Swimmers will not be harmed by the presence of black algae in a swimming pool. The black or bluish-green spots will be a deterrent to swimmers; however, similar to garden weeds, black algae is a nuisance but will not, by itself, cause illness.
Black Algae Does Not Cause Illness
Black Algae Harbors Harmful Organisms
Black algae may harbor organisms, such as E. coli, which can make swimmers quite ill. In addition, swimmers will need to remain out of the water while the pool is being treated; heavy doses of chemicals such as pool shock and algaecide must circulate for several hours prior to the water chemistry reaching safe levels.
Swimmers should think twice before diving into a pool affected by black algae. Algae can attract insects and harbor harmful bacteria which can make swimmers ill. Home owners should take preventative measures, such as regular brushing of pool sides and maintaining balanced chemical, to avoid black algae, if possible.