Thermal pollution decreases the solubility of oxygen and increases the metabolism of fishes thus changing the ecological balance of the aquatic ecosystems.
Thermal Pollution: Detrimental to Aquatic Ecosystems
-Dr. Arvind Singh
Thermal pollution is the dumping of heat into ecological systems like rivers, lakes, ponds, ocean etc. which degrades the water quality.
Sources of Thermal Pollution:
Electricity generating power plants and industries are the main sources of thermal pollution. Thermal pollution occurs when an industry removes water from the source (e.g. river), uses the water for cooling purposes, and then returns the heated water to the source. Electricity generating power plants heat water to convert it into steam to drive the turbines that generate electricity. For efficient functioning of the steam turbines, the steam is condensed into the water after it leaves the turbines. The condensation is done by taking water from a water body to absorb the heat. This hot water which is at least 15-degree celsius higher than the normal is later discharged back into the water body.
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Effects of Thermal Pollution:
The release of heated water into the aquatic bodies changes the average water temperature and concentration of dissolved oxygen. Elevated temperature decreases the level of dissolved oxygen in the water which is harmful to aquatic animals like fishes, amphibians and other aquatic organisms. High temperature limits oxygen dispersion into deeper waters, leading to anaerobic conditions. It can lead to an increased bacterial population when there is sufficient food supply. Several aquatic species fail to reproduce at elevated temperature. The eggs of trout fail to hatch while salmon does not spawn as higher temperature.The elevated temperature favours the growth of blue-green algae hence the other groups of algae (particularly the green algae and diatoms) are replaced by blue-green algae. The dominance of blue-green algae causes bloom formation which deteriorates the water quality by the release of toxins.
Tropical marine animals are generally unable to withstand a temperature increase of 2-3oC and most sponges, molluscs and crustaceans are eliminated at temperatures above 37oC. This result in a change in the diversity of animals, as only those species can live in warmer water will survive.
Thermal pollution may also increase the metabolic rate of aquatic animals, as enzyme activity, resulting in these organisms consuming more food in a shorter time. An increased metabolic rate may result in fewer resources causing a sharp decrease in a population.
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A large increase in temperature can lead to the denaturation of enzymes. Decreased enzyme activity in aquatic organisms can cause problems such as the inability to break down fats, which leads to malnutrition.
Changes in the environment may also result in migration of organisms to another suitable environment and to in-migration of fishes that normally only live in warmer waters elsewhere. This leads to a competition of fewer resources. The more adapted organisms moving in may have an advantage over organisms that are not used to the warmer temperature consequently one has the problem of compromising food chains of the old and new environments. As result biodiversity may be decreased.
Thermal pollution can also be caused by the release of very cold water from the base of reservoirs into warmer rivers. This affects the fish population and river productivity.
Control of Thermal Pollution:
Thermal pollution can be controlled by cooling ponds, cooling towers and cogeneration. Cooling ponds are man-made water bodies designed for cooling of water by evaporation, convection and radiation. Hot water is pumped into one end of the pond and cooler water is removed from the other end. The heat gets dissipated from the pond into the atmosphere.
Cooling towers are the devices which transfer waste heat to the atmosphere through evaporation and heat transfer. Cooling towers take up less area than the ponds. Here most of the heat transfer occurs through evaporation. The warm water coming from the condenser is sprayed downwards over vertical sheets or baffles where the water flows in thin films. Cool air enters the tower through the water inlet that encircles the base of the tower and rises upwards, causing evaporative cooling. The heat is dissipated into the atmosphere about 100 m above the base of the tower. The cooled water is collected at the floor of the tower and recycled back to the power plant condensers.
Cogeneration is a process where waste heat is recycled for domestic and industrial heating purposes.
Conclusion:
Conclusively it can be said that the thermal pollution deteriorates the water quality by reducing the dissolved oxygen content of the water thus adversely affecting the aquatic flora and fauna. Moreover, thermal pollution favours the growth of blue-green algae resulting in bloom formation which further degrades the water quality by the release of toxins.
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Dr. Arvind Singh is M. Sc. and Ph. D. in Botany with an area of specialization in Ecology. He is a dedicated Researcher having more than four dozens of published research papers in the journals of national and international repute. His main area of research is the Restoration of Mined Lands. However, he has also conducted research on the Vascular Flora of Banaras Hindu University-Main Campus, Varanasi (India). Furthermore, he is also an active science writer having more than 10 dozens of published science articles in different periodicals of national repute. His email address is: 
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