For instance, discharge of warm water may cause fish eggs to hatch so early in the spring that the natural food organisms needed by the young fingerlings would not be available. Also trout eggs commonly fail to hatch in water that is too warm, and salmon often fails to spawn in such conditions. Warm water also holds less oxygen and hence its BOD rises. Water at 0°C has approximately 14 ppm of dissolved oxygen whereas that at 20°C has only 9 ppm. High water temperature can adversely affect the daily and seasonal behaviour and metabolic responses and rates of aquatic organisms.

However, proper manipulation and release in a controlled manner can often turn the liability into asset, as it has been shown that waste heat from steam electric stations can be used to stimulate the growth of certain aquatic animals and fish (Ansell, 1962; Shelbourne, 1964). Mihursky and associates (see Mihursky, 1967) have proposed the controlled use of waste heat to eradicate fouling organisms, stimulate sewage decomposition, stimu­late fish growth and check the growth of rooted aquatic vegetation. The biological effects of thermal pollution are mainly local in scale, although a series of effluents discharged on the same watershed could have a much wider effect. A survey of the main effects indicates that they are site- specific (Dickson, 1975), depending on the organisms involved and the physical features of the area. In organically rich waters, chronic exposures to temperatures elevated by a few degrees often cause an increase in productiv­ity. There is also a shift toward more heat-tolerant species and this is often accompanied by a decrease in species diversity.

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These general conclusions are, of course, subject to modification by secondary physical effects. For instance, a thermal plume may have the beneficial effect of preventing the usual winter oxygen depletion from taking place by keeping a lake open. The composition of aquatic communities depends largely on the temper­ature characteristics of their environment. Organisms have upper and lower thermal tolerances and their best growth and activity occurs at the optimum temperature. Individual animals tend to congregate at preferred temperatures in thermal gradients, and have temperature limitations for migration, spawn­ing, and egg incubation. Since each species has its own particular thermal optimum, the balance of interactions between competitors can shift with change in temperature, favouring those species which are more tolerant at the new temperature. This can often lead to replacement of desirable species by an undesirable one. A thermal plume often exerts significant influence on a water body depending on the size of the plume, on the temperature increment it produces, and on its location with respect to the aquatic biota.

Heated water produces more serious effects in those cases where an important biological resource is diminished. Fish and other swimming animals can move freely in and out of a thermal plume but sessile, benthic invertebrates cannot do so and hence in their case a plume can exert a much more profound effect. Discharge of heated effluents into a water body also affects species composition and production of submerged macrophytes, and Goriiam (1975) has reported that the rapidly spreading weed Elodea canadensis has become the dominant species at thermal outfalls and heated areas in Alberta (Canada) and that in such areas this weed has tended to replace species of Myriophyllum, Chara and Potamogeton (Allen and Gorham, 1973). Discharge of waste heat in rivers can not only affect the indigenous fish population but may also affect their migration or spawning. The integrity of a fish population is regulated by balance between recruitment and mortality. The chief form of recruitment involves reproduction-recruitment of fry. Those factors which inhibit reproductive success of fish can exert a highly adverse effect on the population.

Effluents from thermal power plants some time affect the fish population by inhibiting the spawning run which blocks the access of fish to spawning grounds. In a study of the effects of thermal effluents on marine organisms in the USA, Warinner and Brehmer (1966) found that carbon assimilation by natural phytoplanktons was affected by an artificially induced increase in water temperature. Small increases in temperature caused by discharge of hot water into a stream were found to enhance primary production in winter months. However, higher temperature rises (exceeding 5-6°C) resulted in decreased carbon assimilation and primary production. The greater the temperature rise caused by thermal pollution the greater was the depression in primary produc­tion. Many workers have recently described the effects of thermal pollution on microbes, plankton, plants and animals (sec Cravens, 1981).

Vannote and Sweeney (1980) showed that adult body size and fecundity of several aquatic insects depend mainly on thermal conditions during larval growth. They noted that when growing in non-optimal temperature range, the larvae developed into smaller-sized and less fecund adults. Thermophilic algae, cyanobacteria and fungi have been isolated from many hot habitats in the past few years (Ellis, 1980). According to Thorhaug (1980), the effect of the heat from thermal effluents in the tropics is much more severe than from those in low temperature areas. In tropical zones, the summer ambients and maxima for physiological processes lie within a few degrees of their upper lethal limit. Heat effects in these zones are generally most severe on lower food chain organisms such as seagrasses and corals. Thermal effects on consumers have been studied at the levels of repro­duction, development, morphology, feeding and behaviour or activity (Cra­vens, 1981). In general, gonad development and spawning depends strongly on temperature.

The development of embryos and larvae occurs within a narrow temperature range and temperature also regulates the rate of develop­ment of many consumers. May (1980) estimated the grazing rate of a rotifer (Notholca sp.) on the diatom Asterionella formosa. Whereas at 10°C, the rotifer consumed an average of 11 cells per hour, at 6°C, it consumed only 3 cells/hr Besides its direct effects, temperature sometimes controls the rates and extents of action of other stresses or pollutants on aquatic organisms. When exposed to a thermal gradient, motile organisms congregate in a narrow range of temperatures.

This kind of behaviour is called behavioural thermoregula­tion. Different organisms and even different developmental stages of the same species can have different preference temperatures. For instance, the prefer­ence temperatures of adult snails (Goniobasis spp.) lie around 10-25°C, whereas starfish (Asterias forbesii) prefers 20-25°C, American eel (Anguilla rostrata) prefers 16-17°C and Nassarius trivittatus prefers 30-35°C. The preference temperature for adult Poecilia reticulata is 24.5°C whereas that for its juvenile stage is 28.2°C (Johansen and Cross, 1980).