Water bloom is generally caused by
green algae
blue- green algae
bacteria
Hydrilla
B.
blue- green algae
Water blooms are formed by the growth of some micrscopic or semi- microscopic algae, such as, Anabaena, Arthrospira, Nodularia, Nostoc etc.
These are considered to be harmful because they are responsible for the depletion of oxygen in water bodies.