How do colonies of cyanobacteria benefit aquatic organisms how do colonies of cyanobacteria harm aquatic organisms

Colonies of cyanobacteria benefit aquatic organisms because they can be an important source of food to organisms. 

The problem with cyanobacteria is that they can be harmful and in an alga bloom then can cover the surface cutting off any light to other plants and producing toxins that kill fish and other life forms. 

From the topexospherethermospheremesospherestratosphere troposphere