What are Microcystis blooms and why are they a problem?

Microcystis algal blooms are the genus of freshwater cyanobacteria. It is a single-celled planktonic blue-green type of algae that blooms naturally in surface waters. The single cells organize themselves into colonies that begin spherically, but eventually lose shape as they grow in surface area. Their buoyancy is a result of intracellular structures called gas vesicles which allow them to float when water is stable. Colonies range from a few to hundreds of cells, making it difficult to see anything over a couple inches below the water. When they bloom rapidly, they form dense blooms that can be very harmful to the environment. As they grow, they produce toxins that remain inside their cells until they die, at which point the cells break open, releasing the toxins into the water around them. If this toxin containing water is ingested, it can pose serious threats to the health of many different aquatic species, terrestrial organisms, and humans.

Microcystis blooms typically occur in in warm and stagnant waters with low salinity. They are most prevalent and dense in eutrophic waters -  waters containing high levels of phosphorus and nitrogen. Blooms can only occur through photosynthesis, so these algal plants also require sufficient light sources. They bloom during the warmest times of the year, typically from the late months of summer to the end of fall, and live for about two to four months. During hotter years, blooms are more common, live longer, and more severe. Though most blooms remain visible at the surface, they can sink if they become dense enough, making them invisible to the the human eye.