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Organic matter, measured as Biochemical Oxygen Demand (BOD) and ammonium, are key indicators of the oxygen content of water bodies. Severe organic pollution may lead to rapid de-oxygenation of river water, a high concentration of ammonia and the disappearance of fish and aquatic invertebrates. Mainly due to the implementation of secondary biological wastewater treatment under the UWWTD concentrations of BOD and total ammonium have decreased in European rivers in the period 1992 to 2009 (Fig. 7.2a).