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Exercise 26
Extreme and irregular water-level fluctuations occur in reservoirs often as a result of
flooding, land-use practices not conducive to water retention, channelization of the
main inflows, and large, irregular water withdrawals, commonly for hydropower
generation. Large areas of sediments are alternately inundated and exposed. These
manipulations usually prevent the establishment of productive, stabilizing wetland and
littoral flora. Erosion and resuspension of floodplain sediments augment high loadings
from elsewhere in the drainage basin. Sediments may be shifted between aerobic and
anaerobic conditions, which enhances nutrient release. The reduction or elimination of
wetland and littoral communities around many reservoirs minimizes their extensive
nutrient and physical sieving capacities that function effectively in most natural lake
ecosystems (Wetzel, 1979, 1983, 1990b).
A reservoir can be viewed as a very dynamic lake in which a significant portion of its
volume possesses characteristics of and functions biologically as a river (Wetzel, 1990a).
Often the riverine portion of a reservoir functions analogously to large, turbid rivers in
which turbulence, sediment instability, high turbidity, reduced light availability, and
other characteristics limit photosynthesis despite high nutrient availability. Although
phytoplanktonic productivity of riverine sections of reservoirs can be high per unit
water volume, the limited photic zone reduces areal productivity, as is similarly the
case in large rivers [e.g., Wetzel (1975), Minshall (1978), and Bott (1983)]. As turbidity
is reduced and the depth of the photic zone increases in the transition to the lacustrine
(near the dam) regions of the reservoir, areal primary productivity increases
concomitant with greater light penetration and depth of the trophogenic zone.
Likewise, nutrient limitations can vary throughout a reservoir as losses of nutrients
exceed renewal rates.
Fish biology and productivity are variable in dynamic, constantly changing
reservoir ecosystems. High fish productivity often is observed soon after reservoir
formation: this productivity has been related to the high productivity of benthic fauna
associated with greater habitat variability and refugia among inundated terrestrial
vegetation. In addition, high nutrient and organic matter loadings occur during the
early "trophic surge" period following the damming of a river. Many reservoirs are not
cleared completely of forest and shrub vegetation prior to inundation, particularly in
riverine areas. As these habitats decay and decline, fish must shift to alternative,
predominantly pelagic, food sources. High reservoir turbidity can decrease visual
predation on pelagic zooplanktonic food sources. Fluctuating water levels, high
siltation, and heavy predation often result in high mortality of eggs and larval fish in
littoral areas.
The environmental conditions of reservoir ecosystems tend to have large, rapid, and
erratic fluctuations. Often insufficient time exists for complete population growth and
reproduction to occur before a succeeding major disturbance occurs, e.g., a plume
intrusion and disturbance of stratification patterns following a major rainfall. These
instabilities result in biota that tend to be few and well adapted with broad physiological tolerances (low diversity, less specialization, rapid growth). As in all restrictive,
stressed environments, the productivity of adapted organisms can be high, as high or
greater than in more homeostatic natural lakes.
Determine the thermal characteristics of the water by temperature profiles at
different positions in a local reservoir from the primary river input along a gradient
to the deeper water near the dam. Collect water from the riverine, transitional,
and lacustrine sections of the reservoir. Compare the chemical and biological
characteristics at different depths of these areas with methods given in the previous
exercises.
Exercise 26
Extreme and irregular water-level fluctuations occur in reservoirs often as a result of
flooding, land-use practices not conducive to water retention, channelization of the
main inflows, and large, irregular water withdrawals, commonly for hydropower
generation. Large areas of sediments are alternately inundated and exposed. These
manipulations usually prevent the establishment of productive, stabilizing wetland and
littoral flora. Erosion and resuspension of floodplain sediments augment high loadings
from elsewhere in the drainage basin. Sediments may be shifted between aerobic and
anaerobic conditions, which enhances nutrient release. The reduction or elimination of
wetland and littoral communities around many reservoirs minimizes their extensive
nutrient and physical sieving capacities that function effectively in most natural lake
ecosystems (Wetzel, 1979, 1983, 1990b).
A reservoir can be viewed as a very dynamic lake in which a significant portion of its
volume possesses characteristics of and functions biologically as a river (Wetzel, 1990a).
Often the riverine portion of a reservoir functions analogously to large, turbid rivers in
which turbulence, sediment instability, high turbidity, reduced light availability, and
other characteristics limit photosynthesis despite high nutrient availability. Although
phytoplanktonic productivity of riverine sections of reservoirs can be high per unit
water volume, the limited photic zone reduces areal productivity, as is similarly the
case in large rivers [e.g., Wetzel (1975), Minshall (1978), and Bott (1983)]. As turbidity
is reduced and the depth of the photic zone increases in the transition to the lacustrine
(near the dam) regions of the reservoir, areal primary productivity increases
concomitant with greater light penetration and depth of the trophogenic zone.
Likewise, nutrient limitations can vary throughout a reservoir as losses of nutrients
exceed renewal rates.
Fish biology and productivity are variable in dynamic, constantly changing
reservoir ecosystems. High fish productivity often is observed soon after reservoir
formation: this productivity has been related to the high productivity of benthic fauna
associated with greater habitat variability and refugia among inundated terrestrial
vegetation. In addition, high nutrient and organic matter loadings occur during the
early "trophic surge" period following the damming of a river. Many reservoirs are not
cleared completely of forest and shrub vegetation prior to inundation, particularly in
riverine areas. As these habitats decay and decline, fish must shift to alternative,
predominantly pelagic, food sources. High reservoir turbidity can decrease visual
predation on pelagic zooplanktonic food sources. Fluctuating water levels, high
siltation, and heavy predation often result in high mortality of eggs and larval fish in
littoral areas.
The environmental conditions of reservoir ecosystems tend to have large, rapid, and
erratic fluctuations. Often insufficient time exists for complete population growth and
reproduction to occur before a succeeding major disturbance occurs, e.g., a plume
intrusion and disturbance of stratification patterns following a major rainfall. These
instabilities result in biota that tend to be few and well adapted with broad physiological tolerances (low diversity, less specialization, rapid growth). As in all restrictive,
stressed environments, the productivity of adapted organisms can be high, as high or
greater than in more homeostatic natural lakes.
Determine the thermal characteristics of the water by temperature profiles at
different positions in a local reservoir from the primary river input along a gradient
to the deeper water near the dam. Collect water from the riverine, transitional,
and lacustrine sections of the reservoir. Compare the chemical and biological
characteristics at different depths of these areas with methods given in the previous
exercises.