Pfiesteria is a type of phytoplankton known as a dinoflagellate, which means they have a whip-like tail called a flagella which allows them to propel themselves, unlike some other types of algae which can only float. Pfiesteria, though, do not perform photosynthesis and instead survive by eating other organisms, usually algae. Under specific conditions, such as high nutrient levels and the presence of large schools of fish, pfiesteria's population increases. During a bloom, pfiesteria may produce toxins that numb fish, allowing the microbe to feed on them. High concentrations of pfiesteria can cause deep lesions on fish, and may kill them.
Blooms exist only for short periods of time, usually a few hours at most.
Blooms of pfiesteria may not result in fish kills or lesions. Pfiesteria may spend its entire life feeding harmlessly on bacteria and algae. It is not well understood what environmental factors cause pfiesteria to produce harmful toxins. In the laboratory, toxic pfiesteria blooms are induced by nutrient enrichment and large amounts for fresh fish excrement. In most of these fish kills, large schools of menhaden were present in warm, shallow, partially land-locked waters.