Many aquatic algae are directly or indirectly fed to fish. Diatoms, filamentous algae, some planktonic green algae, and numerous cyanobacteria are very commonly found in the gut of various species of freshwater and brackish water fish, and appear to be used directly as fish food. Chlamydomonas, Scenedesmus, Chlorella and Euglena seeds are used in sewage treatment plants to photosynthe sise the oxygen needed for the rapid breakdown of wastewater by bacteria.
During the summer, the phytoplankton in ponds, lakes, and reservoirs become very abundant and very visible. A floating mat of foam may occur. These signs of algal growth are commonly known as "water blooms". Such algae concentrations are very troublesome not only in public water supplies, but also in water bodies used for bathing, fishing, and other recreational purposes. Blue-green algae are most commonly responsible for contamination of water sources, but greens, flagellated tawnies, and diatoms are also occasionally troublesome. Prymnesium parvum, Gymnodinium veneficum and Microcystis spp. It leads to the death of fish and livestock that drink water infested with these algae.