This calculation assumes you bought or used a fertilizer that included an N:P:K ratio on the bag of 16:4:8. This is a mix that has 4% phosphorus (middle number on the bag). It was further assumed that you applied the fertilizer using the recommended 1 pound of nitrogen per 1000 square feet and fertilized three times a year.
The result of this application is about 8.17 pounds of phosphorus per year, per one quarter acre lot. Knowing that one pound of phosphorus can produce 500 pound of wet algae (University of MN Extension Service, 2003), and assuming half of the 8.17 pounds of phosphorus applied runs off and enter the water system, a reduction of 4.09 pounds could result in 2042 pounds of algae saved from our local lakes and streams