Phosphorus consumes a lot of oxygen. Just 1 pound of it can eliminate up to 16 pounds of oxygen.
Phosphorus is essentially a food product for many living organisms. If you use a lot of it and it is part of your discharge, then you can change the local habitat. Algae is probably the biggest reason why we should be looking at reducing phosphorous wherever and whenever we can.
Excess algae not only changes a river, lake, or pond, but it becomes hazardous for boating or swimming. Marine life might eat that algae as well, so if you eat a fish that eats algae, you’re getting that stuff too.