(i) A sharp decline in dissolved oxygen and increase in BOD. Low oxygen levels in the water cause death of fish and other aquatic creatures.
(ii) Algal bloom which results from the presence of organic matter in water. It causes:
(a) deterioration of the water quality.
(b) fish mortality.
(c) toxicity to humans and animals.
(iii) Biological maghification of a few toxic substances, as DDT causing harmful effects in living species.
(iv) Accelerated eutrophication, i.e, ageing of lakes and other water bodies due to human activities.