Too much nutrient such as nitrogen and phosphorus causes algae to grow in excess. The algae can color the water resulting in green, red, or brown water. This is called an algal bloom. The algae reduce the amount of sunlight reaching below the surface. As the algae die and decay, bacteria in the water increase. The bacteria consume oxygen as they break down the algae. In addition, certain types of algae produce toxic chemicals that can kill aquatic life.

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