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Exercise 23
4. Interpreting the results of the chemos tat:
a. Compare and contrast the parameters before and after enrichment.
b. What can be concluded about the relative dependence upon nitrogen and/or
phosphorus for the enrichment of this system (i.e., which seemed to have the
greatest effect on, or which was required the most by, the phytoplankton)? What
can be said about the role of potassium?
c. What can you conclude about the water originally used to operate this
chemostat? Discuss the use of the chemostat in estimating the potential of a
natural system (lake or stream) for studies of the eutrophication process.
5. Relating the chemostat to a natural system:
a. Considering the parameters measured in the chemos tat, how would you collect
the same information in a lake? Would you sample more frequently and at more
sites? If so, why? [See deNoyelles and O'Brien (1974}.J
b. Considering the estimates that you made for growth rates and nutrient uptake
rates, how would you make the same calculations from a lake study? Would there
be different or additional parameters that must be measured, considering the
relative complexity of the lake as compared to the chemostat? Discuss in detail
any additonal information that would have to be collected. Consider some
particular reasons why these estimations would be more difficult to make for a
lake study.
A MICROCOSM APPROACH
Aquatic microcosms may be established by bringing together collections of biota,
sediment, and water. Microcosms may be established conveniently and economically
in 1-gal glass jars and maintained in the laboratory. Ideally, it would be most
informative to compare ecosystems constructed or collected from different kinds of
environments (i.e., oligotrophic and eutrophic). It is critically important that all
replicate microcosms for a given experiment be established at the same time from the
same samples of water and sediment. Why?
At least four major habitats may be recognized in these model ecosystems: (1) the
free-water column containing planktonic organisms; (2) the water surface, where
duckweed (Lemna) and watermeal (W oljfia), both angiosperms, may grow in abundance, (3) the glass wall of the container where algae and bacteria may attach and grow;
and (4) the sediments, containing an abundance of microorganisms, immature insects,
and other invertebrates, and a reservoir of nutrients. Eutrophic microcosms may
develop a large snail population. Planktonic cladocera and copepods may be replaced
by a stable population of seed shrimp (ostracods).
The kinds of microcosms ultimately established will depend on the nature of the
experimental design. It may be appropriate to exclude snails, zooplankton, or
sediments from some microcosms. The composition of the microcosms can be modified
according to the experimental design, but steps necessary to insure replication must be
taken.
Collect large volumes of sediments and overlying water from a nearby lake or pond.
Pass the sediment through one or two coarse-mesh sieves to remove snails and large
animals as well as larger stones and organic debris. Collect this sieved sediment into
another bucket. Sort the animals both by type and by size. After thoroughly mixing the
sediment, add equal aliquots (about 300ml) to the microcosm jars. Stir to allow even
settling. Some of the extra animals may be maintained in stock microcosms for
addition to the experimental microcosms at appropriate times. If they were added, use
Exercise 23
4. Interpreting the results of the chemos tat:
a. Compare and contrast the parameters before and after enrichment.
b. What can be concluded about the relative dependence upon nitrogen and/or
phosphorus for the enrichment of this system (i.e., which seemed to have the
greatest effect on, or which was required the most by, the phytoplankton)? What
can be said about the role of potassium?
c. What can you conclude about the water originally used to operate this
chemostat? Discuss the use of the chemostat in estimating the potential of a
natural system (lake or stream) for studies of the eutrophication process.
5. Relating the chemostat to a natural system:
a. Considering the parameters measured in the chemos tat, how would you collect
the same information in a lake? Would you sample more frequently and at more
sites? If so, why? [See deNoyelles and O'Brien (1974}.J
b. Considering the estimates that you made for growth rates and nutrient uptake
rates, how would you make the same calculations from a lake study? Would there
be different or additional parameters that must be measured, considering the
relative complexity of the lake as compared to the chemostat? Discuss in detail
any additonal information that would have to be collected. Consider some
particular reasons why these estimations would be more difficult to make for a
lake study.
A MICROCOSM APPROACH
Aquatic microcosms may be established by bringing together collections of biota,
sediment, and water. Microcosms may be established conveniently and economically
in 1-gal glass jars and maintained in the laboratory. Ideally, it would be most
informative to compare ecosystems constructed or collected from different kinds of
environments (i.e., oligotrophic and eutrophic). It is critically important that all
replicate microcosms for a given experiment be established at the same time from the
same samples of water and sediment. Why?
At least four major habitats may be recognized in these model ecosystems: (1) the
free-water column containing planktonic organisms; (2) the water surface, where
duckweed (Lemna) and watermeal (W oljfia), both angiosperms, may grow in abundance, (3) the glass wall of the container where algae and bacteria may attach and grow;
and (4) the sediments, containing an abundance of microorganisms, immature insects,
and other invertebrates, and a reservoir of nutrients. Eutrophic microcosms may
develop a large snail population. Planktonic cladocera and copepods may be replaced
by a stable population of seed shrimp (ostracods).
The kinds of microcosms ultimately established will depend on the nature of the
experimental design. It may be appropriate to exclude snails, zooplankton, or
sediments from some microcosms. The composition of the microcosms can be modified
according to the experimental design, but steps necessary to insure replication must be
taken.
Collect large volumes of sediments and overlying water from a nearby lake or pond.
Pass the sediment through one or two coarse-mesh sieves to remove snails and large
animals as well as larger stones and organic debris. Collect this sieved sediment into
another bucket. Sort the animals both by type and by size. After thoroughly mixing the
sediment, add equal aliquots (about 300ml) to the microcosm jars. Stir to allow even
settling. Some of the extra animals may be maintained in stock microcosms for
addition to the experimental microcosms at appropriate times. If they were added, use