Light and Nutrients in the Indian Ocean
63
maximum zone is found to occur beneath the level of the O2 minimum, as recognized
by MOKIEVSKAYA (1961).
66) Observations at 1200 m (Fig. 9)
The maximum phosphate values of 3.00 ,Ltg-at I-I or more are found in the Arabian
Sea at 1200 m (Fig. 9). These readings are slightly higher than the maximum concentrations noted by ROZANOV (1964). Values for the central and equatorial regions are from
2.50 ,Ltg-at I-I to about 2.80 ,Ltg-at 1- 1 • The P04 3 - - P concentration in the Antarctic
Intermediate water south of the Equator is similar to the level found in the Atlantic Ocean
(2.50- 2.70 ,Ltg-at I-I), and these values agree with those reported by ROZANOV (1964).
The O2 minimum has now been traversed, and values at 1200 m in the North Indian
Ocean range from 0.3 to 1.25 mll- 1 • Oxygen values decrease northward in the Antarctic
Intermediate water about 1.50 ml 1- 1 in the area east of the Mascarene Ridge between
the Seychelles and Mauritius.
EE) Observations at 2000 m (Fig. 10)
The zone of the phosphate concentration remains high in the northern areas (more
than 2.60 ,Ltg-at 1- 1 ) and it is only slightly lower to the south. MOSTERT (1966) has found
the area near South Africa to have a maximum concentration of phosphate at 1000 to
1500 m followed by "a thick bottom layer with relatively little change with depth". He
reports that pockets of high P04 3 - - P (up to 2.6 ,Ltg-at I-I ) appear throughout the area
and may perhaps be attributed to the regeneration of phosphate from decaying matter
sinking to the bottom. ORREN (1963) found a region of maximum phosphate concentration at about 2000 m for the SW Indian Ocean.
O 2 concentrations in the deeper waters are increasing, but a north-south gradient
from 2.00 mll- 1 to 4.00 m 1- 1 is still seen near Madagascar in the data of Fig. 10.
W Observations at 3000 m (Fig. 11)
At 3000 m, as shown in Fig. 11, the schematic bottom topography delineates the top
of the mid-ocean ridge system, including the Mascarene Ridge. The Arabian Sea basin
shows O 2 levels of 3.00 to 3.50 mll - 1 at this depth and P04 3 - - P values greater than
2.50 ,Ltg-at 1- 1. The phosphate level decreases to less than 2.00 ,Ltg-at I- I near South Africa.
In this same southern location the O2 content is greater than 5.00 ml - 1 •
rJrJ) Observations at 4000 m (Fig. 12)
The bottom depths of the western Indian Ocean are represented by Fig. 12 at 4000 m,
which is designated by IVANENKOV and GUBIN (1960) as the top of the distinct bottom
water masses. The Arabian Sea area shows O 2 concentrations of 3.75 to 3.80 mll- 1 and
phosphate from 2.40 to 2.50 ,Ltg-at 1- 1 • The South Indian Ocean bottom water (temperature 1.1-1.5 ° C, salinity 34.72-34.73 %0) has an O 2 content of more than 4.00 mIl-I,
increasing southward, while the P04 3 - - P concentration is close to 4.00 ,Ltg-at I-I. South
of the Mozambique Channel is a water mass closely corresponding to Antarctic Bottom
Water (temperature less than 1 ° C, salinity 34.73%0) with an O 2 content of more than
5.00 ml- 1 • The phosphate concentration of this water is 2.10 ,Ltg-at 1-\ which is lower
than in the other regions at this depth.
63
maximum zone is found to occur beneath the level of the O2 minimum, as recognized
by MOKIEVSKAYA (1961).
66) Observations at 1200 m (Fig. 9)
The maximum phosphate values of 3.00 ,Ltg-at I-I or more are found in the Arabian
Sea at 1200 m (Fig. 9). These readings are slightly higher than the maximum concentrations noted by ROZANOV (1964). Values for the central and equatorial regions are from
2.50 ,Ltg-at I-I to about 2.80 ,Ltg-at 1- 1 • The P04 3 - - P concentration in the Antarctic
Intermediate water south of the Equator is similar to the level found in the Atlantic Ocean
(2.50- 2.70 ,Ltg-at I-I), and these values agree with those reported by ROZANOV (1964).
The O2 minimum has now been traversed, and values at 1200 m in the North Indian
Ocean range from 0.3 to 1.25 mll- 1 • Oxygen values decrease northward in the Antarctic
Intermediate water about 1.50 ml 1- 1 in the area east of the Mascarene Ridge between
the Seychelles and Mauritius.
EE) Observations at 2000 m (Fig. 10)
The zone of the phosphate concentration remains high in the northern areas (more
than 2.60 ,Ltg-at 1- 1 ) and it is only slightly lower to the south. MOSTERT (1966) has found
the area near South Africa to have a maximum concentration of phosphate at 1000 to
1500 m followed by "a thick bottom layer with relatively little change with depth". He
reports that pockets of high P04 3 - - P (up to 2.6 ,Ltg-at I-I ) appear throughout the area
and may perhaps be attributed to the regeneration of phosphate from decaying matter
sinking to the bottom. ORREN (1963) found a region of maximum phosphate concentration at about 2000 m for the SW Indian Ocean.
O 2 concentrations in the deeper waters are increasing, but a north-south gradient
from 2.00 mll- 1 to 4.00 m 1- 1 is still seen near Madagascar in the data of Fig. 10.
W Observations at 3000 m (Fig. 11)
At 3000 m, as shown in Fig. 11, the schematic bottom topography delineates the top
of the mid-ocean ridge system, including the Mascarene Ridge. The Arabian Sea basin
shows O 2 levels of 3.00 to 3.50 mll - 1 at this depth and P04 3 - - P values greater than
2.50 ,Ltg-at 1- 1. The phosphate level decreases to less than 2.00 ,Ltg-at I- I near South Africa.
In this same southern location the O2 content is greater than 5.00 ml - 1 •
rJrJ) Observations at 4000 m (Fig. 12)
The bottom depths of the western Indian Ocean are represented by Fig. 12 at 4000 m,
which is designated by IVANENKOV and GUBIN (1960) as the top of the distinct bottom
water masses. The Arabian Sea area shows O 2 concentrations of 3.75 to 3.80 mll- 1 and
phosphate from 2.40 to 2.50 ,Ltg-at 1- 1 • The South Indian Ocean bottom water (temperature 1.1-1.5 ° C, salinity 34.72-34.73 %0) has an O 2 content of more than 4.00 mIl-I,
increasing southward, while the P04 3 - - P concentration is close to 4.00 ,Ltg-at I-I. South
of the Mozambique Channel is a water mass closely corresponding to Antarctic Bottom
Water (temperature less than 1 ° C, salinity 34.73%0) with an O 2 content of more than
5.00 ml- 1 • The phosphate concentration of this water is 2.10 ,Ltg-at 1-\ which is lower
than in the other regions at this depth.