102
D. A. MCGILL: Light and Nutrients in the Indian Ocean
while the sediments near Madras are uniformly poor. The distribution is believed to
reflect the presence of a dominant upwelling followed by a slow rate of deposition of
detrital materials. Here also, however, the influence of the rivers must be cited. It is
suggested that they bring in a sediment load, with resulting absorption of dissolved
phosphate in the water by silts and clays, followed by settling to the bottom. This is
particularly true in the northern region where the Ganges and Brahmaputra carry large
amounts of suspended terrigenous material to the Bay. In more southerly regions, between
Visakhapatnam and Karaikal, differences have been attributed to the variation in upwelling and the corresponding differences in the level of biological productivity.
ROCHFORD (1963) has reported that the organic phosphorus in the upper 200 min
the SE Indian Ocean has its maximum values (0.20 - 0.38 .ug-at 1-1 for mean concentrations) along the southern boundary of the South Equatorial Current, along the Equatorial
divergence, and within the boundary of the Counter Current and North Equatorial
Current. In general, there was great variation in the surface concentration with latitude
throughout the region surveyed. Pronounced maxima were also observed in the vertical
profiles of organic phosphorus. Subsurface organic phosphorus maxima at 500 -700 m
and again at 1000 - 1500 m tended to coincide with the O2 minimum zones. A maximum
in bottom waters near 4320 m at one station consisted of about 40% particulate and
60% dissolved organic phosphorus. In stations between Java and NW Australia, the
organic phosphorus maximum consisted of between 40% and 100% particulate organic
phosphorus. It appears that the ROCHFORD values are the only ones available for particulate phosphorus concentrations in the deeper parts of the water column. More detailed
data on such fractions in deep water for all ocean areas are needed to help understand
the regeneration of nutrients.
It is possible that particulate maxima represent the occurrence of "refractory compounds" (MCGILL, 1963) which are extremely resistant to recycling. REDDY and
SANKARANARAYANAN (1968 b) find evidence in shallow waters of a more rapid release
of some nutrients relative to others. "Nitrates in general show an inverse relationship
with the other nutrients, particularly in the surface ... These differences in maxima of
respective nutrients might be due to more rapid release of certain nutrients earlier than
others and it provides a mechanism by which nitrogen and phosphorus may be fractionated and this may explain the variation in the ratio of these elements in the sea water .
. .. The ratio between elements Si, Nand P ... is highly variable, unlike that of the temperate region. This variation may presumably be due to the differential and rapid rates
of regeneration and consumption of the nutrients accompanied by extensive mixing of
water masses which seems a characteristic features of tropical waters." (REDDY and
SANKARANARAYANAN, 1968 b, p. 219). Others have evaluated the pr?blem in a different
light, attempting to derive ratios between biological decomposition and uptake (MIYAKE,
1966), or expressing the phosphorus variability as related to O2 variation (VISWANATHAN
and GANGULY, 1968).
D. A. MCGILL: Light and Nutrients in the Indian Ocean
while the sediments near Madras are uniformly poor. The distribution is believed to
reflect the presence of a dominant upwelling followed by a slow rate of deposition of
detrital materials. Here also, however, the influence of the rivers must be cited. It is
suggested that they bring in a sediment load, with resulting absorption of dissolved
phosphate in the water by silts and clays, followed by settling to the bottom. This is
particularly true in the northern region where the Ganges and Brahmaputra carry large
amounts of suspended terrigenous material to the Bay. In more southerly regions, between
Visakhapatnam and Karaikal, differences have been attributed to the variation in upwelling and the corresponding differences in the level of biological productivity.
ROCHFORD (1963) has reported that the organic phosphorus in the upper 200 min
the SE Indian Ocean has its maximum values (0.20 - 0.38 .ug-at 1-1 for mean concentrations) along the southern boundary of the South Equatorial Current, along the Equatorial
divergence, and within the boundary of the Counter Current and North Equatorial
Current. In general, there was great variation in the surface concentration with latitude
throughout the region surveyed. Pronounced maxima were also observed in the vertical
profiles of organic phosphorus. Subsurface organic phosphorus maxima at 500 -700 m
and again at 1000 - 1500 m tended to coincide with the O2 minimum zones. A maximum
in bottom waters near 4320 m at one station consisted of about 40% particulate and
60% dissolved organic phosphorus. In stations between Java and NW Australia, the
organic phosphorus maximum consisted of between 40% and 100% particulate organic
phosphorus. It appears that the ROCHFORD values are the only ones available for particulate phosphorus concentrations in the deeper parts of the water column. More detailed
data on such fractions in deep water for all ocean areas are needed to help understand
the regeneration of nutrients.
It is possible that particulate maxima represent the occurrence of "refractory compounds" (MCGILL, 1963) which are extremely resistant to recycling. REDDY and
SANKARANARAYANAN (1968 b) find evidence in shallow waters of a more rapid release
of some nutrients relative to others. "Nitrates in general show an inverse relationship
with the other nutrients, particularly in the surface ... These differences in maxima of
respective nutrients might be due to more rapid release of certain nutrients earlier than
others and it provides a mechanism by which nitrogen and phosphorus may be fractionated and this may explain the variation in the ratio of these elements in the sea water .
. .. The ratio between elements Si, Nand P ... is highly variable, unlike that of the temperate region. This variation may presumably be due to the differential and rapid rates
of regeneration and consumption of the nutrients accompanied by extensive mixing of
water masses which seems a characteristic features of tropical waters." (REDDY and
SANKARANARAYANAN, 1968 b, p. 219). Others have evaluated the pr?blem in a different
light, attempting to derive ratios between biological decomposition and uptake (MIYAKE,
1966), or expressing the phosphorus variability as related to O2 variation (VISWANATHAN
and GANGULY, 1968).