for a continuous performance of phytoplanktonic organisms. Seasonal, diurnal, and geographical variations in temperature and the occurrence of organisms under different, even extreme, conditions indicate that life, in general, is possible in a very wide temperature range. In hot springs, for example, algae may be abundant at temperatures above 70 C (Brock, 1967; Kempner, 1963) and temperature limits for bacterial existence go even higher (Brock, 1967; 1970). Not all forms of life are, of course, adaptable to elevated temperatures. For emotional, aesthetic, or economic reasons, some organisms are particularly desirable in certain habitats. Conservation projects designed to protect endangered species will undoubtedly enlarge in area and in number of organisms included in such programs. Global aspects of thermal addition are kept out of this discussion. As long as the global demand for energy increases, the only avenues open for dealing with calefaction are the location of heat producers and the problem of what to do with calefaction locally. Available appraisals of the overall biological effects of man-made increase in temperature range from harmful to innocuous and even beneficial (Gilluly, 1970). The local situation depends largely on the temperature of the natural water and on reaction of specific organisms to increase in temperature above that level. A prognosis of possible effects, of the anticipated thermal addition, must involve a dissection of ecological communities into their components and an assessment of temperature effects on each component. Nothing in this discussion should be construed as to condone, least of all advocate, an immediate increase in temperature of natural waters as affected by the operation of industrial installations or power plants. Ecological situations are always complex. Temperature is only one, though very important, environmental factor. Change in temperature will probably involve changes in other parameters affecting the existence and competing power of various organisms. Though higher forms of life may be of particular concern under specific conditions, one must realize that all these organisms even if not affected directly by the increase in temperature will ultimately depend on their existence on the lowest link in the food chain operational in natural waters that is, on phytoplankton. The presence or abundance of the lower link in this chain will not ensure the presence or abundance of higher forms of life, but its absence will certainly preclude the existence of higher forms of life.