Special Lake Types
333
Questions
1. How might reservoirs be managed to minimize the extreme variability in physical
and chemical parameters?
2. What means could be invoked to increase the exposure time of incoming water to
the attached micro biota of wetland and littoral communities?
3. What is meant by underflow, overflow, and interflow of influent water? [See Wetzel
(1983).] How could distribution of this inflowing water affect the productivity of
phytoplankton? Bacteria? Zooplankton? Turbidity?
4. How would the removal of effluent water from the hypolimnion rather than from
the surface affect the physical, chemical, and biological characteristics of a reservoir?
Of the river below the dam that is receiving the effluent water?
5. If dissolved organic compounds adsorb to the surfaces of silt and other inorganic
particulate matter suspended in water, could these compounds supplement the
nutrition of zooplankton if ingested?
6. How could rates of siltation/sedimentation is a reservoir be reduced?
MEROMICTIC LAKES
Meromictic lakes are rather rare but, nevertheless, are distributed throughout the
world (Walker and Likens, 1975). Because they lack complete circulation, meromictic
lakes provide unique opportunities for interesting limnological studies. Typically, the
deepest water contains no dissolved oxygen, maintains a relatively constant temperature, and is relatively poorly illuminated. Sediments below this zone are relatively
undisturbed and, therefore, may offer an extraordinarily detailed record of the history
of the lake and its surroundings [see Exercise 27; Gorham and Sanger (1972) and
Frey (1955)]. Often a "plate," or dense accumulation, of photosynthetic bacteria forms
in the chemolimnion of meromictic lakes. In Fayetteville Green Lake in New York
State, the density of green-sulfur bacteria (Chlorobium phaeobacteroides) at a depth of
18 to 20m can be sufficiently high to make the water appear light magenta in color
(Culver and Brunskill, 1969).
The monimolimnion of a meromictic lake represents a harsh environment for most
organisms, because of the absence of dissolved oxygen and the presence of reducing
substances such as H 2 S and NH 3 . However, there are certain organisms that can
survive in this environment, at least temporarily. These forms include anaerobic
bacteria, certain dipteran larvae (e.g., Chaoborus), gastrotrichs, and some oligochaetes.
Collect water from the monimolimnion and measure the dissolved oxygen and
hydrogen sulfide content [see Exercise 6 and Cline (1969)]. Be careful that the water
sample is not contaminated with atmospheric oxygen during collection and analysis for
dissolved oxygen. The human nose is very sensitive to hydrogen sulfide. You should be
able to smell it as it vaporizes from the samples. Bubbles may form in the bottles after
the water sample has been acidified during the analysis for dissolved oxygen. What do
you think these bubbles are composed of?
Observe carefully for the presence of any living organism in the samples of water
from the monimolimnion. How could you test for the presence of anaerobic bacteria in
these samples? See, for example, Jackson (1967).
Collect some Chaoborus larvae and add them to sealed samples of monimolimnetic
water. How long do they survive? What adaptations do you think these organisms have
for surviving in such a harsh environment? See, for example, Brand (1946).
333
Questions
1. How might reservoirs be managed to minimize the extreme variability in physical
and chemical parameters?
2. What means could be invoked to increase the exposure time of incoming water to
the attached micro biota of wetland and littoral communities?
3. What is meant by underflow, overflow, and interflow of influent water? [See Wetzel
(1983).] How could distribution of this inflowing water affect the productivity of
phytoplankton? Bacteria? Zooplankton? Turbidity?
4. How would the removal of effluent water from the hypolimnion rather than from
the surface affect the physical, chemical, and biological characteristics of a reservoir?
Of the river below the dam that is receiving the effluent water?
5. If dissolved organic compounds adsorb to the surfaces of silt and other inorganic
particulate matter suspended in water, could these compounds supplement the
nutrition of zooplankton if ingested?
6. How could rates of siltation/sedimentation is a reservoir be reduced?
MEROMICTIC LAKES
Meromictic lakes are rather rare but, nevertheless, are distributed throughout the
world (Walker and Likens, 1975). Because they lack complete circulation, meromictic
lakes provide unique opportunities for interesting limnological studies. Typically, the
deepest water contains no dissolved oxygen, maintains a relatively constant temperature, and is relatively poorly illuminated. Sediments below this zone are relatively
undisturbed and, therefore, may offer an extraordinarily detailed record of the history
of the lake and its surroundings [see Exercise 27; Gorham and Sanger (1972) and
Frey (1955)]. Often a "plate," or dense accumulation, of photosynthetic bacteria forms
in the chemolimnion of meromictic lakes. In Fayetteville Green Lake in New York
State, the density of green-sulfur bacteria (Chlorobium phaeobacteroides) at a depth of
18 to 20m can be sufficiently high to make the water appear light magenta in color
(Culver and Brunskill, 1969).
The monimolimnion of a meromictic lake represents a harsh environment for most
organisms, because of the absence of dissolved oxygen and the presence of reducing
substances such as H 2 S and NH 3 . However, there are certain organisms that can
survive in this environment, at least temporarily. These forms include anaerobic
bacteria, certain dipteran larvae (e.g., Chaoborus), gastrotrichs, and some oligochaetes.
Collect water from the monimolimnion and measure the dissolved oxygen and
hydrogen sulfide content [see Exercise 6 and Cline (1969)]. Be careful that the water
sample is not contaminated with atmospheric oxygen during collection and analysis for
dissolved oxygen. The human nose is very sensitive to hydrogen sulfide. You should be
able to smell it as it vaporizes from the samples. Bubbles may form in the bottles after
the water sample has been acidified during the analysis for dissolved oxygen. What do
you think these bubbles are composed of?
Observe carefully for the presence of any living organism in the samples of water
from the monimolimnion. How could you test for the presence of anaerobic bacteria in
these samples? See, for example, Jackson (1967).
Collect some Chaoborus larvae and add them to sealed samples of monimolimnetic
water. How long do they survive? What adaptations do you think these organisms have
for surviving in such a harsh environment? See, for example, Brand (1946).