It is the living organisms within the aquatic body which are adversely affected by the additional i heat. Most of the biota in aquatic system consists of poikilotherms whose body temperature is regulated by the temperatures of the outside environment. The adverse effects of heated discharges in an aquatic system may be summarized as follows:

(1) During their passage through the cooling systems, which consist of a series of channels and pipes, small organisms, fish larvae, and plankton have to face abrupt changes in temperatures. Sensitive ones are killed due to the temperature shock.

(2) Mechanical impingement on the side walls and screens, placed to check entry of large organisms and plants which could choke the pipes, may cause mechanical injury and damage to the living organisms. Small fishes drawn in by pumps along with water to be passed through the condenser cooling system may get suffocated to death by pressure exerted on their gill chambers.

(3) The chemicals introduced in these waters to remove slime from the cooling systems can cause toxicity and death to the living organisms.

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(4) At higher temperatures cellular fats may melt, coagulation of cell-proteins may take place and permeability of cell membrane may be affected. The activity of most of the enzymes within the cells of the aquatic organisms is speeded up till a certain optimum level is reached. Any further rise in temperatures causes disruption of the enzymatic machinery.

(5) Metabolic reactions in a biological system are, therefore, speeded up at higher temperatures which cause an increased demand of oxygen. Increased irritability and swimming speed is observed in fishes. The temperatures in which most of the fishes can survive ranges below 35°C. Temperatures beyond 35°C may be lethal to many fish-species. Higher temperatures also cause increased activity which exhausts many aquatic organisms and shortens their life-span. Reproductive processes may be triggered earlier causing pre-mature deposition of eggs in some aquatic animals.

(7) The solubility of oxygen dissolved in water decreases with rise in temperatures. At 5°C about 12.8 mg per litre of oxygen is needed to saturate the aquatic system. However at 35°C only 7.1 mg per litre saturates the water, whereas at 40°C only 6.6 mg per litre of oxygen is sufficient to saturate the system.

So waters at 40°C cannot hold more than 6.6 mg per litre of oxygen. In an aquatic system saturated with oxygen at lower temperatures, rise in water temperature causes the excess oxygen to form bubbles and leave the system. Gas bubbles may be formed within the bodies of aquatic organisms.

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For most of the fishes these bubbles are injurious as they cling to the gills and block oxygen intake. This has virtually the effect of forcing the fishes to breathe in air which they cannot. The phenomenon is known as gas-bubble disease which chokes the fishes to death.

(8) At higher temperatures pathogenic forms become more active while plants and animals become an easy victim to various types of infections.