photosynthesis may exceed the increase in respiration; however, at higher temperatures, the respiration exceeds the increase of photosynthesis. The respiration
increases to a maximum rate at the optimum temperature and at an only slightly
higher temperature reaches a critical or fatal temperature at which the organism is
destroyed. Since individuals and different species have different optimum and
critical temperatures, it may be seen that a change in temperature may seriously
change the character of the biological systems present at different temperatures. As a
rule of thumb, in the growth range, respiration doubles for each 10
C rise. The
increase in the rate of respiration is correlated with the increased rate of metabolism.
Also, the rate of reproduction is increased. This normally is recorded as a shortening
of the time for the reproduction cycle to be completed. Thus, more organisms are
produced that respire and metabolize at a higher rate, thereby utilizing the diminished oxygen reservoir in a stream at a higher rate. This, of course, will reduce the
BOD rapidly, but with a commensurate reduction in DO. As all these factors
increase, the total activity may be limited by the availability of food.
Some organisms have a temperature sensitivity in which they prefer warm or cold
water. In general in cold streams, fish swim toward warm water, whereas in warm
streams, fish swim toward cold water. However, there are also some fish that are
quite temperature insensitive. This can result in an accumulation of fish and other
swimming aquatic organisms at either a warm spot or cold spot in a stream. Here
they may utilize all the food available, ultimately resulting in their demise.
The increase in temperature also adds to the synergistic effects of other harmful
substances, such as toxicity, disease, and so on. In general, organisms have a lower
tolerance to these impacts at a higher temperature. In some instances, for example,
disease may be controlled by a lower temperature. Thus, the higher temperature
merely adds to other antagonisms. Another important factor is the rate of change of
temperature. Many aquatic organisms can tolerate a slow change in temperature, but
not a rapid one. If there is a discharge of hot water into a stream, there can be a
narrow zone of mixing resulting in a rapid change in temperature within a short
distance.
Another problem regarding a rapid change in temperature is that power plants
must shut down at certain times for maintenance and/or accidents. If the thermal
discharge from a power plant has been keeping a stream warm through the winter, a
sudden stoppage of the power generation would allow the stream to cool very
rapidly. This would impact a significant thermal shock on the aquatic organisms.
Therefore, most regulations call for prohibiting routine shutdowns during the winter
months. Obviously, accidental shutdowns may still occur. Thus there are two main
concerns: (a) the impact of the higher temperature and (b) the impact of sudden
changes of temperatures because of operation of the thermal generating system.
There are many combinations of impacts related to increasing aquatic temperature. For example, certain organisms will reproduce only at or above a certain
temperature. If the body of water is maintained above this temperature, they will
reproduce for a longer period of time or constantly. However, their development is
dependent upon the food available to them. If the food on which they rely is not
available because of other temperature impacts, then they cannot survive. If they
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
285
increases to a maximum rate at the optimum temperature and at an only slightly
higher temperature reaches a critical or fatal temperature at which the organism is
destroyed. Since individuals and different species have different optimum and
critical temperatures, it may be seen that a change in temperature may seriously
change the character of the biological systems present at different temperatures. As a
rule of thumb, in the growth range, respiration doubles for each 10
C rise. The
increase in the rate of respiration is correlated with the increased rate of metabolism.
Also, the rate of reproduction is increased. This normally is recorded as a shortening
of the time for the reproduction cycle to be completed. Thus, more organisms are
produced that respire and metabolize at a higher rate, thereby utilizing the diminished oxygen reservoir in a stream at a higher rate. This, of course, will reduce the
BOD rapidly, but with a commensurate reduction in DO. As all these factors
increase, the total activity may be limited by the availability of food.
Some organisms have a temperature sensitivity in which they prefer warm or cold
water. In general in cold streams, fish swim toward warm water, whereas in warm
streams, fish swim toward cold water. However, there are also some fish that are
quite temperature insensitive. This can result in an accumulation of fish and other
swimming aquatic organisms at either a warm spot or cold spot in a stream. Here
they may utilize all the food available, ultimately resulting in their demise.
The increase in temperature also adds to the synergistic effects of other harmful
substances, such as toxicity, disease, and so on. In general, organisms have a lower
tolerance to these impacts at a higher temperature. In some instances, for example,
disease may be controlled by a lower temperature. Thus, the higher temperature
merely adds to other antagonisms. Another important factor is the rate of change of
temperature. Many aquatic organisms can tolerate a slow change in temperature, but
not a rapid one. If there is a discharge of hot water into a stream, there can be a
narrow zone of mixing resulting in a rapid change in temperature within a short
distance.
Another problem regarding a rapid change in temperature is that power plants
must shut down at certain times for maintenance and/or accidents. If the thermal
discharge from a power plant has been keeping a stream warm through the winter, a
sudden stoppage of the power generation would allow the stream to cool very
rapidly. This would impact a significant thermal shock on the aquatic organisms.
Therefore, most regulations call for prohibiting routine shutdowns during the winter
months. Obviously, accidental shutdowns may still occur. Thus there are two main
concerns: (a) the impact of the higher temperature and (b) the impact of sudden
changes of temperatures because of operation of the thermal generating system.
There are many combinations of impacts related to increasing aquatic temperature. For example, certain organisms will reproduce only at or above a certain
temperature. If the body of water is maintained above this temperature, they will
reproduce for a longer period of time or constantly. However, their development is
dependent upon the food available to them. If the food on which they rely is not
available because of other temperature impacts, then they cannot survive. If they
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
285