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These little one celled plants and animals are called "algae". They are normally in the river water and are important to the river's health. They help produce oxygen for the river through a process called photosynthesis, and serve as food for other animals. But when there is too much nitrogen in the river, they gobble it up, and begin to reproduce like crazy! Pretty soon there is a huge population explosion, and then you can see them, piled on top of another, looking like a green "mat" on the surface of the water.
This is bad for several reasons. One big reason is that when there are so many of these algae, they begin to suck the oxygen out of the water, which fish need to breathe.
But wait, I thought we said that the algae MADE the oxygen in the river!
They do... but when Trillions of them die at almost the same time, they begin to decay at the same time. And when a living creature (an "organism") begins to decay, oxygen is used! And its a lot of oxygen, if you have trillions and trillions of organisms decaying! This means that they are sucking oxygen up, oxygen that the fish need to breathe! And remember the green "mat" that was on the surface of the water? Well, that mat keeps plants which are not on the surface from getting any sunlight -- which means they can not produce oxygen, either!
Would this lack of oxygen be enough to kill the fish? Sometimes, if conditions were just right. But not usually. However, it would make the fish much weaker...
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