Both Phosphorous and Nitrogen are needed by algae. If you remove...

Both Phosphorous and Nitrogen are needed by algae. If you remove the phosphorous I don't think you need to remove the Nitrogen but I believe Phoslock uses Zeolites to remove some of the Nitrogen in some applications.
"PHOSLOCK is a unique water treatment product that permanently binds phosphorus in water bodies such as lakes, dams and canals. Binding the dissolved phosphorus before it is used by blue-green algae (BGA) will starve the algae of its food source and will cause BGA death."

An analogy would be that plants need both sunlight and water. You only need to remove one to cause the plants to die.