Algal blooms are a result of eutrophication. Eutrophication involves change in biological productivity and nutrient content of a water body. Eutrophication literally means 'well nourished or enriched'. Eutrophication become excessive, when abnormally high amount of nutrients from sewage, fertilizers, animal wastage and detergent, enter streams and lakes causing excessive growth or blooms of microorganisms. They are harmful for both humans and aquatic organisms. Eutrophication results in higher BOD and reduced dissolved oxygen in water body. It causes death of fish and other aquatic animals.