What happens if fertilizer goes in streams or lakes?

Most times, fertilizers are dangerous to living organisms. If it were to get into streams and lakes where an animal is there, they could die because of the poison. Think about it, fertilizers try to get rid of bugs, so of course it would be hurtful to animals as well. If one species of an animal dies in a place, others start to die if the organism that died was their way to live, or one organism can excel in size because they aren't being eaten.