Water dissolves the fertilisers and pesticides that we use on our farms. So some percentage of these substances are washed into the water bodies. Sewage from our towns and cities and the waste from factories are also dumped into rivers or lakes. Specific industries also use water for cooling in various operations and later return this hot water to water-bodies. Another manner in which the temperature of the water in rivers can be affected is when water is released from dams. The water inside the deep reservoir would be colder than the water at the surface which gets heated by the Sun.
All this can affect the life-forms that are found in these water bodies in various ways. It can encourage the growth of some life-forms and harm some other life-forms. This affects the balance between various organisms which had been established in that system.

So ,.we use the term water-pollution to cover the following effects:

  1. The addition of undesirable substances to water -bodies. These substances could be the fertilisers and pesticides used in farming or they
    could be poisonous substances, like mercury salts which are used by paper-industries. These could also be disease-causing organisms, like the
    bacteria which cause cholera.
  2. The removal of desirable substances from water-bodies. Dissolved oxygen is used by the animals and plants that live in water. Any change that reduces the amount of this dissolved oxygen would adversely affect these aquatic organisms. Other nutrients could also be depleted from the water bodies.
  3. A change in temperature. Aquatic organisms are used to a certain range of temperature in the water -body where they live, and a sudden marked
    change in this temperature would be dangerous for them or affect their breeding. The eggs and larvae of various animals are particularly susceptible to temperature changes.

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