Answer:
An algal bloom.
Explanation:
The input of sewage, fertilizer runoff, and animal waste can cause the formation of an algal bloom (abundances of cyanobacteria) because of the excess nutrients (nitrogen or phosphorus). If there are too much nutrients in the water (caused by the sewage, fertilizer runoff, and animal waste) it creates a perfect environment for excess growth, being an algal bloom. Algal blooms are toxic and are known to be harmful to the animals that rely on this source for their survival and humans as well.
