Theory:

Any form of undesirable substance present in water is known as water pollution.
The pollutants may be organic, inorganic, biological, radiological, and heat, degrading water quality and becoming unfit for use.
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Causes of water pollution:
The sources of pollutants can be natural and human-made.
The natural sources of pollutants are due to soil erosion, decomposition, etc.
 
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The man-made pollutants are due to modernisation. The causes of water pollution can be from different sources such as industrial effluents, surface runoff, oil spills, thermal pollution, radioactive waste, refuse dumps, mining activities etc.
 
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These are caused due to improper handling of wastewater and the carelessness of humans towards their environment, which directly affects the aquatic ecosystem.
 
The consequence of water pollution:
 
The different water sources (inland water pollution, underground water, marine or ocean pollution) are polluted by various factors that may be harmful to many aquatic species (plankton and fish etc.) because of reduced dissolved oxygen levels.
 
Annelid worm and some larvae species survive in highly polluted water with low dissolved oxygen levels, which indicates contaminated water. Hot, polluted water from industries also reduces dissolved oxygen.
 
The gradual increase in nutrient enrichment due to ageing promotes the growth of algae, aquatic plants and fauna. This process is known as eutrophication.
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This eutrophic water body increases plants' growth, killing animal life by depriving it of oxygen. Eutrophication leads to the loss of coral leaves, changing the species composition of the ecosystem.
 
When algal blooms die or are eaten, they release neuro and hepatotoxins, which can kill aquatic organisms and cause human illness, such as shellfish poisoning. Drinking contaminated water can cause diarrhoea, cholera, typhoid, dysentery, or skin disorders in humans.
 
To this prevent, we need to implement wastewater treatment, green agriculture (limiting the use of chemicals), stormwater management (reduce runoff water), plastic reduction, water management.