(a) The addition of undesirable substances like fertilizers and pesticides, poisonous substances mercury salts in water. These could also be disease - causing organisms, like the bacteria which cause cholera.
(b) The removable of desired substances like oxygen from the water adversely affect the aquatic organisms.
(c) A sudden change in temperature in water bodies would be dangerous and affect their breeding. The eggs and larvae are affected by the change in temperature.