Nitrogen

A plant nutrient that can cause an overabundance of bacteria and algae when high amounts are present, leading to a depletion of oxygen and fish kills. Several forms occur in water, including ammonia, nitrate, nitrite or elemental nitrogen. High levels of nitrogen in water are usually caused by agricultural runoff or improperly operating wastewater treatment plants. Also see phosphorous.


Source: Corrosion-Doctors.org

Wikipedia:  https://en.wikipedia.org/wiki/Nitrogen