PHOSPHATE PO₄^3-

The high concentration of phosphate ion is due to agricultural pollution and partially to domestic pollution too.
It increases the aquatic flora trophism called eutrophization. The first signal of this phenomenon is the percentage of oxygen in the water higher than the saturation.
This is an apparent improvement of the quality water because the algae have an abnormal proliferation and a lot of them die and go to the river-bottom, where the anaerobe oxidation processes begin producing ammonia, sulphydric acid and phosphin. These substances poison the water course.