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Exercise 23
ANALYTICAL PROCEDURES
Because of the relatively small volumes involved, results must be based on in situ
measurements or on analyses of very small samples from a microcosm.
To determine the numbers of bacterioplankton, phytoplankton, or zooplankton,
collect 5 ml of water from the middle of the water column with a large-bore pipet.
Transfer this sample to a small bottle and add 1 drop of Lugol's solution to preserve.
Follow the enumeration procedures described in Exercises 10, 11, and 19.
Periphyton may be removed from the surface of the sediment or glass walls with a
Pasteur pipet. Take no more than 5 mljsample. Store and preserve as was done with the
plankton samples. Analyze according to the procedures given in Exercise 22.
Follow the procedures given in Exercises 2 (light and temperature), 7 (inorganic
nutrients), 8 (pH and alkalinity), 9 (dissolved organic matter), and 10 (plant pigments)
for the various physical and chemical analyses. Use the smallest sample possible for
chemical analyses of the water column and/or sediment. In establishing the sampling
interval, bear in mind the total amount of water or sediment to be removed from the
microcosm during the course of the experiment.
In situ procedures have been used successfully in studies of ecosystem metabolism in
microcosms [e.g., Beyers (1963, 1965)]. Changes in pH or in dissolved oxygen
concentration may be recorded continuously, and without consumptive sampling from
the microcosms, with electronic probes (see Exercises 6 and 7). For example, pH has
been used to measure the change in total dissolved carbon dioxide concentration in an
application of the carbon dioxide diurnal rate of change curve method in microcosms
(Beyers, 1963, 1965). The microcosms may be sealed to minimize the diffusion of gases
across the air-water interface. Whole microcosms may be darkened to estimate
ecosystem respiration in short-term experiments.
These ecosystems are not static. Changes occur rapidly, especially during the initial
phases of manipulation.
Questions
1. Compare and contrast your microcosm and the pond(s) from which it was
established, in terms of physical, chemical, and biological parameters (e.g.,
stratification, food webs, and so on). What are the properties peculiar to the
microcosm?
2. Can you draw any conclusions from your data about ponds or ecosystems in
general? Has the microcosm approach provided "relevant" insight concerning
aquatic ecosystems, or is the observed behavior of the microcosm simply an
anomaly?
3. What do you view as being the major advantages and disadvantages of working with
microcosms? [See Beyers (1964).]
4. What is the difference between a control and a reference jar in microcosm studies?
[See Likens (1985).]
References
Barlow, J.P., W.R. Schaffner, F. deNoyelles, Jr., and B. Peterson. 1973. Continuous flow nutrient
bioassays with natural phytoplankton populations. pp. 299-319. In: Bioassay Techniques and
Environmental Chemistry. Ann Arbor Science Pub!., Ann Arbor, MI.
Exercise 23
ANALYTICAL PROCEDURES
Because of the relatively small volumes involved, results must be based on in situ
measurements or on analyses of very small samples from a microcosm.
To determine the numbers of bacterioplankton, phytoplankton, or zooplankton,
collect 5 ml of water from the middle of the water column with a large-bore pipet.
Transfer this sample to a small bottle and add 1 drop of Lugol's solution to preserve.
Follow the enumeration procedures described in Exercises 10, 11, and 19.
Periphyton may be removed from the surface of the sediment or glass walls with a
Pasteur pipet. Take no more than 5 mljsample. Store and preserve as was done with the
plankton samples. Analyze according to the procedures given in Exercise 22.
Follow the procedures given in Exercises 2 (light and temperature), 7 (inorganic
nutrients), 8 (pH and alkalinity), 9 (dissolved organic matter), and 10 (plant pigments)
for the various physical and chemical analyses. Use the smallest sample possible for
chemical analyses of the water column and/or sediment. In establishing the sampling
interval, bear in mind the total amount of water or sediment to be removed from the
microcosm during the course of the experiment.
In situ procedures have been used successfully in studies of ecosystem metabolism in
microcosms [e.g., Beyers (1963, 1965)]. Changes in pH or in dissolved oxygen
concentration may be recorded continuously, and without consumptive sampling from
the microcosms, with electronic probes (see Exercises 6 and 7). For example, pH has
been used to measure the change in total dissolved carbon dioxide concentration in an
application of the carbon dioxide diurnal rate of change curve method in microcosms
(Beyers, 1963, 1965). The microcosms may be sealed to minimize the diffusion of gases
across the air-water interface. Whole microcosms may be darkened to estimate
ecosystem respiration in short-term experiments.
These ecosystems are not static. Changes occur rapidly, especially during the initial
phases of manipulation.
Questions
1. Compare and contrast your microcosm and the pond(s) from which it was
established, in terms of physical, chemical, and biological parameters (e.g.,
stratification, food webs, and so on). What are the properties peculiar to the
microcosm?
2. Can you draw any conclusions from your data about ponds or ecosystems in
general? Has the microcosm approach provided "relevant" insight concerning
aquatic ecosystems, or is the observed behavior of the microcosm simply an
anomaly?
3. What do you view as being the major advantages and disadvantages of working with
microcosms? [See Beyers (1964).]
4. What is the difference between a control and a reference jar in microcosm studies?
[See Likens (1985).]
References
Barlow, J.P., W.R. Schaffner, F. deNoyelles, Jr., and B. Peterson. 1973. Continuous flow nutrient
bioassays with natural phytoplankton populations. pp. 299-319. In: Bioassay Techniques and
Environmental Chemistry. Ann Arbor Science Pub!., Ann Arbor, MI.