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Nitrogen is an element needed by all plants and animals to build protein. It is also important in determining water quality.

As aquatic plants and animals die, bacteria break down large protein molecules into ammonia, which is oxidized to form nitrites and nitrates. Like the problems with phosphate, eutrophication problems occur with the increase of nitrogen in the water. However, plants are not as sensitive to increase in ammonia and nitrate levels as they are with phosphates.

Sewage is the main source of nitrates added by humans to rivers and can cause eutrophication.