340 ESCH AND HAZEN 



given functional attribute (more commonly). In the latter course, the 

 new steady state may be a close facsimile of the original, but again 

 this depends on the nature, magnitude, and duration of the 

 perturbing force. For example, let us consider the consequences of 

 cultural eutrophication and its reversal. It is well known that 

 enrichment causes an oligotrophic ecosystem to undergo eutrophica- 

 tion and that this process can be reversed if the source of enrichment 

 is diverted or stopped, in which case the original "pristine" character 

 of the system will be restored. Many of the functional characteristics 

 of the original system may be restored, but the system still will not 

 be the same. The explanation is simple when we consider that some 

 species in the system will be unable to withstand conditions of stress 

 associated with eutrophication and will become locally extinct. If 

 this occurs, many of the species interactions that were characteristic 

 of the original ecosystem are not reestablished in the new one. The 

 result is a new steady state, or equilibrium, even though functionally 

 the two oligotrophic systems are similar in many ways. 



Thermal Ecology and Stress 



Temperature is a universal influence on the normal physiological, 

 metabolic, and behavioral processes of individual plants and animals, 

 especially those living as heterotherms in aquatic systems. Most 

 species of plants and animals have evolved a strategy not only of 

 coping with normal environmental temperatures but also of exploit- 

 ing temperature variations to such an extent that they are either 

 partially or totally dependent on annual, seasonal, or diel fluctua- 

 tions. With the construction of electricity-generating facilities pro- 

 ducing heated effluent, many species of plants and animals that have 

 evolved successful strategies for dealing with normal temperature 

 cycles are now faced with levels of temperature that can be 

 considered excessive. In responding to these elevated temperatures, 

 each species has several alternative courses of action: (1) It can 

 migrate away from or toward the high temperatures; (2) it can 

 become locally extinct or flourish; or (3) it can cope until forced to 

 follow one of the first two courses of action or until such time as it 

 can evolve a new strategy to permit it to survive. Under any of these 

 situations, the individual is subjected to stress and will behave in a 

 manner consistent with Fig. 3. 



Stress may be manifested in a number of insidious ways, none 

 more so perhaps than increased susceptibility to disease of both 

 organic and external origin. Our own specific interest over the years 

 has been the relation between thermal effluent and parasitism and 

 the various ways in which host— parasite relationships can be 



