EXERCISE 12
Benthic Fauna of Lakes
The animals living on and in the sediments and
large plants of lakes are usually highly diverse.
Much emphasis in the study of benthic fauna
has been given to the immature stages of insects
that often make up the dominant part of the
total animal biomass of these habitats. Nearly
all insect orders are represented. Some orders of
insects are entirely aquatic; others inhabit fresh
waters only during certain life stages. Segmented
worms (oligochaetes and leeches), microcrustacea
(ostracods), and macrocrustacea (mysids, isopods,
decapods, and amphipods) often form major
components of benthic fauna of fresh waters.
The sediment composition and characteristics
of the water adjacent to the sediments are also
highly variable. Attached microflora (bacteria,
fungi, and algae) may occur in great abundance
on sediments. These organisms and associated
detrital organic matter often provide the
predominant energy sources for the benthic
fauna. Consequently, a variety of types offeeding
and of reproductive dynamics occur among the
benthic fauna. It is, therefore, essential in
limnological analyses to obtain reliable quantitative estimates of the sizes and distributions of
populations constituting these communities.
Quantitative estimations of benthic fauna
require effective sampling procedures, separation
of the organisms from the substratum, indentification, and evaluation of biomass of species
and of their life history stages. Methods to
accomplish these tasks are not totally satisfactory,
and the taxonomy of many immature stages of
benthic invertebrates is difficult and incompletely
understood. Nonetheless, in spite of these
problems, often it is important to analyze the
response of population growth and survival and
physiological characteristics (e.g., respiration,
excretion, and assimilation) of the benthic fauna
to environmental variations, such as temperature,
oxygen concentrations, and food quantity and
quality.
The living biomass (g/m2) of a population of
animals at an instant in time and in a given
habitat represents the net result of reproduction
and growth and the opposing processes of loss
(respiration, predation, mortality, emigration,
and so on). Although the measurement of biomass
gives an estimate of the extent of population
development, from these data alone nothing can
be said about the growth and reproduction of
the organisms. More information is needed to
evaluate production rates (g/m2/time). This
information can be obtained only by detailed
analyses of reproductive and physiological
characteristics or survivorship and growth.
There are some general direct correlations
between the overall productivity of fresh waters
and benthic animal productivity [cf., summary
of Wetzel (1983)]. Moreover, some general
insight into benthic faunal composition and
distribution in relation to lake characteristics can
be obtained from simple quantitative analyses.
The following exercise is directed toward the
understanding of problems of sampling, sorting,
and quantitative estimates of population size.
Obviously, much more detailed investigations
would be needed to determine secondary
179
Benthic Fauna of Lakes
The animals living on and in the sediments and
large plants of lakes are usually highly diverse.
Much emphasis in the study of benthic fauna
has been given to the immature stages of insects
that often make up the dominant part of the
total animal biomass of these habitats. Nearly
all insect orders are represented. Some orders of
insects are entirely aquatic; others inhabit fresh
waters only during certain life stages. Segmented
worms (oligochaetes and leeches), microcrustacea
(ostracods), and macrocrustacea (mysids, isopods,
decapods, and amphipods) often form major
components of benthic fauna of fresh waters.
The sediment composition and characteristics
of the water adjacent to the sediments are also
highly variable. Attached microflora (bacteria,
fungi, and algae) may occur in great abundance
on sediments. These organisms and associated
detrital organic matter often provide the
predominant energy sources for the benthic
fauna. Consequently, a variety of types offeeding
and of reproductive dynamics occur among the
benthic fauna. It is, therefore, essential in
limnological analyses to obtain reliable quantitative estimates of the sizes and distributions of
populations constituting these communities.
Quantitative estimations of benthic fauna
require effective sampling procedures, separation
of the organisms from the substratum, indentification, and evaluation of biomass of species
and of their life history stages. Methods to
accomplish these tasks are not totally satisfactory,
and the taxonomy of many immature stages of
benthic invertebrates is difficult and incompletely
understood. Nonetheless, in spite of these
problems, often it is important to analyze the
response of population growth and survival and
physiological characteristics (e.g., respiration,
excretion, and assimilation) of the benthic fauna
to environmental variations, such as temperature,
oxygen concentrations, and food quantity and
quality.
The living biomass (g/m2) of a population of
animals at an instant in time and in a given
habitat represents the net result of reproduction
and growth and the opposing processes of loss
(respiration, predation, mortality, emigration,
and so on). Although the measurement of biomass
gives an estimate of the extent of population
development, from these data alone nothing can
be said about the growth and reproduction of
the organisms. More information is needed to
evaluate production rates (g/m2/time). This
information can be obtained only by detailed
analyses of reproductive and physiological
characteristics or survivorship and growth.
There are some general direct correlations
between the overall productivity of fresh waters
and benthic animal productivity [cf., summary
of Wetzel (1983)]. Moreover, some general
insight into benthic faunal composition and
distribution in relation to lake characteristics can
be obtained from simple quantitative analyses.
The following exercise is directed toward the
understanding of problems of sampling, sorting,
and quantitative estimates of population size.
Obviously, much more detailed investigations
would be needed to determine secondary
179