Nitrogen and phosphorous represents nutrients that can be seen as pollutants, as large concentations of those increase eutrophication. Nutrients are present in primarily commercial fertilisers and in sewage. A measure of eutrophication is biochemical oxygen demand (BOD), which measures the activity in the water column. High BOD means that there is low water quality (in terms of eutrophication) and high activity of bacteria in the water.
From collection: Vital Caspian Graphics
Philippe Rekacewicz, UNEP/GRID-Arendal