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Water temperature changes resulting in degraded water quality

Thermal pollution, sometimes called "thermal enrichment", is the degradation of water quality by any process that changes ambient water temperature. Thermal pollution is the rise or drop in the temperature of a natural body of water caused by human influence. Thermal pollution, unlike chemical pollution, results in a change in the physical properties of water. A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers. Urban runoff—stormwater discharged to surface waters from rooftops, roads, and parking lots—and reservoirs can also be a source of thermal pollution. Thermal pollution can also be caused by the release of very cold water from the base of reservoirs into warmer rivers. When water used as a coolant is returned to the natural environment at a higher temperature, the sudden change in temperature decreases oxygen supply and affects ecosystem composition. Fish and other organisms adapted to particular temperature range can be killed by an abrupt change in water temperature (either a rapid increase or decrease) known as "thermal shock". Warm coolant water can also have long term effects on water temperature, increasing the overall temperature of water bodies, including deep water. Seasonality effects how these temperature increases are distributed throughout the water column. Elevated water temperatures decrease oxygen levels, which can kill fish and alter food chain composition, reduce species biodiversity, and foster invasion by new thermophilic species.

During warm weather, urban runoff can have significant thermal impacts on small streams. As storm water passes over hot rooftops, parking lots, roads and sidewalks it absorbs some of the heat, an effect of the urban heat island. Storm water management facilities that absorb runoff or direct it into groundwater, such as bioretention systems and infiltration basins, reduce these thermal effects by allowing the water more time to release excess heat before entering the aquatic environment. These related systems for managing runoff are components of an expanding urban design approach commonly called green infrastructure. Retention basins (stormwater ponds) tend to be less effective at reducing runoff temperature, as the water may be heated by the sun before being discharged to a receiving stream.

Elevated temperature typically decreases the level of dissolved oxygen and of water, as gases are less soluble in hotter liquids. This can harm aquatic animals such as fish, amphibians and other aquatic organisms. 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 than if their environment were not changed. Temperature changes of even one to two degrees Celsius can cause significant changes in organism metabolism and other 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. 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. Increased water temperature can also increase the solubility and kinetics of metals, which can increase the uptake of heavy metals by aquatic organisms. This can lead to toxic outcomes for these species, as well as build up of heavy metals in higher trophic levels in the food chain, increasing human exposures via dietary ingestion.

Releases of unnaturally cold water from reservoirs can dramatically change the fish and macroinvertebrate fauna of rivers, and reduce river productivity. In Australia, where many rivers have warmer temperature regimes, native fish species have been eliminated, and macroinvertebrate fauna have been drastically altered. Survival rates of fish have dropped up to 75% due to cold water releases.

Water warming effects, as opposed to water cooling effects, have been the most studied with regard to biogeochemical effects. Much of this research is on the long term effects of nuclear power plants on lakes after a nuclear power plant has been removed. Overall, there is support for thermal pollution leading to an increase in water temperatures. When power plants are active, short term water temperature increases are correlated with electrical needs, with more coolant released during the winter months. Water warming has also been seen to persist in systems for long periods of time, even after plants have been removed. Similar to effects seen in aquatic systems due to climatic warming of water, thermal pollution has also been seen to increase surface temperatures in the summer. This can create surface water temperatures that lead to releases of warm air into the atmosphere, increasing air temperature. Many ecological effects will be compounded by climate change as well, as ambient temperature rises in water bodies. Rivers present a unique problem with thermal pollution. As water temperatures are elevated upstream, power plants downstream receive warmer waters. Evidence of this effect has been seen along the Mississippi River, as power plants are forced to use warmer waters as their coolants. This reduces the efficiency of the plants and forces the plants to use more water and produce more thermal pollution.