Large growths of algae are called algal blooms and they can severely reduce or eliminate oxygen in the water, leading to illnesses or the death of large numbers of fish. Some algal blooms, such as cyanobacteria, also known as blue-green algae, are harmful because they produce toxins that can make people and animals sick if they come into contact with the polluted water, consume tainted fish or shellfish, or drink contaminated water. Look Out for Harmful Algal Blooms - PUB2889