176
N.M. SHAH:
2 or 3 times at the same depth and empty the water into a plastic bucket. This water was
then poured into 2 or 3 one-liter polyethylene screw-cap bottles for determining the
amount of plant pigments and nutrients. Samples for pigment determinations were stored
in a cool dark place after the addition of 1 ml magnesium carbonate suspension. On reaching the shore laboratory these samples were filtered through Whatman GFIC glass fibre
filters, which were then stored in a desiccator in a refrigerator. The analytical procedures
for salinity were described in BARNES (1959) and the rest as in STRICKLAND and PARSONS
(1965 and 1968). Most of the phytoplankton pigment measurements were made with
10 cm path-length, low-volume cuvettes which were specially fabricated for this purpose.
Trichromatic equations designated "P.S." were used for calculating the quantity of
pigments.
II. Results
1. Variability Studies
The following studies were made to explore the statistical significance of the collections
and analyses of seawater samples for phytoplankton pigments.
One quadruplicate and 15 duplicate sets of analyses were made. Combining the
results of these, the maximum coefficients of variation were found to be 12 % for
chlorophyll a and plant carotenoids, and 20% for chlorophyll c. To estimate the extent
of sampling variations of individual collections, 2 sets of 7 replicate collections were
made alternately from surface and 15 m depth within a lO-minute period in April and
October 1968, and 1 set of 4 replicates in April 1969. Each sample of the replicate was
treated separately and absorbancies at 750 nm and 665 nm were measured. The
coefficient of variation for both surface and 15 m samples was about 10 -12 % .
HUMPHREY (1960) observed greater variation in deeper (30 m and 50 m) samples
compared to surface samples. It is concluded that determinations of phytoplankton
pigments by a single analysis using mixed water from 2 or 3 Nansen bottle casts in quick
succession, as was the practice in most of the cruises, would give values within 12%
for chlorophyll a and plant carotenoids, and within 20% for chlorophyll c.
To find out the extent to which the regular station represented the conditions of the
area under study, in October 1968,seven collections were made from 5 stations located in
a grid in areas, 1, 2, 4, 5 and 6. The first, last and middle collections of the series were
taken from the regular station (area 2) so as to eliminate as far as possible, changes in
temperature, chlorophylls, tide etc., with lapse of time. Temperature, salinity and 02
saturation distribution (Fig. 2) show that the general flow in the area was northerly on
that day, and that some run-off from the backwaters reached the regular station, mostly
in the surface layer, and joined the main flow. However, the regular station does
represent the conditions of the area fairly well. The distribution of chlorophyll a also
supports this. Profiles of 3 stations located in areas 1, 2 and 3 were worked out in
December 1968, and January, February and April 1969. On the basis of the distribution
of temperature, salinity and O 2 saturation values, it may be inferred that the 3 stations
are more or less alike and are typically marine during these months. However, the
influence of backwater run-off can also be seen from the chlorophyll a values, which are
higher, and the Secchi disk depths, which are lower, towards the shore.
N.M. SHAH:
2 or 3 times at the same depth and empty the water into a plastic bucket. This water was
then poured into 2 or 3 one-liter polyethylene screw-cap bottles for determining the
amount of plant pigments and nutrients. Samples for pigment determinations were stored
in a cool dark place after the addition of 1 ml magnesium carbonate suspension. On reaching the shore laboratory these samples were filtered through Whatman GFIC glass fibre
filters, which were then stored in a desiccator in a refrigerator. The analytical procedures
for salinity were described in BARNES (1959) and the rest as in STRICKLAND and PARSONS
(1965 and 1968). Most of the phytoplankton pigment measurements were made with
10 cm path-length, low-volume cuvettes which were specially fabricated for this purpose.
Trichromatic equations designated "P.S." were used for calculating the quantity of
pigments.
II. Results
1. Variability Studies
The following studies were made to explore the statistical significance of the collections
and analyses of seawater samples for phytoplankton pigments.
One quadruplicate and 15 duplicate sets of analyses were made. Combining the
results of these, the maximum coefficients of variation were found to be 12 % for
chlorophyll a and plant carotenoids, and 20% for chlorophyll c. To estimate the extent
of sampling variations of individual collections, 2 sets of 7 replicate collections were
made alternately from surface and 15 m depth within a lO-minute period in April and
October 1968, and 1 set of 4 replicates in April 1969. Each sample of the replicate was
treated separately and absorbancies at 750 nm and 665 nm were measured. The
coefficient of variation for both surface and 15 m samples was about 10 -12 % .
HUMPHREY (1960) observed greater variation in deeper (30 m and 50 m) samples
compared to surface samples. It is concluded that determinations of phytoplankton
pigments by a single analysis using mixed water from 2 or 3 Nansen bottle casts in quick
succession, as was the practice in most of the cruises, would give values within 12%
for chlorophyll a and plant carotenoids, and within 20% for chlorophyll c.
To find out the extent to which the regular station represented the conditions of the
area under study, in October 1968,seven collections were made from 5 stations located in
a grid in areas, 1, 2, 4, 5 and 6. The first, last and middle collections of the series were
taken from the regular station (area 2) so as to eliminate as far as possible, changes in
temperature, chlorophylls, tide etc., with lapse of time. Temperature, salinity and 02
saturation distribution (Fig. 2) show that the general flow in the area was northerly on
that day, and that some run-off from the backwaters reached the regular station, mostly
in the surface layer, and joined the main flow. However, the regular station does
represent the conditions of the area fairly well. The distribution of chlorophyll a also
supports this. Profiles of 3 stations located in areas 1, 2 and 3 were worked out in
December 1968, and January, February and April 1969. On the basis of the distribution
of temperature, salinity and O 2 saturation values, it may be inferred that the 3 stations
are more or less alike and are typically marine during these months. However, the
influence of backwater run-off can also be seen from the chlorophyll a values, which are
higher, and the Secchi disk depths, which are lower, towards the shore.