hanges in temperature affect aquatic life. Temperature determines which organisms will thrive and which will diminish in numbers and size. For each organism there is a thermal death point. Also there is a range of temperature of that produces optimal abundance. The effects of temperature upon life of a cold blooded or poikilotherm are profound. Poikilothermic animals, such as fish, are those whose body temperatures follow closely the temperature of their medium.
These animals have coped with temperature problems in different ways. Not only the organism survival, but growth and reproduction of each organism have critical temperature ranges. Each organism must be favored by the proper temperature if the individual or its population are going to survive. For instance, temperature influences enzymatic reactions through hormonal and nervous control to digestion, from respiration and osmoregulation to all aspects of an organism’s performance and behavior.
High and low temperatures that are lethal to individual organism of a species determines the distribution and abundance it’s populations. However, more often the distribution and abundance of populations is determined by less than lethal temperatures interacting with other environmental factors that either tend to favor or not to favor reproduction and growth.
Increased water temperature is an important consideration when toxic substances are present in water. Many substances (i.e. cyanides, phenol, xylene, zinc) exhibit increased toxicity at elevated temperatures. These toxicities and other physiological interactions are also influenced by temperature acclimation or history of the species.
We can gain a clearer understanding of these interactions through consideration of lethal temperature levels. In relation to the survival of individual organisms, the upper and lower lethal temperatures define the total temperature gradient. Within this temperature gradient, there is a range as shown in Figure 4 in which the species can function at or near optimum. In this range, growth and reproduction temperature requirements are met and the species will be found in greatest abundance. Outside the optimum range, there are zones of physiological stress. In these zones, organisms become infrequent because activities are limited more by temperatures that produce discomfort or stress. The period of time an organism can live under physiological stress is a function of how far the temperature is from the lethal level.
Most changes in water temperature as a result of land use activity generally trend upward. An exception is the release of cold bottom water from stratified artificial impoundments that may alter the flora and fauna for many miles downstream from a dam. Most other activities generally raise the temperature of receiving waters with the following effects:
(a) Higher temperatures diminish the solubility of dissolved oxygen and thus decrease the availability of this essential gas.
(b) Elevated temperatures increase the metabolism, respiration and oxygen demand of fish and other aquatic life, approximately doubling the respiration for a 10° C. rise in temperature. Hence the demand for oxygen is increased under conditions where oxygen supply is lowered.
(c) The solubility of many toxic substances is increased as well as intensified as the temperature rises.
(d) Higher temperatures militate against desirable fish life by favoring the growth of sewage fungus and the putrification of sludge deposits, and finally
(e) even with adequate dissolved oxygen, there is a maximum temperature that each species of fish or other organism can tolerate. Higher temperatures produce death. The maximum temperatures that adult fish can tolerate vary with the species of fish, prior acclimatization, oxygen availability and the synergistic effects of other pollutants.
Median Tolerance Limits (MTL)have been reported are shown in the following table. Species have been arranged in the order of heat tolerance. As shown by this table, Goldfish, Bass and Carp are relatively tolerant of high temperatures, whereas Trout and Salmon are more sensitive. These temperatures, however, apply to adult fish. For spawning and hatching of eggs, much lower temperatures are required. Many species spawn only above or below certain temperatures. Several species of fish and their spawning temperature ranges are shown in Figure (Cooper).