what causes algal blooms in freshwater

Algal blooms are the overgrowth of microscopic algae or algae-like bacteria in fresh, salt, or brackish waters. They can occur when a combination of suitable environmental conditions exist for abundant algal growth, such as increased nutrients, warmer temperature, still water, and climate change. Some human activities, such as agricultural run-off or poorly functioning septic systems, can make blooms more likely, and excessive amounts of nutrients within a lake can markedly increase the risk and severity of algal blooms. However, excessive nutrients may not lead to a bloom if other factors are unfavorable. The key nutrients that encourage the growth of algae in waterbodies are phosphorus and nitrogen, and increased concentrations of nutrients often lead to increased algal production and increased risk of a bloom forming. Cyanobacteria, a type of photosynthetic bacteria also known as blue-green algae, are often the cause of algal blooms in freshwater and occasionally in marine water. A combination of environmental factors such as the presence of nutrients, warm temperatures, and lots of light encourage the natural increase in the numbers of cyanobacteria. Harmful algal blooms tend to form in warm water with high levels of nutrients such as nitrogen and phosphorus.