82
D. A. MCGILL:
(3) Nitrogen Distribution Patterns
aa) Distribution of Inorganic Nitrate-Nitrogen
The distribution of nitrate in the western Indian Ocean was determined for the same
"Atlantis II" survey. The shallower levels of this distribution are represented in Fig. 13.
The only significant nitrate concentration at the surface occurs near Socotra in the upwelling of the SW monsoon. Values from 5 to more than 15 ,ug-at I-I are seen in this
area, while negligible amounts are detected elsewhere at the surface. KABANOVA (1964)
found the surface waters in the Gulf of Aden and Somali Current region were still enriched
by nitrite- and nitrate-nitrogen to some extent in the post-monsoon season (October to
April). Some increases of the primary production to the west of the Maldive Island and in
the eastern portion of the Arabian Sea could also be correlated by KABANOVA with a level
of nitrate concentration from 0.5 to 1.5 ,ug-at I-I at that period. In the central part of the
Arabian Sea she found only a negligible surface nitrate concentration, however.
The western Arabian Sea shows the highest nitrate levels at 200 m (over 25 ,ug-at I-I);
but there is not so wide a separation from the concentrations of other regions. A maximum
of over 30 ,ug-at I-I is found near South India and Ceylon, which may be an indication
of coastal conditions there. The equatorial area is distinguished by values less than
20 ,ug-at I-I. The concentration of nitrate at 200 m decreases to less than 5 ,ug-at I-I in
the waters to either side of Madagascar.
At 600 m there is a small nitrate maximum on the East African coastal waters and
also near the Seychelles Bank. A large area of high concentration is found near the South
Indian coast. At 1200 m the whole Arabian Sea shows nitrate levels of about 35 ,ug-at r\
lower concentrations are seen in the intrusive Antarctic Intermediate water east of the
Mascarene Ridge
The concentrations of N03 --N in the deeper water layers are shown in Fig. 14. At
2000 m, there is a general concentration of 30 to 35 ,ug-at I-I. Nitrate values near
30 ,ug-at I-I persist at 3000 m and at 4000 m, such values cover nearly the entire region
of the western Indian Ocean basin.
f3f3) Evaluation of Organic Nitrogen Fractions
FRAGA (1966) detected a high concentration of dissolved organic nitrogenous matter
in the surface layers of the equatorial region from 0 to 8 0 N and found this extending
down to the deep layers. At latitudes 20 0 Nand 20 0 S, he showed two zone,s of lower
nitrogen content, extending from the bottom up to a depth of 500 to 300 m. Values for
both dissolved and particulate organic nitrogen indicated a convergence in the equatorial
region but this was not constant since there was no continual decrease in the organic
nitrogen content with depth. FRAGA suggested that pockets may be formed at about
1000 m which could be due to seasonal variations in the organic matter produced at the
surface with the change of the monsoon or transported horizontally from other richer
areas.
"The v~rtical distribution of particulate nitrogen had a maximum at 10 m and a very
clear diurnal variation extending down to 340 m. During the night the nitrogen increased
from 0 to 60 m and decreased from 60 to 340 m. It is supposed that this was due to
zooplankton migration. The vertical distribution of dissolved organic nitrogen had a
peak at 20 m and another very large one between 100 and 170 m." (FRAGA, 1966, p. 413).
D. A. MCGILL:
(3) Nitrogen Distribution Patterns
aa) Distribution of Inorganic Nitrate-Nitrogen
The distribution of nitrate in the western Indian Ocean was determined for the same
"Atlantis II" survey. The shallower levels of this distribution are represented in Fig. 13.
The only significant nitrate concentration at the surface occurs near Socotra in the upwelling of the SW monsoon. Values from 5 to more than 15 ,ug-at I-I are seen in this
area, while negligible amounts are detected elsewhere at the surface. KABANOVA (1964)
found the surface waters in the Gulf of Aden and Somali Current region were still enriched
by nitrite- and nitrate-nitrogen to some extent in the post-monsoon season (October to
April). Some increases of the primary production to the west of the Maldive Island and in
the eastern portion of the Arabian Sea could also be correlated by KABANOVA with a level
of nitrate concentration from 0.5 to 1.5 ,ug-at I-I at that period. In the central part of the
Arabian Sea she found only a negligible surface nitrate concentration, however.
The western Arabian Sea shows the highest nitrate levels at 200 m (over 25 ,ug-at I-I);
but there is not so wide a separation from the concentrations of other regions. A maximum
of over 30 ,ug-at I-I is found near South India and Ceylon, which may be an indication
of coastal conditions there. The equatorial area is distinguished by values less than
20 ,ug-at I-I. The concentration of nitrate at 200 m decreases to less than 5 ,ug-at I-I in
the waters to either side of Madagascar.
At 600 m there is a small nitrate maximum on the East African coastal waters and
also near the Seychelles Bank. A large area of high concentration is found near the South
Indian coast. At 1200 m the whole Arabian Sea shows nitrate levels of about 35 ,ug-at r\
lower concentrations are seen in the intrusive Antarctic Intermediate water east of the
Mascarene Ridge
The concentrations of N03 --N in the deeper water layers are shown in Fig. 14. At
2000 m, there is a general concentration of 30 to 35 ,ug-at I-I. Nitrate values near
30 ,ug-at I-I persist at 3000 m and at 4000 m, such values cover nearly the entire region
of the western Indian Ocean basin.
f3f3) Evaluation of Organic Nitrogen Fractions
FRAGA (1966) detected a high concentration of dissolved organic nitrogenous matter
in the surface layers of the equatorial region from 0 to 8 0 N and found this extending
down to the deep layers. At latitudes 20 0 Nand 20 0 S, he showed two zone,s of lower
nitrogen content, extending from the bottom up to a depth of 500 to 300 m. Values for
both dissolved and particulate organic nitrogen indicated a convergence in the equatorial
region but this was not constant since there was no continual decrease in the organic
nitrogen content with depth. FRAGA suggested that pockets may be formed at about
1000 m which could be due to seasonal variations in the organic matter produced at the
surface with the change of the monsoon or transported horizontally from other richer
areas.
"The v~rtical distribution of particulate nitrogen had a maximum at 10 m and a very
clear diurnal variation extending down to 340 m. During the night the nitrogen increased
from 0 to 60 m and decreased from 60 to 340 m. It is supposed that this was due to
zooplankton migration. The vertical distribution of dissolved organic nitrogen had a
peak at 20 m and another very large one between 100 and 170 m." (FRAGA, 1966, p. 413).