Phosphates are necessary to aquatic plant life.  Increases in phosphates can lead to excessive algae growth.  Forms of phosphate that occur in water are organic (needed for animal life) and inorganic (needed for plant life.)  Excess phosphates can lead to algae blooms, which increase turbidity and water temperature.  When the algae dies, it is consumed by bacteria that significantly decrease the amount of oxygen in water.  All of these affects can make habitat uninhabitable for aquatic life.  Sources of phosphates include sewage treatment plants, septic systems, fertilizers, industrial discharges, and animal manure.