56
D. A. MCGILL:
WICKSTEAD (1962) correlates plankton populations from the East African area of the
Indian Ocean with Secchi disk readings, showing a related decline in plankton (dry weight
m- 3 ) in the topmost 200 m with increase in transparency. "The bulk of the plankton
reacts according to the degree to which the transparency of the water permits penetration
of light. This reaction modifies vertical behavior movements due to negative geotropism,
inherent vertical migration patterns, etc. Variations of the plankton are not indicative of
richer or poorer areas but of the relative transparency at the time of sampling." (WICKSTEAD, 1962, pp. 1225).
b) Color
VOYTOVand DEMENT'YEVA (1970) list a total of 638 observations of color, with over
90% of these from the winter period corresponding to the main concentration of transparency observations. However, "color scale values of I to II predominate over the entire
northern equatorial part of the Indian Ocean, except for the littoral regions". Such values
correspond to a maximum irradiance transmittance in the blue wavelengths, according
to JERLOV (1965). This blue color of the open ocean is due to selective absorption by the
water itself, while multiple scattering by the water molecules also favors short wave
length. In turbid waters, selective absorption by particles and yellow substance leads to a
shift of color towards longer wave lengths (JERLOV, 1965). Thus, WICKSTEAD (1961 b)
has noted the bottle-green color of the sea over the North Kenya banks and compared this
with temperate waters such as the North Sea. Such an area is markedly different from
the usual clear blue of the offshore East African waters and also the "dirtier blue or pale
green of the inshore waters". Increasing organic production is commonly associated
with a change toward longer wave lengths.
2. Extinction (= Attenuation) Measurements
The light extinction is a measure of the reduction of light intensity in a vertical distance
in the sea and may be defined by a coefficient, k:
k = 2.30 (LoglOhz- LOglOhz+1)
where IAz and hz+1 are the illuminations of a wave length (A) at depths z and z+ 1. The
use of the terms attenuation and attenuation coefficient has replaced the concept of
extinction in recent accounts of optical studies (JERLOV, 1963; 1968), but most Indian
Ocean data in the literature has been expressed as extinction.
The early studies of T. S. S. RAO (1957) related the extinction coefficient for the
visible light rays to the Secchi disk values by use of the formula:
k = 1. 7/D (in m)
where D is the maximum depth of visibility in m, as determined from Secchi disk readings.
This numerical conversion was originally derived by POOLE and ATKINS (1929) in the
English Channel for blue light and is believed to be suitable for moderately clear oceanic
water. MURPHY (1959) has suggested, however, that there may be considerable differences among the spectral transmittance of various waters. Although GRAHAM (1966)
found a statistically significant correlation between Secchi disk observations and extinction coefficients, he warns that caution should be used when extrapolating a relationship
from one oceanic environment to another. Thus, QASIM, BHATTATHIRI and ABIDI (1968)
D. A. MCGILL:
WICKSTEAD (1962) correlates plankton populations from the East African area of the
Indian Ocean with Secchi disk readings, showing a related decline in plankton (dry weight
m- 3 ) in the topmost 200 m with increase in transparency. "The bulk of the plankton
reacts according to the degree to which the transparency of the water permits penetration
of light. This reaction modifies vertical behavior movements due to negative geotropism,
inherent vertical migration patterns, etc. Variations of the plankton are not indicative of
richer or poorer areas but of the relative transparency at the time of sampling." (WICKSTEAD, 1962, pp. 1225).
b) Color
VOYTOVand DEMENT'YEVA (1970) list a total of 638 observations of color, with over
90% of these from the winter period corresponding to the main concentration of transparency observations. However, "color scale values of I to II predominate over the entire
northern equatorial part of the Indian Ocean, except for the littoral regions". Such values
correspond to a maximum irradiance transmittance in the blue wavelengths, according
to JERLOV (1965). This blue color of the open ocean is due to selective absorption by the
water itself, while multiple scattering by the water molecules also favors short wave
length. In turbid waters, selective absorption by particles and yellow substance leads to a
shift of color towards longer wave lengths (JERLOV, 1965). Thus, WICKSTEAD (1961 b)
has noted the bottle-green color of the sea over the North Kenya banks and compared this
with temperate waters such as the North Sea. Such an area is markedly different from
the usual clear blue of the offshore East African waters and also the "dirtier blue or pale
green of the inshore waters". Increasing organic production is commonly associated
with a change toward longer wave lengths.
2. Extinction (= Attenuation) Measurements
The light extinction is a measure of the reduction of light intensity in a vertical distance
in the sea and may be defined by a coefficient, k:
k = 2.30 (LoglOhz- LOglOhz+1)
where IAz and hz+1 are the illuminations of a wave length (A) at depths z and z+ 1. The
use of the terms attenuation and attenuation coefficient has replaced the concept of
extinction in recent accounts of optical studies (JERLOV, 1963; 1968), but most Indian
Ocean data in the literature has been expressed as extinction.
The early studies of T. S. S. RAO (1957) related the extinction coefficient for the
visible light rays to the Secchi disk values by use of the formula:
k = 1. 7/D (in m)
where D is the maximum depth of visibility in m, as determined from Secchi disk readings.
This numerical conversion was originally derived by POOLE and ATKINS (1929) in the
English Channel for blue light and is believed to be suitable for moderately clear oceanic
water. MURPHY (1959) has suggested, however, that there may be considerable differences among the spectral transmittance of various waters. Although GRAHAM (1966)
found a statistically significant correlation between Secchi disk observations and extinction coefficients, he warns that caution should be used when extrapolating a relationship
from one oceanic environment to another. Thus, QASIM, BHATTATHIRI and ABIDI (1968)