EXERCISE 11
Collection l Enumeration l
and Biomass of Zooplankton
Zooplankton usually are distributed throughout
a lake or reservoir. Most forms are motile, and
thus their distribution both vertically and
horizontally may be quite variable. Species have
different habitat preferences that further accentuate spatial and temporal heterogeneity. Zooplankton are major herbivores as well as
important predators in aquatic ecosystems.
Therefore, to understand lake metabolism it is
necessary to evaluate the biomass and the role
of zooplankton in the ecosystem.
Quantitative collection of aquatic organisms
within their natural habitats usually is difficult.
The foremost problem is that only relatively
small subsamples can be obtained from a
population, which is often mobile, changing in
size, and distributed in patches.
If zooplankton were distributed unevenly
within a lake ecosystem, then discrete samples
would grossly overestimate or underestimate the
true population size. Thus characterization of
the distribution of individuals is a prerequisite
for quantitative sampling. Individuals may be
dispersed randomly, contagiously, or uniformly
(or various combinations of these) throughout
the volume of a lake or reservoir (Fig. 11.1).
Errors associated with enumeration may be
categorized into two types: (1) error associated
with counting the individuals in a sample; and
(2) errors in obtaining a representative sample
of the population from a site within a lake, from
the entire lake, or both. The purpose of this
exercise is to examine these problems while, at
the same time, attempting to characterize a
zooplankton population for a lake.
Apparently random or uniform distributions
are rather uncommon in zooplankton populations as distributions tend to be patchy (Tonolli,
1971; Levin and Segal, 1976). However, various
scales of vertical and horizontal distance must
be considered. A chi-square test may be used to
test for contagion. This test is done by
determining the significance of the difference
between the observed distribution of individuals
in unit volumes and a theoretical Poisson
distribution for the same number of individuals
and volumes (see Appendix 2). When the
distribution is contagious, the degree of
contagion can be estimated by the ratio of the
variance to the mean. Ratios of 1 indicate
randomness, > 1 indicate contagion, and < 1
indicate evenness.
The objectives of this exercise are
1. Evaluate counting procedures.
2. Compare various collection devices.
3. Characterize the population size, distribution,
and biomass for a site and for an entire lake.
167
Collection l Enumeration l
and Biomass of Zooplankton
Zooplankton usually are distributed throughout
a lake or reservoir. Most forms are motile, and
thus their distribution both vertically and
horizontally may be quite variable. Species have
different habitat preferences that further accentuate spatial and temporal heterogeneity. Zooplankton are major herbivores as well as
important predators in aquatic ecosystems.
Therefore, to understand lake metabolism it is
necessary to evaluate the biomass and the role
of zooplankton in the ecosystem.
Quantitative collection of aquatic organisms
within their natural habitats usually is difficult.
The foremost problem is that only relatively
small subsamples can be obtained from a
population, which is often mobile, changing in
size, and distributed in patches.
If zooplankton were distributed unevenly
within a lake ecosystem, then discrete samples
would grossly overestimate or underestimate the
true population size. Thus characterization of
the distribution of individuals is a prerequisite
for quantitative sampling. Individuals may be
dispersed randomly, contagiously, or uniformly
(or various combinations of these) throughout
the volume of a lake or reservoir (Fig. 11.1).
Errors associated with enumeration may be
categorized into two types: (1) error associated
with counting the individuals in a sample; and
(2) errors in obtaining a representative sample
of the population from a site within a lake, from
the entire lake, or both. The purpose of this
exercise is to examine these problems while, at
the same time, attempting to characterize a
zooplankton population for a lake.
Apparently random or uniform distributions
are rather uncommon in zooplankton populations as distributions tend to be patchy (Tonolli,
1971; Levin and Segal, 1976). However, various
scales of vertical and horizontal distance must
be considered. A chi-square test may be used to
test for contagion. This test is done by
determining the significance of the difference
between the observed distribution of individuals
in unit volumes and a theoretical Poisson
distribution for the same number of individuals
and volumes (see Appendix 2). When the
distribution is contagious, the degree of
contagion can be estimated by the ratio of the
variance to the mean. Ratios of 1 indicate
randomness, > 1 indicate contagion, and < 1
indicate evenness.
The objectives of this exercise are
1. Evaluate counting procedures.
2. Compare various collection devices.
3. Characterize the population size, distribution,
and biomass for a site and for an entire lake.
167