major sites of seasonal upwelling appear to be located outside the
zone of intense denitrification. The locus of the most intense
denitrification appears to extend southwest from the shelf break off
Gujarat. In the northernmost and western parts of the Arabian Sea,
denitrification seemed to be inhibited by the supply of oxygen from the
surface to subsurface layers due to the convective overturning of surface
waters from the equatorial region, however, maintained strong stratification
in the eastern and the central Arabian Sea. It was suggested that the
development of reducing conditions in the Arabian Sea may be closely related
to this feature.
Measurements of N20 at 16 stations in the Arabian Sea reveal high degrees
of surface saturation (186+/- 17%) and consequently large atmospheric fluxes
of N20 just below the mixed layer as the vertical exchange coefficients (0.55
+ 0.32 cm^2/sec) required to sustain the observed fluxes, are not very high.
The mechanisms proposed so far f or the production of N20 are re-evaluated
in the light of recent data on the nitrogen isotopic composition of N20 from
the western north Pacific Ocean. It is proposed that -a nitrification-
denitrification couple represented by the pathway NH4 - NO - N20 may be a
potentially dominant mechanism for N20 production in the ocean. The
diffusive losses of N20 to the denitrifying waters and marginal sediments
are estimated as >2 Tg. N./year. The Arabian Sea as a whole appears to be a
net source of N2O. Estimates of global oceanic N20 production have been
revised upward as 10 Tg N/year).
Pioneering measurements of the activity of the respiratory electron
transport system (ETS), at 17 stations during two cruises in the Arabian Sea
yielded very high respiration rates which did not accord to the trends in
primary productivity. The rates could not be sustained by the supply of
carbon, associated with the sinking particles alone, and seemed to point to
a major role of the dissolved organic carbon in fuelling oxygen consumption
and denitrification. The data were utilized to compute the rate of
denitrification in the Arabian Sea as 32.6 Tg N/year which is in excellent
agreement with the estimate based on the exports of nitrate deficits outside
the denitrification zone.
(b) Phosphorus and nitrogen in sediments (M.D. Paiagopal)
Studies on the distribution of phosphorus and nitrogen compounds in the
sediments collected from the nearshore regions of Karwar, Tiracol and Malvan
indicated a wide variation of its concentrations. Ranges of concentrations
of the various fractions are given below:-
Karwar Tiracol Malvan
Total P(mg/g) 0.92-2.22 1.31-3.06 0.49-1.05
dry wt.
Org. P 0.04-0.59 0.06-0.74 0.11-0.33
Inorganic P 0.88-1.63 1.25-2.32 0.38-0.72
Inorganic P has been fractionated to the following serial extraction and the
occluded fraction has been computed by difference.
Karwar Tiracol Malvan
Aluminium phosphate 0.24-0.49 0.21-0.42 0.19-0.22
Iron phosphate 0.23-0.38 0.36-0.88 0.04-0.11
Calcium phosphate 0.07-0.21 0.11-0.24 0.07-0.22
Reductant soluble P 0.13-0.19 0.19-0.23 0.02-0.08
Occluded P 0.21-0.36 0.38-0.55 0.06-0-09
(c) Relationship of DOC with oxygen in the Arabian Sea (A. Rajendran and
M. Dileep Kumar)
In the euphotic zone of the northern Arabian Sea, the DOC concentration
showed a maximum of 119 micromol dm^3 whereas in the deeper layers it was
only 94 micromol/dm^3. The DOC was maximum (310 micromol/dm^3) in both the
upper 150 m and the deeper layers of the southern Arabian Sea. The central
Arabian Sea showed a gradient between the north and the south. The northern
Arabian Sea surface (upper 150 m) layers showed apparent oxygen utilization
(AOU) values up to 28 micromol/dm^3 whereas the southern Arabian Sea did not
show any AOU. In the deeper layers AOU varied between 203 and 262 micromol/
dm^3 for the northern Arabian Sea and the same was 47-223 micromol/dm^3 for
the southern Arabian Sea.
A single relationship was observed between DOC and AOU for the north, which
was AOU = 547 - 4.9 DOC. In the southern Arabian Sea, of the two
relationships, one showed AOU = 1.82 - 0.06 DOC and the other showed AOU =
29.07 + 0.52 DOC.
(d) Inorganic carbon components in the Arabian Sea (M. Dileep Kumar and
A. Pajendran)
Titration Alkalinity (TA) decreases from north to south and also from west
to east. The distribution, in general, depicts an initial decrease from
surface to 500 m before showing an increasing trend with depth. TA values
in the north are higher than those in the west. TC02 also behaved similar to
TA but with a very clear decreasing trend southward at all depth levels.
TC02 was found to be around 2400 micromol/dm^3 at equator but in the north
it was more than 2600 micromol/dm^3, with a maximum of 2650 micromol/dm^3
in the deeper layers. Salinity normalized TC02 (NTC02) when plotted against
potential temperature, showed decreasing trend northward. The most sig-
nificant phenomenon was the deepening of isopleths (2300 micromol/dm^3)
around 12 N. This could be due to the influx of the Red Sea water and its
influence could be felt even in very deep layers.
(e) River inputs to the Arabian Sea and Bay of Bengal (M. Dileep Kumar and
M. D. George)
Indian rivers contribute about 4% of the global annual discharge, of which
the major rivers contribute 126 x 10^10 m^3/year to the Bay of Bengal which
is approximately four times of that to the Arabian Sea (29.7 x 10^10 m^3/year.
The Bay of Bengal and the Arabian Sea receive 4% of the global river runoff,
though they occupy only 3% of the oceanic area. The major ion contents are
inversely proportional to the river runoff, specifically for the rivers
entering the Arabian Sea. The Bay of Bengal receives nearly four times more
total dissolved solids (TDS, 207 x 10^12 9 g/year), which also is the case for
major ions. On an average Indian rivers are enriched in major ions, except
Na, compared to the average world rivers.
(f) Inorganic carbon in the Arabian Sea sediments (M. D. George and M.
Dileep Kumar)
Carbonate-carbon analyses were done in two marine sediment cores using
carbon coulometer. Carbonate did not vary much vertically in a core off
Ratnagiri but for a minimum at 14 cm below the interface, after which it
increased again. However, off Goa carbonate increased from 44 mg C/g at the
interface to 54 mg C/g at 14 cm depth but these values were much lower than
those observed in the core, off Ratnagiri. Aragonite carbon constituted
larger portion of inorganic carbon whose percentage generally decreased
with depth, while that of calcite increased.
(g) Humic acids in the mangrove sediments (S. Sardessai)
Humic acids showed distinct seasonal variations in the mangrove ecosystem.
The highest concentration of humic acids in dissolved particulate and
sedimentary forms was found during the monsoon (June-Sept.) when the
salinity was minimum while it was the lowest in the premonsoon (Feb.-May) when
the salinity was maximum. Sedimentary humic acids were relatively higher
than the dissolved and particulate acids except in the monsoon when the
properties of particulate humic acids exceeded that of sedimentary. The
percentage of organic matter associated with humic acids in the sediments
was maximum in the monsoon and minimum in the premonsoon which might be due
to the addition of humic acids to the sediments by the freshwater inflow from
the estuarine upstream region. The percentage of organic matter in the
sediments varied from 1.8 - 9.7% which was high compared to the values
reported elsewhere. A positive correlation (r = 0.736) was also observed
between organic carbon and humic acids. Among the three stations studied,
the one with very thick population of mangroves, was found to be very rich
in humic acids.
(h) Studies on boron chemistry in the Arabian Sea (P. V. Shirodkar and S.
Y. S. Singbal)
Boron concentration in the central eastern Arabian Sea varied from 3.96 to
5.30 mg/kg in the coastal and from 3.0 to 4.74 mg/kg in the off shore waters.
Boron-chlorinity ratios also showed a wide variation from 0.202 to 0.269 in
the coastal region while significantly low ratios ( 27 C) and relatively
saline (salinity >35.3 ppt) and having low density (22.9 - 23-3 sigma units),
low concentration of nutrients and low dissolved oxygen content and (ii)
south of 13 S centred at 14 to 16 S Lat. is another water which is
characterized by low temperature (<25 C), low salinity (<34.8 ppt), low density
(23.2-23.5 sigma units), low nutrients and high dissolved oxygen.
At intermediate depth three main watermasses could be identified, i.e.: (i)
Salinity maximum water, (ii) Oxygen maximum water and (3) Low salinity water.
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