Eutrophication of water bodies leading to killing of fishes is mainly due to non-­availability of:

Explanation

Eutrophication is the excessive growth of algae, plants and animals in water bodies due to the nutrient enrichment particularly with nitrogen and phosphate. Nutrients present in sewage, agricultural wastes and fertilizers cause dense growth of plants and planktonic algae. The excessive growth of planktonic algae that cause coloration of water is called algal bloom, which is toxic to animals and humans. Eutrophic water bodies also support excessive growth of floating plants. Algal blooms and floating plants cut off light form submerged plants, resulting in their death. There is drastic decrease in oxygen replenishment inside water. Non-­availability of oxygen results in the death of aquatic animals such as fish, which further adds to organic loading of water. Decomposition is replaced by putrefraction which is anaerobic, leading to absence of oxygen in water and death of aquatic animals.