A discussion of the effect of heated water on aquatic life needs little introduction. During the past several years there has been much interest in this subject within the scientific community and also more recently by the public itself. Federal and state water quality legislation has increased this interest, especially with regard to timetables of progress.

In almost any field of endeavor, an awareness of a need usually evolves before adequate data have been accumulated for making im­portant decisions. Biologists in general and those of the Federal Water Pollution Control Administration in particular now know what answers are needed and how they might be determined. In the interim between the present and some future time when sufficient data are available for making highly accurate and predictive statements, we must speak to some extent in generalities rather than of precise temperature levels. In addi­tion, we must recognize that the effects of heated water on aquatic life are the same whether the source is a nuclear power plant, a fossil fuel plant, or some other process in which water is used as a coolant.

There are two major categories of heated water effects on aquatic life. Direct effects are usually unrelated to another parameter of the aquatic environment. Indirect effects involve a stepwise procedure where­by some other condition, changed by the addition of heat, becomes dele­terious to aquatic life. This distinction will become clear as specific cases are discussed.

The most obvious direct effect is lethality. The lethal temperatures for several species of fish native to and important in Minnesota, for ex­ample, are: walleye, 86° F; yellow perch, 84-88° F; white sucker, 84-85° F; and the fathead minnow, 93° F. No salmon or trout are listed

because there are few cold water environments suitable for these species that are large enough to satisfy the cooling water requirements for any kind of power plant — especially a nuclear plant, which is less efficient than other kinds. Also, there is usually little temperature rise required to replace trout and salmon with walleye, smallmouth bass, and other fish. For perspective, it may be stated that the Columbia River, one of the largest single cold water environments in the country, is only a few degrees from this conversion of fish populations.

A consideration of lethal conditions must include the fact that fish exposed to a lethal temperature do not die immediately. Several horns or several days may be required before stress becomes evident. This situation results in statements that fish were found at a temperature that should have caused mortality on the basis of published data. The presence of fish in heated discharges is often interpreted to mean that these effluents provide desirable or optimum conditions for that species. A little thought would suggest that many organisms, including man, are at times and under cer­tain conditions attracted to environments that are clearly not optimal. This example is one of many that have resulted in confusion and the drawing of conclusions from apparently contradictory events and ob­servations.

The blocking of spawning migrations of fish is another example of direct effects of heat on aquatic life. In this connection, mixing zones as well as larger heated areas must be considered. If a thermal barrier is pro­duced that will prevent one or several fish species from reaching spawning grounds, the eventual result will be the same as if the effect were directly lethal. This adverse effect can be aggravated when the thermal discharge is quickly mixed with cooler stream water, instead of permitting some heat dissipation to the atmosphere before complete mixing occurs. With imme­diate mixing, the temperature of the entire cross-section of the stream becomes higher than if complete mixing occurred more slowly and some distance downstream.

Related to this potential effect on reproduction is a direct effect on the spawning process itself. As mentioned earlier, the temperature that kills 50 per cent of fathead minnows in 96 hours is 93° F. However, it has been shown under laboratory conditions that a lower temperature (86° F) almost completely prevented spawning by this important minnow. More recent research with other aquatic species at the National Water Quality Laboratory has given comparable results, indicating that there are adverse thermal effects on reproduction several degrees below a lethal tempera­ture.

The most important ecological considerations, as always, are the

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most difficult to investigate since they can rarely be studied under satis­factory conditions. These are the potential alterations in species compo­sition and food relationships. If we consider the geographic distribution of important fish species, it is apparent that when an environment is at the southern boundary of a particular geographic distribution, any increase in water temperature would probably result in removal of that species. Con­versely, a species at its northern limit would probably benefit from a rise in temperature. It would be simple to consider desirable species and their distribution if fish were the only factor. However, fish must feed to survive, and if the principal food, or an irreplaceable item in the food chain of fish fry, is removed, the fish species would also be removed even though other­wise thermally adapted. This consideration of the total effects of a change in temperature cannot be completed quickly.

Some investigators have found dense populations of fish near or in power plant effluents and have somewhat illogically concluded that heated water is therefore beneficial. A critical evaluation of such data frequently shows that these dense populations are not of desirable game species but are less desirable, rough fish species. These same data, in addition to in­dicating an abundance of rough fish, also indicate a scarcity or absence of desirable game or commercial fish. This kind of study rarely considers sublethal deleterious conditions such as reduced spawning.

The point in the environment at which the temperature is recorded, although frequently considered a minor problem, may be important. Many times when biological collections are being made, a hand-held thermom­eter is used and records a surface temperature that is actually higher than that in deeper water where fish or other biological forms were collected. This difference could be very significant; the resultant conclusion would have fish existing or thriving at a higher temperature than actually existed where they are found.

The food chain interrelationships of aquatic environments are not static. Everyone knows that at different times of each year there are differ­ent sizes of fish and that feeding habits vary with size, especially with newly hatched fry. Not everyone knows that there are great seasonal variations in the types and abundance of food organisms for these fish. The fry stage of fish is the most critical period because the necessary food for most fry is limited to organisms as small or smaller than immature copepods and cladocerans. Any significant increase in the thermal regime of an aquatic environment could result in the absence of this food when fish fry are totally dependent on them. This absence could be caused by removal of the food species from the system or merely by a change in the time of their availability. Either permanent removal or seasonal displace­ment would place great stress on the fry.

Research at the National Water Quality Laboratory with inverte­brates involving emergence from an aquatic larval form to a flying adult has shown that elevated water temperature causes earlier than normal emergence; this might not be critical except for the fact that these adults thus are ready to enter the aerial environment when air temperatures are still below normal. Under such a condition, many newly emerged adults are unable to leave the water’s surface and complete their life cycle.

It has been stated also that fish grow faster in heated waters. This, like many similar conclusions, is true, within certain limits. At some upper levels of temperature, which are still below lethal temperature, growth is reduced below normal. Even under the conditions which fostered more rapid growth under experimental conditions, we cannot be sure that the increased food required will be available in the aquatic environment. Such results emphasize the need for total environmental considerations and indicate also that subtle changes, many as yet unknown, can have im­portant implications.

One potentially important factor that could be the most critical of all has yet to go much beyond the discussion stage between biologists and engineers. In many plants the cooling water requirements may require up to the total stream flow. Certainly then, the microscopic and slightly larger food organisms would pass through the plant and any associated cooling processes and would be subjected to maximum temperatures which will exceed those of the stream. Even though the obviously desir­able species such as fish are not subjected to this exposure, they are still dependent upon their food chain. The direct, adverse thermal effect on this very important microscopic part of the food chain is measurable; the few times it has been investigated, contradictory results were obtained — ranging from no effect to 95 per cent mortality of the plankton.

The last of the direct effects of heated water on aquatic life is actually a summation of several that are best discussed together. Although up to a certain level, increased temperature can result in increased fish growth, it may also cause proliferation of algal growths that have a deleterious effect on justifiable uses of water other than that for aquatic life. Conse­quently, when excessive algal growth, caused by thermal pollution alone or in combination with municipal or agricultural fertilization of surface waters, results in windrows of rotting algae on shorelines or beaches, the use of these areas for recreational purposes is certainly limited. Not only are swimming and pleasure boating opportunities reduced, but any fisher­man who consistently catches string of algae on hook and line would be upset. This consideration is not marginal —and in fact can be carried further, because decaying algae would create problems of air pollution control.

Another justifiable use of water is for the industrial requirements of nonmunicipal water. Almost any pretreatment of this water for industrial use would have to be increased when heated water causes increased growth of slimes and algae.

A final effect of excessive algal growth is felt by municipal water supplies. Most inhabitants of larger cities are aware of the unpleasantness of drinking highly chlorinated water. The need for chlorination to control bacteria and other undesirable materials in drinking water will increase with increasing temperature, since the need for chlorination is accentuated when the temperature is raised.

The indirect effects of heated water may be considered incidental or secondary in nature, but nonetheless can have a significant impact on aquatic environments. Adverse effects attributed to other factors may be aggravated by heat, or the factor itself may be a result of increased tem­perature. Disease is probably one of the better examples. In relatively unconfined environments, fish diseases rarely reach catastrophic propor­tions, killing large numbers of fish rapidly. Disease organisms in the water and a nominal incidence of infected fish commonly occur. Disease organ­isms multiply more rapidly at elevated temperatures, just as the rates of most biological processes increase with increasing temperature. When temperatures are higher, the virulence of the disease increases, and the resistance of the fish decreases. Together, these may result in a significant loss of fish from disease — but in effect, the mortality is caused by a tem­perature that permits a normally low incidence of disease to become an epidemic. Such situations have occurred “naturally” in the confined en­vironments of hatchery ponds and small farm ponds. Only recently has this condition been shown to be a potential hazard in unconfined, artificially heated waters such as the Columbia River.

Everyone is aware of fish kills which occur from the addition of toxic materials to the aquatic environment. In a few instances the cause is found, and remedial action taken. Again, the addition of toxic materials may seem unrelated to thermal discharge, but further scrutiny uncovers a sig­nificant relationship. The toxicity of most materials — whether pesticides, solvents, heavy metals, or others — increases at higher temperatures. The important point is that water quality criteria can be determined for mate­rials toxic to aquatic life under desirable temperatures, but when tempera­tures are elevated above optimum, toxicity is increased and following these criteria may no longer protect aquatic life.

Again, even though the immediate cause of the damage is a toxic material, the addition of heat to the environment may have been the factor which brought about the undesirable effect. With regard to toxicity, the addition of anti-fouling chemicals in cooling towers and other industrial processes must be considered. The periodic addition of biocides or fungi­cides to the cooling water of power plants and industrial facilities is intend­ed to control and destroy slimes, algae, or scales composed of living cells. Besides the target organisms, these chemicals also, to some extent, affect the food chain organisms. Unfortunately, the frequency of use of these chemicals is directly related to need; since the growth of these target or­ganisms would be greater in warmer weather, the frequency of this addi­tional stress would increase when summer thermal stress is at a maximum. The use of copper sulfate for the control of algae in domestic water supply reservoirs will certainly be increased when raised temperatures increase algal growth. More frequent use of copper could also cause deleterious effects on aquatic environments.

As mentioned earlier, increased temperature causes an increase in physiological activity, which, in turn, increases the oxygen demand of aquatic organisms. This fact is true not only for higher living cells but also for decaying organic matter. Most power plants or other processes requiring cooling water are located near centers of population, where the greatest amounts of organic matter in water also accrue. The effect of these two conditions is at least additive. Either condition alone might be endured by the aquatic fauna, but when both are present, as they fre­quently are, a problem is created.

An example of a computed situation on the White River below Indianapolis can provide a solution to this problem. A sewage treatment plant operates at 92 per cent efficiency at 81° F in order to maintain a minimum dissolved oxygen concentration of 5.0 mg/1. A temperature rise to 86° F will require the treatment to be improved to nearly 95 per cent. This slight increase in required efficiency may seem negligible, but those who are knowledgeable state that this solution would be expensive.

It is not at all uncommon to have significant stretches of streams or rivers below municipalities nearly devoid of oxygen. These undesirable conditions are certainly enhanced by the discharge of heated water in the same general area. Federal and state legislation is resulting in the im­provement of treatment plant efficiencies and an increase in the percentage of the population served by sewer systems. It would be inappropriate to require improved treatment to the point of correcting not only the prob­lem of domestic sewage but that of the power industry as well. The cost would probably be prohibitive, and the treatment would require further improvement every time a heat source is added.

A slight digression here to consider the implications of combined stresses in general would be appropriate. Several specific examples have been mentioned; the list of others would be limited only by one’s imagina­tion. Dr. Auerbach has discussed radiological considerations with regard to the environment. To the stresses he mentioned would have to be added the stresses of elevated temperature as discussed above. The stresses of any other adverse conditions — whether related to toxicants, domestic sewage, or other factors — must be considered and added to the subtotal of artificial stresses. Too often, specialists do not view the environment through wide-angle lenses. They can no longer refuse to accept the just responsibility of providing sound recommendations that will protect the aquatic environment. Continued narrow-mindedness can result only in slower steps toward an ultimate goal that in some instances may no longer exist by the time we have completed the first halting progress.

A single temperature criterion for aquatic life would be simple and efficient. But the complexity and variety of environments in this country and in nearly every state do not permit a single number. Even if all the basic data needed were available, there would have to be a consideration of existing environmental quality. A stretch of river already borderline for the existence of desirable aquatic life (itself a difficult state to agree upon) certainly cannot accept another stress. The “natural” water temperature associated with latitude would also be an important factor.

Economics and public preference about the intended uses of the aquatic environment must be evaluated. Certainly, it is inadequate to consider only the need for electrical power, physical location and cost, and whether or not there is sufficient cooling water available. Existing stresses — low dissolved oxygen, toxic materials, disease, and so forth — must be acknowledged and must influence decision-making. Too often we have been unable to define environmental problems adequately because experts attempt too fine a dissection that does not consider all phases of a problem. There will be redundancy of effort and delays in solution of major pollution problems if experts adhere to narrow-minded problem­solving. The full impact of a pollution source can be determined only by adding up the total effects on many qualities and uses of the aquatic environment. Perhaps no other environmental condition demonstrates such a wide range of effects as does temperature.

For years biologists have been accused of being idealists willing to accept only pristine, unaltered conditions. In most of this country one sees many examples of changed environment caused by impoundments,

fish management, and many other activities. Demands for pristine condi­tions will not result in progressive pollution control. In most instances, a compromise between the optimum and the unacceptable is the only fea­sible means for progress. Recent recommendations for the Ohio River by the Ohio Basin Region of the Federal Water Pollution Control Adminis­tration provide an excellent example of current thought.

Those recommendations designated several specific classes of warm water fishes that required several different temperature criteria. The most restrictive temperature criteria would permit the continued existence of all present fish species. The next set of temperature criteria in terms of quality would not protect the most sensitive species, in this case, the sauger. A third step would eliminate such fish as the smallmouth bass, emerald shiner, and white sucker. Comparable sets of criteria were estab­lished for the cold water fisheries in the Ohio River watershed. Each set of criteria is different and gives different degrees of protection. The final resolution of water quality standards will involve much cooperation with representatives of all directly and indirectly involved parties.

The need for power production is urgent and obvious. Planning or construction delays are unfortunate. An awareness and understanding of each party’s problems and considerations are essential to constructive efforts to provide the necessary electrical power without usurping a basic public right to desirable aquatic life and recreation. Both masters may be served, but not without careful, mutual cooperation.