The negative significance of algae in human life and nature?

Algae: 1. Produce harmful toxic (poisoning other organisms) substances in the water, stopping growth and causing various diseases 2. They provoke (when warm) the “blooming” of the reservoir – the proportion of oxygen becomes less (death of fish, as a result) 3. Dead algae so are also dangerous for fish. Both for animals and for humans (toxic, cause allergic reactions of different etymologies (origins) and diseases) 4. They are associated in a “reaction” (generating carcinogens (poisons)) with chlorine. 5. They can clog filters during forced water purification