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The change in temperature impacts organisms by
(a) decreasing oxygen supply, and
(b) affecting ecosystem composition.
Warm water contains less oxygen. Elevated temperature typically decreases the level of dissolved oxygen (DO) in water. So there is decrease in rate of decomposition of organic matter. Green algae are replaced by less desirable blue green algae. Many animals fail to multiply.
It also increases the metabolic rate of aquatic animals results in consumption of more food in a shorter time than if their environment were 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 fishes that normally only live in warmer waters elsewhere. This leads to competition for fewer resources; the more adapted organisms moving in may have an advantage over organisms that are not used to the warmer temperature. As a result one has the problem of compromising food chains of the old and new environments. Biodiversity can be decreased as a result.
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.
Primary producers are affected by warm water because higher water temperature increases plant growth rates, resulting in a shorter life span and species overpopulation. This can cause an algae bloom which reduces the oxygen levels in the water. The higher plant density results in reduced light intensity, decreases photosynthesis and leads to an increased plant respiration rate. This is similar to the eutrophication.
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.