Cyanobacteria (blue-green algae) at the edge of a eutrophic lake. The picture shows a bloom of cyanobacteria, appearing as a green granular scum on the surface of the water at the shoreline.Lines within the scum suggest it is filamentous, but are caused by wind and wave action.Cyanobacteria are simple prokaryotic organisms lacking a cell nucleus, and are not correctly named algae. Many genera of cyanobacteria produce toxins which can affect the nerves, skin and liver of humans. The most commonly encountered freshwater genus is Microcystis; It produces a hepatotoxins called microcystins, which are cyclic peptides. Blooms occur after periods of hot weather, and are associated with eutrophication - an increase in nutrient levels in the water caused by factors such as pollution from wastewater, agricultural run-off, or large water bird populations