Temperature mediates how animals respond to other environmental factors.
Temperature mediates how animals respond to other environmental factors.

The vast majority of marine animals cannot regulate their internal body temperature directly. The temperature of surrounding water controls everything from how much oxygen they can absorb and how fast they can digest food to when and whether they become sexually mature. Thermal pollution also alters plant and microbial populations and interferes with the feeding, migratory and breeding behavior of marine mammals, reptiles and birds.

Getting Hot in Here

Thermal pollution is causing an unnatural change in temperature. The most common and well-studied form in water is effluent from power-plant cooling systems, but it may also be caused or enhanced by erosion, loss of forest cover, or eutrophication -- anything that changes a body of water's capacity to absorb light and heat.

Dependence and Tolerance

Ocean invertebrates, fish and marine reptiles are obligate poikilotherms -- their body temperatures depend entirely on the surrounding water. Each species is adapted to a single range of temperatures and many pass through several different life stages, each with an individual range of tolerance. The ocean's vast populations of microbes, fungi and sea plants also rely on narrow temperature ranges for optimal growth. Thermal pollution often temporarily increases aquatic plant populations. Other life forms, including microbes and animals, move into these regions to exploit the higher oxygen levels, but when photosynthesis stops at night or upon the death of plants, dissolved oxygen levels plummet, leading to massive animal die-offs. Excess heat can also cause unnaturally large microbial blooms, which kill animals by depleting local oxygen or producing toxins.

Direct Effects

The most obvious effects of thermal pollution are direct -- hot and cold kills. These massive fish die-offs result from sudden temperature change. They're common at point-source pollution transition zones, such as power plant discharge where a river meets the ocean. Animals migrating through these zones, such as spawning salmon, are particularly vulnerable. Hot kills happen when they enter the effluent, and cold kills may happen when survivors pop back out into normal-temperature water.

Altered Movement

Unnatural warmth and cold can also delay and redirect migration, influence when and whether breeding occurs, and decrease survival of young in marine mammal, reptile and bird populations. Heat pollution that leads to an overabundance of organisms in a region can draw in excessive numbers of creatures that feed on them, while depletion of prey populations by heat stress forces their predators to encroach on regions outside their normal range. Both situations lead to unnatural competition between and within species.

Indirect Damage

Effects of small temperature increases can be just as devastating as dramatic hot-kills. Warmer water contains less dissolved oxygen. More energy spent in respiration means less available for other functions. Heat-stressed organisms are more vulnerable to changes in light and salinity and to predators and parasites. Outside of ideal temperatures, digestive enzymes begin to fail, altering organisms' rate of growth and ultimate size. Altered temperature can cause the need for more food and farther travel farther to find it, placing stress on surrounding populations as hungry interlopers encroach. Excess heat can confuse animals' ability to determine season and even direction. If they manage to overcome these challenges, animals in breeding condition are even more susceptible to heat stress than non-breeders, which directly impacts the number of viable young they can produce.