Organic pollutants in water have adverse effect on ecology system. How?

Organic pollutants are source of food for bacteria and other microbiological organisms in water. Depletion of these pollutants need oxygen resulting in deficiency of oxygen in water thus reducing BOD. The same oxygen is necessary for survival of aquatic animals.

Poisoning nature of some pollutants also has adverse effects on aquatic life.

In this way ecological system is disturbed.