Bright green blooms in freshwater systems are frequently a result of cyanobacteria (colloquially known as "blue-green algae" as a result of their confusing taxonomical history) such as Microcystis.
Of particular note are harmful algal blooms (HABs), which are algal bloom events involving toxic or otherwise harmful phytoplankton such as dinoflagellates of the genus Alexandrium and Karenia, or diatoms of the genus Pseudo-nitzschia.