It's phosphorus, which, likely as not, ran off a farm field somewhere in the watershed. A spectrometer calibrated to detect that wavelength displays the concentration of phosphorus.
Drainpipes spewing untreated industrial wastes into waterways are largely a thing of the past.
Suburbia contributes its share of lawn chemicals and construction siltation, but the U.S. EPA has cited agricultural runoff as the culprit in more than half of the rivers and lakes that have water-quality problems. Phosphorus is usually the offender. From fields to waterways Runoff and erosion carry farmland phosphorus into waterways. In most waters, phosphorus is the limiting nutrient for plant growth. When there is no shortage of phosphorus, plants and algae bloom.