Heated waters are less dense and hence are lighter. They float over the water surface and cover it causing stratification.

This often insulates, effectively, the lower layers of water from the atmosphere above. Exchange of gases from water surface stops. At high temperatures the solubility of oxygen in water also decreases while rapid metabolic activity requires more oxygen.

Species intolerant to higher temperatures migrate to lower layers which are cooler and so also are many plank tonic forms which settle down from lighter water above to denser water below. All this cumulatively results in creation of oxygen deficit in both upper and lower layers.

If the heated waters contain some organic matter microbial activity is stepped up for its degradation. Various toxic chemicals such as organic acids, hydrogen supplied, ammonia etc. accumulate and damage the plant and animal life which is already under duress due to the depletion of oxygen.

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Solubility’s of many organic and inorganic chemicals are higher in heated waters than in cooler waters, while the solubility’s of gases decrease with rise in temperatures. This results in dissolution of more salts while the amount of dissolved gases diminishes. This causes an altogether changed chemical picture which either kills or forces some organisms to migrate to more suitable habitats.

The elimination of some species or their migration to other places leaves the tolerant forms to grow and multiply without much competition. The diversity in species composition^ lost. Number of species present in the system decreases while the number of individuals of the few tolerant species which are present rises abruptly. This often causes disappearance of economically desirable forms. The entire biological spectrum is, therefore, changed as a consequence of discharge of heated waters or effluents from various thermal power plants and industries.