Algal bloom refers to an increase in the population of algae or blue-green algae in water due to an enrichment of nutrients, resulting in discoloration of the water body. This causes an increase in the biological oxygen demand (BOD), resulting in the death of fishes and other aquatic animals.
Red tides are caused by red dinoflagellates (Gonyaulax) that multiply rapidly. Their large numbers in the sea makes the sea appear red in colour. These release a large amount of toxin which results in the death of other marine organisms. 
Both of these signify the pollution of the water bodies.