Light and Nutrients in the Indian Ocean
57
have preferred a value of k = 1.5/D in the turbid estuarine waters near Cochin, while
STRICKLAND (1958) cites an expression, k = 1.9/D, for very clear oceanic waters. Such
evaluations result in an expression of extinction (or attenuation) for any water layer of
1 m thickness as a percent of the incident daylight radiation.
a) Applications of Extinction Values
WYRTKI (1962) correlates the transparency of East Australian waters with the seasonal
upwelling, noting that subtropical water has an extinction below 0.12 m -t, which is
close to the extinction of pure water (thus indicating the absence of suspended matter).
"Upwelling areas south of Java have a much higher extinction coefficient, indicating an
abundance of particles, probably plankton. The highest values are found off the coast
of Australia, where strong tidal currents are responsible for an enrichment of the water
with particulate matter from the shelf bottom. In general, the distribution of the extinction
coefficients very closely resembles that of the plankton biomass." (WYRTKI, 1962,
pp. 223 - 224.) Similar observations in other areas of Indian Ocean upwelling would be
valuable but are not available in the literature.
Results such as those above may, however, be compared with findings of GRAHAM
(1966) in the NW Pacific, where cells of high extinction coefficient (greater than 0.16)
were associated with areas of summer upwelling or river discharge. "In offshore locations
where seasonal warming produced sharp thermoclines and shallow surface layers during
the summer, extinction coefficients were high and sometimes exceeded the highest values
obtained (in costal waters)." (GRAHAM, 1966, p. 184).
b) Irradiance Measurements
A transparency meter was used on INS "Kistna" by RAMAM and MURTHY (1968)
to determine percent transparency as the difference of overall readings and ambient
readings. For depths from 0 to 20 m, transparencies increased toward 100% from a low
of 40% in inshore waters and near the bottom. Waters off Madras (Coromandel coast)
were somewhat more transparent than near Cochin (Malabar coast).
CLARKE and KELLY (1964) have determined transparency in the western Indian
Ocean at midday for radiation at wavelengths centered at 480 nm, using a deep sea photomultiplier photometer. Attenuation coefficients were found as follows:
0- 30 m: k = 0.70 to 0.192
30 -100 m: k = 0.023 to 0.066
100 - 900 m: k = Low values, to a minimum of 0.021
The variations of attenuation are related to the complexity of water movements in the
equatorial regions. Thus, extremely optically clear water corresponded roughly with
the tongue of Indian Ocean central water in the section at 60 ° E longitude. There is a
tendency throughout for rates of attenuation of visible light to become less with increasing
depth. In most cases, the authors found no further change in transparency below 300 m.
One value at 900 m (k = 0.0021; "Anton Bruun" station 154,22° 58'S, 59° 45'E)
represents an irradiance of 6 X 10- 9 % of the surface value, a range 10 times greater than
that for comparable observations in other areas (JERLOV, 1951). It thus represents the
most transparent water ever recorded below 100 m. It is estimated that a deep sea fish
could probably detect tbe presence of daylight at 1300 m at noon in this part of the Indian
57
have preferred a value of k = 1.5/D in the turbid estuarine waters near Cochin, while
STRICKLAND (1958) cites an expression, k = 1.9/D, for very clear oceanic waters. Such
evaluations result in an expression of extinction (or attenuation) for any water layer of
1 m thickness as a percent of the incident daylight radiation.
a) Applications of Extinction Values
WYRTKI (1962) correlates the transparency of East Australian waters with the seasonal
upwelling, noting that subtropical water has an extinction below 0.12 m -t, which is
close to the extinction of pure water (thus indicating the absence of suspended matter).
"Upwelling areas south of Java have a much higher extinction coefficient, indicating an
abundance of particles, probably plankton. The highest values are found off the coast
of Australia, where strong tidal currents are responsible for an enrichment of the water
with particulate matter from the shelf bottom. In general, the distribution of the extinction
coefficients very closely resembles that of the plankton biomass." (WYRTKI, 1962,
pp. 223 - 224.) Similar observations in other areas of Indian Ocean upwelling would be
valuable but are not available in the literature.
Results such as those above may, however, be compared with findings of GRAHAM
(1966) in the NW Pacific, where cells of high extinction coefficient (greater than 0.16)
were associated with areas of summer upwelling or river discharge. "In offshore locations
where seasonal warming produced sharp thermoclines and shallow surface layers during
the summer, extinction coefficients were high and sometimes exceeded the highest values
obtained (in costal waters)." (GRAHAM, 1966, p. 184).
b) Irradiance Measurements
A transparency meter was used on INS "Kistna" by RAMAM and MURTHY (1968)
to determine percent transparency as the difference of overall readings and ambient
readings. For depths from 0 to 20 m, transparencies increased toward 100% from a low
of 40% in inshore waters and near the bottom. Waters off Madras (Coromandel coast)
were somewhat more transparent than near Cochin (Malabar coast).
CLARKE and KELLY (1964) have determined transparency in the western Indian
Ocean at midday for radiation at wavelengths centered at 480 nm, using a deep sea photomultiplier photometer. Attenuation coefficients were found as follows:
0- 30 m: k = 0.70 to 0.192
30 -100 m: k = 0.023 to 0.066
100 - 900 m: k = Low values, to a minimum of 0.021
The variations of attenuation are related to the complexity of water movements in the
equatorial regions. Thus, extremely optically clear water corresponded roughly with
the tongue of Indian Ocean central water in the section at 60 ° E longitude. There is a
tendency throughout for rates of attenuation of visible light to become less with increasing
depth. In most cases, the authors found no further change in transparency below 300 m.
One value at 900 m (k = 0.0021; "Anton Bruun" station 154,22° 58'S, 59° 45'E)
represents an irradiance of 6 X 10- 9 % of the surface value, a range 10 times greater than
that for comparable observations in other areas (JERLOV, 1951). It thus represents the
most transparent water ever recorded below 100 m. It is estimated that a deep sea fish
could probably detect tbe presence of daylight at 1300 m at noon in this part of the Indian