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Thermal pollution is a temperature change in natural water bodies caused by human influence. The main cause of thermal pollution is the use of water as a coolant, especially in power plants. Water used as a coolant is returned to the natural environment at a higher temperature. Increases in water temperature can alter aquatic organisms by (a) decreasing oxygen supply, (b) killing fish juveniles which are vulnerable to small increases in temperature, and (c) affecting ecosystem composition.

Thermal pollution typically decreases the level of dissolved oxygen in the water. The decrease in levels of dissolved oxygen can harm aquatic animals such as fish, amphibians and copepods. Thermal pollution may also increase the metabolic rate of aquatic animals, as enzyme activity, meaning that these organisms will consume more food in a shorter time than if their environment was not changed. An increased metabolic rate may result in food source shortages, causing a sharp decrease in a population. Changes in the environment may also result in a migration of organisms to another, more suitable environment, and to in-migration of organisms that normally only live in cooler waters elsewhere. This leads to competition for lesser resources; the more adapted organisms moving in may have an advantage over organisms that are not used to the warmer temperatures. As a result one has the problem of compromising food chains of the old and new environments. Biodiversity can be decreased as a result.

Fish kill in receiving waters

It is known that temperature changes of even one to two degrees Celsius can cause significant changes in organism metabolism and in adverse cellular biology effects. Principal adverse changes can include rendering cell walls less permeable to necessary osmosis, coagulation of cell proteins, and alteration of enzyme metabolism. These cellular level effects can adversely affect mortality and reproduction.

Primary producers are affected by thermal pollution because higher water temperature increases plant growth rates, resulting in a shorter lifespan and species overpopulation. This can cause an algae bloom which reduces the oxygen levels in the water. The higher plant density leads to an increased plant respiration rate because the reduced light intensity decreases photosynthesis. This is similar to the eutrophication that occurs when watercourses are polluted with leached agricultural inorganic fertilizers.

A large increase in temperature can lead to the denaturing of life-supporting enzymes by breaking down hydrogen- and disulphide bonds within the quaternary structure of the enzymes. Decreased enzyme activity in aquatic organisms can cause problems such as the inability to break down lipids, which leads to malnutrition.

In limited cases, thermal pollution has little deleterious effect and may even lead to improved function of the receiving aquatic ecosystem. This phenomenon is seen especially in seasonal waters and is known as thermal enrichment. An extreme case is derived from the aggregational habits of the manatee, which often uses power plant discharge sites during winter. Projections suggest that manatee populations would decline upon the removal of these discharges.

The added heat lowers the dissolved oxygen content and may cause serious problems for the plants and animals living there. In extreme cases, major fish kills can result. Thermal pollution may also increase the metabolic rate of aquatic animals, as enzyme activity, meaning that these organisms will consume more food in a shorter time than if their environment was not changed.

In the 1970s there was considerable activity from scientists in quantifying effects of thermal pollution. Hydrologists, physicists, meteorologists, and computer scientists combined their skills in one of the first interdisciplinary pursuits of the modern environmental science era. First came the application of gaussian function dispersal modeling that forecasts how a thermal plume is formed from a thermal point source and predicts the distribution of aquatic temperatures. The ultimate model was developed by the U.S. Environmental Protection Agency introducing the statistical variations in meteorology to predict the resulting plume from a thermal outfall.