Experimental Manipulation of Model Ecosystems
309
equal biomass and size distributions. Add equal amounts of water to each microcosm,
so that the water level is near the shoulder of the jar. Each jar holds about 3.51.
Note that adult snails may begin to lay eggs within days after they are brought
into the laboratory (they are hermaphroditic). To avoid unwanted explosions of
snail populations (incubation time is a couple of weeks), use smaller, immature
snails.
Place the microcosms in an area of the laboratory where environmental variation
will be minimal. For example, microcosms of a single experiment should receive the
same sun exposure (same side of the room). Cross seed samples of water and sediment
for the first two weeks to insure replication between the experimental and control
microcosms. Allow the microcosms to equilibrate in the laboratory for three weeks.
During this time, a relatively stable community and metabolic state should be
established. During the following week, make initial measurements on the microcosms
and initiate experimental manipulations. Then monitor the effects of the experimental
manipulations for a period of two weeks.
POTENTIAL EXPERIMENTAL MANIPULATIONS
The research ideas presented below are straightforward possibilities. However, these
ideas should not limit you; they are some of the more practical, but may not be the
"best," nor the only, possibilities for microcosms experiments. These ideas include
experiments that relate to ecological and environmental problems in limnology.
1. Alteration of decomposition rate due to chemical changes in water. Sample
hypotheses:
a. An increased availability of nitrogen and phosphorus stimulates decomposition
of organic matter.
b. The presence oflow pH levels (pH 3 or 4) inhibits bacterial decomposition of dead
organic matter (detritus).
2. Changes in primary productivity and/or species composition due to nutrient
availability, pH, and temperature. Sample hypotheses:
a. An increased availability of nitrogen and phosphorus stimulates levels of
phytoplankton production.
b. Increases in temperature cause a change in both species composition (perhaps
from diatoms to bluegreens) and rates of primary production.
c. High pH levels due to high carbonate alkalinity cause changes in species
composition and rates of production.
d. Low pH levels cause changes in species composition and rates of production.
3. Variation of N/P ratio. Sample hypothesis: A decrease in the N/P ratio causes the
dominant phytoplankton taxa to shift from a diatom/green algal assemblage to a
blue-green (nitrogen-fixing) assemblage.
4. Effects of herbivory on aquatic ecosystems. Sample hypotheses:
a. The presence of herbivores (snails or zooplankton) decreases primary
production.
b. The presence of herbivores alters the size composition of the phytoplankton, such
that small or large phytoplankton are excluded selectively.
5. Effects of oil and/or pesticides on aquatic ecosystems. Sample hypotheses:
a. The presence of oil or pesticides decreases rates of bacterial mineralization.
b. The presence of oil slicks or pesticides decreases/increases rates of primary
production and/or of secondary production.
309
equal biomass and size distributions. Add equal amounts of water to each microcosm,
so that the water level is near the shoulder of the jar. Each jar holds about 3.51.
Note that adult snails may begin to lay eggs within days after they are brought
into the laboratory (they are hermaphroditic). To avoid unwanted explosions of
snail populations (incubation time is a couple of weeks), use smaller, immature
snails.
Place the microcosms in an area of the laboratory where environmental variation
will be minimal. For example, microcosms of a single experiment should receive the
same sun exposure (same side of the room). Cross seed samples of water and sediment
for the first two weeks to insure replication between the experimental and control
microcosms. Allow the microcosms to equilibrate in the laboratory for three weeks.
During this time, a relatively stable community and metabolic state should be
established. During the following week, make initial measurements on the microcosms
and initiate experimental manipulations. Then monitor the effects of the experimental
manipulations for a period of two weeks.
POTENTIAL EXPERIMENTAL MANIPULATIONS
The research ideas presented below are straightforward possibilities. However, these
ideas should not limit you; they are some of the more practical, but may not be the
"best," nor the only, possibilities for microcosms experiments. These ideas include
experiments that relate to ecological and environmental problems in limnology.
1. Alteration of decomposition rate due to chemical changes in water. Sample
hypotheses:
a. An increased availability of nitrogen and phosphorus stimulates decomposition
of organic matter.
b. The presence oflow pH levels (pH 3 or 4) inhibits bacterial decomposition of dead
organic matter (detritus).
2. Changes in primary productivity and/or species composition due to nutrient
availability, pH, and temperature. Sample hypotheses:
a. An increased availability of nitrogen and phosphorus stimulates levels of
phytoplankton production.
b. Increases in temperature cause a change in both species composition (perhaps
from diatoms to bluegreens) and rates of primary production.
c. High pH levels due to high carbonate alkalinity cause changes in species
composition and rates of production.
d. Low pH levels cause changes in species composition and rates of production.
3. Variation of N/P ratio. Sample hypothesis: A decrease in the N/P ratio causes the
dominant phytoplankton taxa to shift from a diatom/green algal assemblage to a
blue-green (nitrogen-fixing) assemblage.
4. Effects of herbivory on aquatic ecosystems. Sample hypotheses:
a. The presence of herbivores (snails or zooplankton) decreases primary
production.
b. The presence of herbivores alters the size composition of the phytoplankton, such
that small or large phytoplankton are excluded selectively.
5. Effects of oil and/or pesticides on aquatic ecosystems. Sample hypotheses:
a. The presence of oil or pesticides decreases rates of bacterial mineralization.
b. The presence of oil slicks or pesticides decreases/increases rates of primary
production and/or of secondary production.