1.5
Light and Nutrients in the Indian Ocean
D.A.McGILL
I. Light Measurements
Light in the marine environment presents many difficulties. There is a lack of agreement about the kind of light parameter to measure in the sea and about the means to
employ to obtain measurements of the greatest value. Early workers have emphasized
transparency, as determined by simple instruments such as the Secchi disk, but other
aspects of the spectral distribution are now studied with a variety of specialized instrumentation.
1. Transparency
While the Secchi disk specifically measures the depth of visibility of the instrument,
variations in boundary conditions of the sea surface may act to reduce the precision of
measurements in comparison with photoelectric determinations of transparency (MURPHY,
1959). Although individual observations of the depth of visibility with the Secchi disk
have only limited value, the average of a series of measurements has been accepted as
valid (TYLER, 1968). While the size and reflectance of the disc are significant factors,
there is marked variation in the diameter of the disk used for obtaining much of the data
currently available.
a) Observations of Transparency in the Indian Ocean
The records of the U.S. National Oceanographic Data Center (World Data Center A)
for the HOE list 738 observations of transparency for the Indian Ocean made from
1 September 1959 through 31 December 1965. Spatial distribution of these observations
is shown in Fig. 1. Over half of the total observations, 409 stations or 55.43% of the
total, were obtained in the "post-monsoon months" (QASIM, BHATTATHIRl and ABIDI,
1968) of November, December and January. The remainder of the observations are
about equally spaced throughout the other quarters of the year. For examplt, a further
126 stations or 17.17% of the total, were taken in May, June and July, corresponding
to the onset of the SW monsoon. In all instances the concentration of observations is
notably massed in the eastern Indian Ocean and shows some preference for inshore
locations.
VOYTOV and DEMENT'YEVA (1970) have compiled a chart of relative transparency
for the Indian Ocean and the related sector of Antarctica, based on a total of 824 observations with the Secchi disk and having the greatest frequency of sampling (704 observations) in the winter period from November to March. Their resulting distribution for
average transparency in winter is redrawn as Fig. 2. A relatively uniform transparency
Light and Nutrients in the Indian Ocean
D.A.McGILL
I. Light Measurements
Light in the marine environment presents many difficulties. There is a lack of agreement about the kind of light parameter to measure in the sea and about the means to
employ to obtain measurements of the greatest value. Early workers have emphasized
transparency, as determined by simple instruments such as the Secchi disk, but other
aspects of the spectral distribution are now studied with a variety of specialized instrumentation.
1. Transparency
While the Secchi disk specifically measures the depth of visibility of the instrument,
variations in boundary conditions of the sea surface may act to reduce the precision of
measurements in comparison with photoelectric determinations of transparency (MURPHY,
1959). Although individual observations of the depth of visibility with the Secchi disk
have only limited value, the average of a series of measurements has been accepted as
valid (TYLER, 1968). While the size and reflectance of the disc are significant factors,
there is marked variation in the diameter of the disk used for obtaining much of the data
currently available.
a) Observations of Transparency in the Indian Ocean
The records of the U.S. National Oceanographic Data Center (World Data Center A)
for the HOE list 738 observations of transparency for the Indian Ocean made from
1 September 1959 through 31 December 1965. Spatial distribution of these observations
is shown in Fig. 1. Over half of the total observations, 409 stations or 55.43% of the
total, were obtained in the "post-monsoon months" (QASIM, BHATTATHIRl and ABIDI,
1968) of November, December and January. The remainder of the observations are
about equally spaced throughout the other quarters of the year. For examplt, a further
126 stations or 17.17% of the total, were taken in May, June and July, corresponding
to the onset of the SW monsoon. In all instances the concentration of observations is
notably massed in the eastern Indian Ocean and shows some preference for inshore
locations.
VOYTOV and DEMENT'YEVA (1970) have compiled a chart of relative transparency
for the Indian Ocean and the related sector of Antarctica, based on a total of 824 observations with the Secchi disk and having the greatest frequency of sampling (704 observations) in the winter period from November to March. Their resulting distribution for
average transparency in winter is redrawn as Fig. 2. A relatively uniform transparency