1980 - National Institute of Oceanography

calculated to be about 3.2 x 10 12 g/yr · This represents about 5% of the annual global
denitrification.
3. Major nutrients of the Mandovi and Zuari river systems
Seasonal studies in the Mandovi and Zuari rivers show that the stations at the
marine-end have two peaks of nitrate and phoshate corresponding to monsoon and post
monsoon seasons, while the stations at the river-end showed only one peak during the
monsoon. Silicate showed a well-defined pattern of distribution with only one peak
during the monsoon.
Applying a 'simple mixture' relation using salinity as an indicator it was observed
that in the pre-monsoon season, nitrate, phosphate and silicate are removed from the
water column of the two estuaries. During the monsoon season, however, only phosphate
was removed while nitrate and silicate show a near-conservative behaviour. In the
post-monsoon season there is an addition of nitrate and phosphate to the estuaries while
silicate shows a reduction of about 50%.
4. Studies on the carbon dioxide system in sea water
Distribution of different components of carbon dioxide system in the northern
Indian Ocean, is being studied from the data collected during cruises 66 to 68 of R.V.
Gaveshani. A seasonal study has also been completed in the Mandovi-Zuari estuarine
system to understand the carbon dioxide equilibria in the estuarine process.
5. Silicon cycle in the sea
Studies on the dissolved and particulate organic forms of silicon showed that the
organic fraction of silicon was not detectable in appreciable quantities at any depth of the
Arabian Sea. The particulate silicon, however, in the surface samples varied from 10.7
to 27.9μg-at/l, and it decreased from the surface to a depth of 200 m. Below this depth, no
systematic variation was observed upto a depth of 2050 m where the values ranged from
15.1 to 75.2 μg-at/l. The particulate silicon formed a small proportion of the total
silicon as the concentrations of dissolved silicon increased with depth.
6. Calcium phosphate saturation in sea water
Water samples collected during the cruises of R.V. Gaveshani from the Andaman
Sea were analysed and the preliminary examination of the data indicated that the trend
of variation in the degree of saturation of sea water with respect to calcium phosphate is
similar to that encountered elsewhere, i.e., about 50% saturation occurs in the surface
layer and an undersaturation (about 10-15%) is found in deeper layers.
Variations in calcium and magnesium along 5°N and between 86° and 99°E were
also studied. The average values of Ca, Ca/Cl, Mg and Mg/Cl were found to be 409,
21.36x10 -3
, 1268 and 66.16xl0 -3
mg/kg respectively. Higher Mg/Cl ratio was
observed in this region as compared to the other areas of the northern Indian Ocean,
presumably because of lower salinity. The calcium values, on the other hand, were
comparatively low.
29