Phosphorus from human activities can cause lake pollution. In Sapporo, Japan, researchers studied how different forms of phosphorus in urban rivers changed as they flowed into a eutrophic lake. They found that the rivers had mostly soluble and particulate inorganic phosphorus, linked to iron. Downstream, these forms decreased, while organic phosphorus stayed constant. Lake water had more diverse phosphorus forms, with organic phosphorus increasing. This shows that phosphorus in the water shifted from inorganic to organic forms as it moved from the river to the lake.