How does algal bloom destroy the quality of a fresh water body? Explain.

An algal bloom means the excessive growth planktonic form in the water which is rich in nutrients. .These planktons multiply and reproduce forming a layer on the surface of water. This layer blocks the penetration of sunlight inside the water body . When the light is not able to penetrate inside then the aquatic plants that grow under water are not able to perform the process of photosynthesis which decreases the productivity of the water body and eventually leads to the pollution . The excreta of the planktonic forms accumulate over the surface of water causing a bad odour .
Excretory substances released by planktons build up in concentration and pollute the
water. Some algal species even release substances that are toxic for other life . There will be high respiratory needs of such a huge concentration of biomass on the surface , the BOD of the freshwater increases causing death of many aquatic species.
The dead bodies of the aquatic animals further deteiorate the quality of the freshwater.