Algae are free-floating, microscopic plants that play an important role in the function of a lake. They are fed upon by tiny animals called zooplankton which are a critical food item for fish. Algae occur in virtually all lakes, but their concentration can vary considerably through the year and with location. There are literally thousands of species of algae and they all require phosphorus to live and reproduce. Algae color the water green or brown and can reach nuisance levels in lakes with uncontrolled sources of phosphorus.