Organic matter in rivers

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Organic matter, measured as biochemical oxygen demand, and ammonium are key indicators of the oxygen content of water bodies. Concentrations of these determinants are normally raised as a result of organic pollution, caused by discharges from waste water treatment plants, industrial effluents and agricultural run-off. High biochemical oxygen demand indicates poor chemical and biological quality of river water and may reduce the biodiversity of aquatic communities and microbiological quality.

Increased industrial and agricultural production, coupled with more of the population being connected to sewerage systems, has resulted in increases in discharges of organic waste in most European countries since the 1940s. In many major European rivers, the oxygen decreased to low levels and the ecological quality was heavily affected. For example, the River Thames had no resident fish in the London reaches in the 1950s.

Ammonium and biochemical oxygen demand generally decreased in the 1990s, by 20-30 % and 40-60 % respectively. This reduction in organic pollution in the EU countries during the 1990s was largely due to the urban waste water treatment directive, which increased the level of treatment of waste water. There has also been some investment in improving waste water treatment in the accession countries but the decline in organic pollution in these countries is probably mainly due to declines in industry discharging organic matter.

The concentrations of biochemical oxygen demand and ammonium are lower in the EU countries than in the accession countries. The largest decreases in ammonium have been observed in those countries with highest concentrations at the beginning of the 1990s. However, concentrations of ammonium are still way above background concentrations.

The concentrations of ammonium have decreased in the rivers of the EU and accession countries in the 1990s. The lowest concentrations of ammonium are found in Finland, with the new Baltic States, the UK and Denmark also having ammonium concentrations generally below 100 mg N/l. The highest ammonium concentrations are found in Bulgaria, Germany, Hungary and Poland, where significant improvements were made in the 1990s with ammonium concentrations being more than halved.