An algal bloom or marine bloom is a rapid increase in the production of algae in an aquatic system. They may occur in freshwater as well as marine environments. Algae are considered to be blooming at concentrations of hundreds of thousands of cells per milliliter, depending on the causative species.
The colors observed are green, yellowish-brown, or red.
Bright green blooms may also occur. These are a result of blue-green algae which are actually bacteria (cyanobacteria).