174
Exercise 11
EVALUATION OF BIOMASS
The mass of an organism can be estimated from its volume or it can be determined
directly by weighing (Table 11.3). Many workers estimate average dry weight biomass
of zooplankters from estimates of average length and regressions of length versus
weight. This subject, developed by Dumont et al. (1975), has been reviewed critically
and in great detail by McCauley (1984). Volumetric determinations are difficult to
make for small organisms of irregular shape. Dimensions may be measured (see
Exericse 10) and volume calculated, assuming the organism approximates a cube,
sphere, cylinder, or some other geometrical form [cf., R uttner-Kolisko, 1977 a, 1977b].
Alternatively, volume displacement of water may be used as an approximation. Since
most zooplankton organisms are very small, it would be necessary to add numerous
individuals to the water volume of a small volumetric chamber with fine graduations
to obtain a reliable estimate of the mean volume for the organisms.
Before obtaining the weight of zooplankton they should be thoroughly dried.
Freeze-dry or dry at 60°C until a constant weight is obtained (usually one to two
days). Transfer from the drying oven to a desiccator and allow to cool for at least
1 h before weighing. When available, freeze-drying should be used to dry the zooplankton. Drafts and humidity fluctuations while weighing small organisms can
produce serious errors in these measurements; appropriate precautions should be
taken.
In most cases it will be necessary to weigh several individuals (a few to several
hundred) at once and determine the mean weight. This measurement can be done
by adding several animals to tared small aluminum foil pans or to a small coverglass.
A sensitive electro balance (e.g., Cahn Gram electro balance) is useful for determining
very small weights.
Volume and weight of zooplanktonic species, may vary by an order of magnitude
or more depending on habitat and season (Dumont et aI., 1975). For example, a
relatively small change in length or width may result in a large change in volume for an
individual.
For further details on sampling, enumeration, mass, and chemical determinations
of zooplankton, see Edmondson and Winberg (1971), Winberg (1972), Tranter and
Fraser (1968), de Bernardi (1984), and McCauley (1984).
EXERCISES
OPTION 1.
This exercise is designed for two or more field trips to a small lake or reservoir.
A. Comparison of Collection Apparatus
1. Using at least one of the metered-type collectors (e.g., Clarke-Bumpus sampler) and one of
the volumetric-type collectors (e.g., Schindler trap or Likens-Gilbert technique), obtain three
replicate samples for each device from a depth of 3 m near the center of the lake.
The Clarke-Bumpus sampler is towed from a boat at a constant speed at a specified
depth. The speed should be maintained between 0.9 and 3.2 km/h so that the meter undergoes
40 to 140re'!/min (McNaught, 1971). This rate can be determined from the total number
Exercise 11
EVALUATION OF BIOMASS
The mass of an organism can be estimated from its volume or it can be determined
directly by weighing (Table 11.3). Many workers estimate average dry weight biomass
of zooplankters from estimates of average length and regressions of length versus
weight. This subject, developed by Dumont et al. (1975), has been reviewed critically
and in great detail by McCauley (1984). Volumetric determinations are difficult to
make for small organisms of irregular shape. Dimensions may be measured (see
Exericse 10) and volume calculated, assuming the organism approximates a cube,
sphere, cylinder, or some other geometrical form [cf., R uttner-Kolisko, 1977 a, 1977b].
Alternatively, volume displacement of water may be used as an approximation. Since
most zooplankton organisms are very small, it would be necessary to add numerous
individuals to the water volume of a small volumetric chamber with fine graduations
to obtain a reliable estimate of the mean volume for the organisms.
Before obtaining the weight of zooplankton they should be thoroughly dried.
Freeze-dry or dry at 60°C until a constant weight is obtained (usually one to two
days). Transfer from the drying oven to a desiccator and allow to cool for at least
1 h before weighing. When available, freeze-drying should be used to dry the zooplankton. Drafts and humidity fluctuations while weighing small organisms can
produce serious errors in these measurements; appropriate precautions should be
taken.
In most cases it will be necessary to weigh several individuals (a few to several
hundred) at once and determine the mean weight. This measurement can be done
by adding several animals to tared small aluminum foil pans or to a small coverglass.
A sensitive electro balance (e.g., Cahn Gram electro balance) is useful for determining
very small weights.
Volume and weight of zooplanktonic species, may vary by an order of magnitude
or more depending on habitat and season (Dumont et aI., 1975). For example, a
relatively small change in length or width may result in a large change in volume for an
individual.
For further details on sampling, enumeration, mass, and chemical determinations
of zooplankton, see Edmondson and Winberg (1971), Winberg (1972), Tranter and
Fraser (1968), de Bernardi (1984), and McCauley (1984).
EXERCISES
OPTION 1.
This exercise is designed for two or more field trips to a small lake or reservoir.
A. Comparison of Collection Apparatus
1. Using at least one of the metered-type collectors (e.g., Clarke-Bumpus sampler) and one of
the volumetric-type collectors (e.g., Schindler trap or Likens-Gilbert technique), obtain three
replicate samples for each device from a depth of 3 m near the center of the lake.
The Clarke-Bumpus sampler is towed from a boat at a constant speed at a specified
depth. The speed should be maintained between 0.9 and 3.2 km/h so that the meter undergoes
40 to 140re'!/min (McNaught, 1971). This rate can be determined from the total number