Zooplankton Biomass and its Relation to Particulate Matter
241
appears extraordinarily high considering that the average figure usually adopted for
this ratio is 6 (CUSHING et aI., 1958). The explanation lies in the large proportion of
salps and medusae present in many samples.
IV. Organic Matter
In the absence of an exact determination of the calorimetric value, the amount of
organic matter in a zooplankton sample may serve as a rough measure for the estimation
of energy transfer to the next level in the food chain, small fish, for instance. It indicates
how much of the ingested food the predators are able to digest and to convert into
energy for the increment of body weight etc. In the 91 samples analysed, dry organic
matter ranged from 1.2 to 39.1 mg m -3 or 0.2 to 7.8 g m -2 for the upper 200 m layer.
The percentage of organic matter in the total dry weight has a mean val ue of 77 ± 7%,
ranging from 54% to 93%. As with the ratio of volume to dry weight, this proportion
changes with the composition of the samples.
V. Zooplankton Biomass and Particulate Matter
The dry weight of particulate matter, called seston, was measured by filtering 2 - 4 I
of water. Over 90% of the seston usually consists of plankton and detritus particles in the
size group approximately 1 to 330 fl. As the mesh size of the IOSN is 330 fl, there is
only a slight overlapping of the 2 size fractions. The integrated seston values for the
upper 200 m of the area investigated vary between 33 and 281 mg m- 3 or between
6.6 and 56.2 g m- 2 • In this layer a mean of 25% of the seston may be ascribed to the
biomass of phytoplankton and small zooplankton. On average the amount of seston
exceeds the zooplankton biomass by a factor of 11, with a standard deviation of ± 60%,
which means that the biomass of phytoplankton and small zooplankton is about 3 times
higher than that of the larger zooplankton. However, there are significant deviations
from this ratio in 2 areas. The first area is the section across the Equator in the middle of
the Arabian Sea; it represents the open sea with a small zooplankton stock. The ratio of
seston to zooplankton is 16: 1, this is above average. Because of this low concentration,
the grazing effect of the zooplankton on the seston components does not seem high.
In the second area near the Gulf of Oman the zooplankton is on average 4 times as
abundant, but the increase of seston is only slight, so the ratio becomes 4: 1. It being
March and the end of the NE monsoon period in this region, we probably encountered
a situation where an originally rich phytoplankton stock had been much reduced by the
grazing of a growing zooplankton population. Alternatively the increase of zooplankton
in this area may have been due to fish predation being lower than elsewhere, the fish
having been repelled by the low O2 concentration in the deeper layers.
241
appears extraordinarily high considering that the average figure usually adopted for
this ratio is 6 (CUSHING et aI., 1958). The explanation lies in the large proportion of
salps and medusae present in many samples.
IV. Organic Matter
In the absence of an exact determination of the calorimetric value, the amount of
organic matter in a zooplankton sample may serve as a rough measure for the estimation
of energy transfer to the next level in the food chain, small fish, for instance. It indicates
how much of the ingested food the predators are able to digest and to convert into
energy for the increment of body weight etc. In the 91 samples analysed, dry organic
matter ranged from 1.2 to 39.1 mg m -3 or 0.2 to 7.8 g m -2 for the upper 200 m layer.
The percentage of organic matter in the total dry weight has a mean val ue of 77 ± 7%,
ranging from 54% to 93%. As with the ratio of volume to dry weight, this proportion
changes with the composition of the samples.
V. Zooplankton Biomass and Particulate Matter
The dry weight of particulate matter, called seston, was measured by filtering 2 - 4 I
of water. Over 90% of the seston usually consists of plankton and detritus particles in the
size group approximately 1 to 330 fl. As the mesh size of the IOSN is 330 fl, there is
only a slight overlapping of the 2 size fractions. The integrated seston values for the
upper 200 m of the area investigated vary between 33 and 281 mg m- 3 or between
6.6 and 56.2 g m- 2 • In this layer a mean of 25% of the seston may be ascribed to the
biomass of phytoplankton and small zooplankton. On average the amount of seston
exceeds the zooplankton biomass by a factor of 11, with a standard deviation of ± 60%,
which means that the biomass of phytoplankton and small zooplankton is about 3 times
higher than that of the larger zooplankton. However, there are significant deviations
from this ratio in 2 areas. The first area is the section across the Equator in the middle of
the Arabian Sea; it represents the open sea with a small zooplankton stock. The ratio of
seston to zooplankton is 16: 1, this is above average. Because of this low concentration,
the grazing effect of the zooplankton on the seston components does not seem high.
In the second area near the Gulf of Oman the zooplankton is on average 4 times as
abundant, but the increase of seston is only slight, so the ratio becomes 4: 1. It being
March and the end of the NE monsoon period in this region, we probably encountered
a situation where an originally rich phytoplankton stock had been much reduced by the
grazing of a growing zooplankton population. Alternatively the increase of zooplankton
in this area may have been due to fish predation being lower than elsewhere, the fish
having been repelled by the low O2 concentration in the deeper layers.