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Describe the effects of thermal pollution on aquatic ecosystems.

Thermal Pollution

An industrial power plant with three large cooling towers emitting steam and a distinctive dome-shaped structure with red stripes. — ***Over 50% of the water used in the United States is used in power plants or factories to cool industrial processes.***

Thermal pollution is excess heat from cooling machinery. Factories and power plants produce a lot of heat when they operate. They often suck in water from surrounding surface waters to cool their machinery. This water is then released back into the environment after it has absorbed heat from the factory or power plant, which means that it is at least several degrees warmer than the surrounding waters.

Thermal pollution is a type of pollution that doesn’t seem too dangerous when it’s considered at face value. Looking deeper, we see that heated water can directly impact many aspects of the aquatic ecosystem and the organisms residing in those ecosystems. It’s not that the water is simply higher in temperature; it’s that the water is unusually warm where it shouldn’t be warm.

Essential Knowledge

Thermal pollution occurs when heat released into the water produces negative effects on the organisms in that ecosystem.

Impacts of Thermal Pollution

The environmental impact of thermal pollution on aquatic ecosystems: starting with ‘low dissolved oxygen,’ leading to ‘suffocating fish,’ followed by ‘decreased fish populations,’ and resulting in ‘altered food webs.’ — ***Thermal pollution can cause a cascade of effects in aquatic ecosystems.***

Many aquatic organisms depend on the ambient temperature of water for their own body’s heat and metabolism. The enzyme system of organisms may only operate within a very narrow range of temperatures. These stenothermic organisms (only capable of living or surviving within a narrow temperature range) can be killed by sudden temperature changes that are beyond the tolerance limits of their metabolic systems. When the temperature of water suddenly increases, many organisms can go into thermal shock, a condition in which the organism dies or becomes severely incapacitated to the point of being unable to carry on basic life processes.

In addition to thermal shock, many aquatic organisms are impacted by thermal pollution through the lowering of dissolved oxygen levels. Water has an inverse relationship between dissolved oxygen and temperature. As temperature increases, dissolved oxygen levels decrease. The graph below shows water temperature (blue line) and dissolved oxygen (red line) over the course of one year in the San Francisco Bay estuary.

A graph shows fluctuations in both temperature and oxygen levels throughout the year in San Francisco Bay — ***As temperature rises, dissolved oxygen declines and vice versa.***

If the dissolved oxygen levels decline too much or too quickly, massive fish kills can occur. Species such as trout and salmon are especially impacted by thermal pollution. These species require high levels of dissolved oxygen in water and if the oxygen level drops too low, they will die.

Coral is another aquatic organism that is susceptible to thermal pollution. Coral reefs that have been impacted by thermal pollution will display coral bleaching, a condition in which the coral expels the algae living in their tissues causing the coral to bleach, making it more susceptible to disease and death. The coral can survive if the temperature of the water returns to normal quickly. If the water does not return to normal, the coral will die.

Various types of plankton and invertebrates such as mussels can be impacted as well.

Essential Knowledge

Variations in water temperature affect the concentration of dissolved oxygen because warm water does not contain as much oxygen as cold water.

Mitigating Thermal Pollution

Spray ponds for cooling warm water — ***Spray ponds try to cool the water before it is released back into the environment.***

To try to lessen the impact of heated water on aquatic ecosystems, many power plants and manufacturing plants employ strategies to cool the heated water before releasing it back into the local waterway.

Cooling towers allow the heat from heated water to radiate into the atmosphere prior to the water being released back into the environment.
Cooling ponds are man-made bodies of water designed for cooling heated water and supplying it to nearby industrial facilities like power plants or refineries. They work by allowing warm water from the facility to flow into the pond, where it cools through evaporation, convection, and radiation before being recirculated back into the system.
Spray ponds are a type of reservoir where warmed water is cooled for reuse by spraying it into the air in a mist form, enhancing evaporation and heat exchange with the cooler ambient air. This method is efficient in terms of space and cooling performance compared to a traditional cooling pond.

Some power plants use a "fish-chase" procedure in which the water temperature is increased gradually, instead of rapidly, to drive fish away from the intake pipes before the temperature reaches lethal levels.

The production of heated water from industrial and power processes is difficult to avoid. Research continues on better ways to mitigate thermal pollution. Of course, using less electricity lowers the demand on power plants and lowers the occurrence of thermal pollution so think of conserving electricity as one way that we can all help in this area.

AP Exam Tip

You need to understand why thermal pollution is problematic for aquatic organisms.

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