That’s interesting question!
 
It’s true that both the Cyanobacteria and Green algae bloom in polluted water.They not only bloom but do undergo various changes inorder to produce toxins in the water.They even deplete the level of oxygen dissolved in the water body.It would be surprising to know that the dead green algae settled at the bottom of the water bodies also depletes oxygen along with polluting the water with production of toxins and a bad odour.This may causes to the death of many aquatic life.
						

							
True. Oraginism that blooms in polluted water body is cyano bacteria.
Cyanobacteria (blue-green algae) are microscopic bacteria found in freshwater lakes, streams, oceans, damp soil, moistened rocks and even sloth fur.  When algae grows too much it can form “blooms”, which can cause various problems. Cyanobacterial Harmful Algal Blooms (CyanoHABs) are dangerous to people, animals or the environment and can lead to a depletion of oxygen in the water and a release of toxins, as well as taste and odour problems.
Most cyanobacterial blooms are clearly visible by naked eye. They form a layer on top of the water, which can be of various colours including green, blue, brown or red.