The truly spectacular manifestation of pollution in the metropolitan Potomac is the periodic growth of algae there in summertime. When conditions are right--when sun, summer temperatures, low inflows from the river above, and a heavy concentration of nitrate and phosphate nutrients all combine to make of the upper estuary one vast inspired pool of fertility--the whole surface of the river may be covered with a thick bright emerald mat, and boats that pass at speed leave wakes of green instead of white. The infestation may extend downstream for thirty or forty miles, in various degrees of concentration, and even if the water were bacterially safe this "bloom," as it is called, would prohibit its recreational use by anyone without a strong stomach. It further disrupts aquatic life balances, and periodically dies and decays aromatically, setting off whole new cycles of oxygen depletion, fish kills, stink, and fertilization. The problem is one of fertility, of course, and stems from the huge quantities of nitrogen and phosphorus perennially present in the water. Some of this comes down from the upper river--where, as we noted, much of it derives from land runoff--but by far the greatest part of it originates at the metropolis and enters the river through the effluent of waste treatment plants. Efficiency of operation has hardly anything to do with it, for even the best standard treatment has little effect on nutrients. Eutrophication is the scientific name of this kind of overenrichment. It is occurring in many places, Lake Erie being the best-known single example in this country. Though its causes are mainly known, the process itself is still not fully understood, particularly in regard to the function of nitrogen and the way it works. But the other key element, phosphorus, has been more amenable to study and to possible action. It occurs in body wastes, in artificial fertilizers, as a by-product of natural decay, and very notably in detergents. Some eight tons of it are released into the estuary each day from the treatment plants in addition to the undetermined but much smaller amounts arriving from upriver, and the usual overall accumulation is enough to make the river's phosphorus content exceed that considered desirable all the way from Theodore Roosevelt Island to Quantico, Virginia, and below, which represents the general extent of the summertime "blooms."