Cyanobacteria (blue-green algae) at the edge of a eutrophic lake. The picture shows patches of cyanobacteria, appearing as a green granular scum stranded on the surface of mud at the shoreline. 5-15cm in size, each patch contains millions of living cells 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.