Light and Nutrients in the Indian Ocean
83
The latter peak is presumed to be produced by phytoplankton which had died and sunk
below the compensation depth, excreting soluble parts of the protoplasm. "Also augmenting the soluble nitrogen in this layer are the metabolites excreted by the zooplankton
during the day. Thus, there is a daily transport of soluble nitrogen from the superficial
layer where the zooplankton feeds to the deeper waters where it retreats to spend the day.
Hence, an accumulation of soluble organic substances produced both by chemical and
by bacterial decomposition occurs at the deeper level." (FRAGA, 1966, p. 417).
ROZANOV and BYKOVA (1964) found a thin layer of N02--N (up to a concentration
of 2 ,ug-at 1- 1 ) in the thermocline and a second nitrite maximum that reached a concentration of 5 ,ug-at 1- 1 in depths from 150 to 1500 m. They related this to the reducing conditions of the Arabian Sea waters in their area of observation.
KABANOVA (1961, 1964) has correlated the replenishment of nitrogen compounds with
the productivity of Indian Ocean areas. DUGDALE, GOERING and RYTHER (1964) report
high rates of nitrogen fixation in the northern Arabian sea, which they related to the
abundance of Trichodesmium. DUGDALE and GOERING (1967) have also estimated the
concentration of the regenerated fractions of available nitrogen, as measured by the
ammonia uptake. They obtain a ratio of carbon to nitrogen of 7.1 : 1; there is some
suggestion from their data that the ratio of light-to-dark uptake of ammonia and nitrate
is lower in tropical than in temperature waters.
b) Eastern Indian Ocean and Bay of Bengal
Numerous investigators have concerned themselves with the eastern region of the
Indian Ocean. Results of the various surveys may best be summarized on a regional basis.
a) Offshore Nutrient Concentrations in the Northern and Equatorial Waters of the
Eastern Indian Ocean
ROZANOV (1964) has found low surface P04 3 --P values of 0.1 ,ug-at 1- 1 in the Bay
of Bengal and the Andaman Sea, increasing to a maximum of 2.90 ,ug-at 1- 1 at depth.
KABANOVA (1964) cites an extremely low surface value for phosphate in the Bay of Bengal,
while in the Andaman Sea the increase of primary production causes the exhaustion of
the phosphate to analytical zero. The dissolved nitrate and nitrite values in the Bay of
Bengal are likewise low in the surface waters, but then increase sharply to 15 to 20 ,ug-at
1- 1 in the thermocline and to 22 of 26 ,ug-at 1- 1 with depth. After achieving these concentrations, the content of N03 - - N changes little in the deep waters, according to
ROZANOV and BYKOVA (1964). Greater concentrations of nitrate (1.5-2.0 ,ug-at 1- 1 )
are reported in the euphotic zone in the central part of the Andaman Sea than in the
other regions investigated by KABANOVA (1964).
The distribution of dissolved silicate is characterized by a constant increase from the
thermocline layer to the bottom. While the maximum content of silicate is 150 to
160 ,ug-at 1- 1 in the Arabian Sea, it is 140 ,ug-at 1- 1 in the Bay of Bengal and 130 to
140 ,ug-at 1- 1 in the northern equatorial part of the Indian Ocean (ROZANOV, 1964).
RAKESTRAW (1964) reported a range of silicate in stations south of the Equator from 60° E
to 100 0 E that went from 4 to 148 ,ug-at 1- 1 between the surface and depth. The silicate
maximum is very much greater in the Indian Ocean than in the Atlantic, but somewhat
less than that found in the Pacific. No intermediate maximum of silicate was observed,
and the concentration below about 2000 m is nearly constant.
83
The latter peak is presumed to be produced by phytoplankton which had died and sunk
below the compensation depth, excreting soluble parts of the protoplasm. "Also augmenting the soluble nitrogen in this layer are the metabolites excreted by the zooplankton
during the day. Thus, there is a daily transport of soluble nitrogen from the superficial
layer where the zooplankton feeds to the deeper waters where it retreats to spend the day.
Hence, an accumulation of soluble organic substances produced both by chemical and
by bacterial decomposition occurs at the deeper level." (FRAGA, 1966, p. 417).
ROZANOV and BYKOVA (1964) found a thin layer of N02--N (up to a concentration
of 2 ,ug-at 1- 1 ) in the thermocline and a second nitrite maximum that reached a concentration of 5 ,ug-at 1- 1 in depths from 150 to 1500 m. They related this to the reducing conditions of the Arabian Sea waters in their area of observation.
KABANOVA (1961, 1964) has correlated the replenishment of nitrogen compounds with
the productivity of Indian Ocean areas. DUGDALE, GOERING and RYTHER (1964) report
high rates of nitrogen fixation in the northern Arabian sea, which they related to the
abundance of Trichodesmium. DUGDALE and GOERING (1967) have also estimated the
concentration of the regenerated fractions of available nitrogen, as measured by the
ammonia uptake. They obtain a ratio of carbon to nitrogen of 7.1 : 1; there is some
suggestion from their data that the ratio of light-to-dark uptake of ammonia and nitrate
is lower in tropical than in temperature waters.
b) Eastern Indian Ocean and Bay of Bengal
Numerous investigators have concerned themselves with the eastern region of the
Indian Ocean. Results of the various surveys may best be summarized on a regional basis.
a) Offshore Nutrient Concentrations in the Northern and Equatorial Waters of the
Eastern Indian Ocean
ROZANOV (1964) has found low surface P04 3 --P values of 0.1 ,ug-at 1- 1 in the Bay
of Bengal and the Andaman Sea, increasing to a maximum of 2.90 ,ug-at 1- 1 at depth.
KABANOVA (1964) cites an extremely low surface value for phosphate in the Bay of Bengal,
while in the Andaman Sea the increase of primary production causes the exhaustion of
the phosphate to analytical zero. The dissolved nitrate and nitrite values in the Bay of
Bengal are likewise low in the surface waters, but then increase sharply to 15 to 20 ,ug-at
1- 1 in the thermocline and to 22 of 26 ,ug-at 1- 1 with depth. After achieving these concentrations, the content of N03 - - N changes little in the deep waters, according to
ROZANOV and BYKOVA (1964). Greater concentrations of nitrate (1.5-2.0 ,ug-at 1- 1 )
are reported in the euphotic zone in the central part of the Andaman Sea than in the
other regions investigated by KABANOVA (1964).
The distribution of dissolved silicate is characterized by a constant increase from the
thermocline layer to the bottom. While the maximum content of silicate is 150 to
160 ,ug-at 1- 1 in the Arabian Sea, it is 140 ,ug-at 1- 1 in the Bay of Bengal and 130 to
140 ,ug-at 1- 1 in the northern equatorial part of the Indian Ocean (ROZANOV, 1964).
RAKESTRAW (1964) reported a range of silicate in stations south of the Equator from 60° E
to 100 0 E that went from 4 to 148 ,ug-at 1- 1 between the surface and depth. The silicate
maximum is very much greater in the Indian Ocean than in the Atlantic, but somewhat
less than that found in the Pacific. No intermediate maximum of silicate was observed,
and the concentration below about 2000 m is nearly constant.