. When fertilisers leach through soil from farms and run off into bodies of water they over nourish the water forming algae blooms, this is called Eutrophication
. The algae blooms block out sunlight preventing underwater plants from photosynthesising
. The plants all die out and the fish die from starvation
. The saprobionts/decomposers use the remaining oxygen to decompose the fish via aerobic respiration
. This depletes the oxygen levels in the water and increases the carbon dioxide levels
. This causes a dead body of water
