It is one of the essential minerals to sustain life as we know it. In lakes and oceans, the relative amounts of phosphorus and nitrogen are important. In the Great Lakes, phosphorus is the nutrient that is usually least abundant. Most of it is readily used by the algae, and any additional amount that enters the water is soon used to create more algae. Rivers naturally transport phosphorus that has weathered from rocks or that has been transported out of soils.
Human waste contains high amounts of phosphorus that can pass through STPs.
Until the mid-1970's or so, additional phosphorus in laundry detergents was also making its way through STPs. Phosphorus is one of the basic components of agricultural fertilizer, and rainstorms easily transport it from fields to streams that eventually flow into the Great Lakes. Over the next few years, a series of mathematical models were constructed and compared. The results suggested that if no more than 11,000 metric tons of phosphorus per year were added to Lake Erie, nuisance algae growth would be curtailed, and a sufficiently small amount of algae would be produced in the central basin so that the bottom waters would retain enough oxygen in late summer to support fish and aquatic insects. To put these findings into perspective, from 1967 to 1972, annual loadings of phosphorus averaged about 24,000 metric tons, more than double the maximum allowable amount.