A dense concentration of algae, also known as eutrophication, that usually occurring under high nutrient concentrations such as phosphorous from fertilizer or from cleaning supplies, especially laundry detergent as one species becomes so abundant that it obliterates all other species, making the water appear cloudy (often brown, yellow, or pea-soup green). Sometimes causing scum and odor, an algae bloom is often blown into shallow areas; during a bloom each ounce of water contains millions of microscopic algae cells. Although a bloom may indirectly provide food for fish, it is an indication that a problem exists in the body of water, and to solve it the source of nutrients must be reduced.

Such blooms occur in both fresh water and salt water environments. There are more than one type of algae that can bloom in this manner. They can be minor in scope wherein they are merely a nuisance all the way to highly toxic. In the ocean, the famous red tide is an example of an algae bloom.