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This image shows an Upper Mississippi River marina covered by excessive duckweed.
Too much nitrogen and phosphorus in water could lead to an overgrowth of free-floating plants such as duckweed and filamentous algae. These overgrowths can result in dense layers of scum on the surface of the waters, and can damage below-surface plants, fish and other lake organisms by depriving them of the oxygen and sunlight they need to survive.