1. Sewage discharge, pollutes the water and makes it unfit for human use, decomposers in the aquatic ecosystem, decompose these pollutants and makes the water foul smelling. 

2. The organic substances present in sewage are decomposed by decomposers aerobically as a result the oxygen content of the water decreases. This leads to biochemical oxygen demand (BOD). It kills large number of aquatic organisms. 

3. Agricultural runoff Or excess wash from agricultural fields, carry pesticides and fertilizers to water bodies like lakes. The nutrient enrichment of lakes promotes the enormous growth of algae. This is called algal bloom; it blocks the free flow of water and O₂ content and eventual death of aquatic organisms. 

4. Industrial effluents discharged into ponds or lakes contain toxic substances, like mercury, DDT, etc. It is neither excreted nor metabolized, gradually they tend to accumulate in organisms through food chain. This is called biomagnification. Biomagnifieation of DDT disturbs calcium metabolism in birds, which lay eggs with thin egg shell, it results in premature breaking and decline in bird population. 

5. Oil slicks formed as a result of leakage of oils from ships carrying huge quantities of crude oil via sea routes, spread to several kilometers. It does not allow oxygen to penetrate into water and causes mass death of aquatic organisms. 

6. The hot water discharged from thermal power stations or nuclear power stations into water bodies, increases the temperature; this results in evaporation of oxygen from the water surface leading to death of aquatic animals like fishes. 

7. Sewage water contains many undesirable pathogenic microorganisms, when this water is discharged into nearby water bodies without proper treatment may cause outbreak of serious diseases like Jaundice, cholera, dysentery, typhoid, etc. 

8. Industrial effluents released into surrounding land or into pits, percolate along with its chemicals into soil and contaminate ground water.