Indicator: Eutrophication of the North Sea/Baltic Sea by nitrogen | Umweltbundesamt

At a glance

• To achieve the objectives for the protection of the marine environment, Germany has committed to achieve target values for total nitrogen at the limnic-marine transition points.
• For the flow-weighted 5-year ⁠moving average⁠ of all rivers, these target values are achieved for the North Sea, but are still exceeded for the Baltic Sea.
• Some of the North and Baltic Sea tributaries still have very high concentrations of total nitrogen.
• In order to fulfil the objectives, it is necessary that each river achieves the target value.
• In order to achieve further reductions of nitrogen concentrations in the rivers, measures need to be taken, especially in agriculture and waste water management.

Environmental importance

Coastal and transitional waters of the German North and Baltic Sea still fail to achieve “good ecological status”. The main cause of this are the excessive nutrient loads of nitrogen and phosphorous (eutrophication). The negative effects of eutrophication are described by the indicator ‘Ecological status of transitional and coastal waters’.

Nutrients are transported to the sea mainly via rivers. This indicator considers the concentration of total nitrogen in the major rivers entering the North and Baltic Sea in Germany (including the Rhine river, excluding the Oder river). These concentrations may fluctuate significantly depending on the weather and hydrological conditions, because in years with high precipitation more nitrogen leaches out of the soils. For phosphorous, it is assumed that the achievement of the orientation values that have been set for rivers is sufficient to achieve the good status of the coastal and marine waters (cf. ‘River eutrophication by phosphorus’).

Assessing the development

In order to achieve the EU Water Framework Directive (WFD, 2000/60/EG) and the EU Marine Strategy Framework Directive (MSFD, 2008/56/EG), the German Surface Waters Ordinance sets management target values for the rivers running into the North Sea and Baltic Sea (OGewV 2016, in German only): 2.6 milligrammes of total nitrogen per litre (mg/l) for rivers flowing into the Baltic Sea and 2.8 mg/l for those flowing into the North Sea. These target values were also adopted for the German Sustainable Development Strategy.

The average flow-weighted nitrogen concentrations for North and Baltic Sea have decreased, mainly due to improved wastewater treatment. However, concentrations have stagnated in recent years. While the moving-average of all rivers is approaching or has already achieved the target value, the concentrations of individual rivers are still well above. Rivers discharging into the Baltic Sea have generally higher maximum concentrations than the North Sea rivers. In recent years, the minimum nitrogen concentrations of the rivers are already below the target values. However, to assess the achievement of the target, the discharge-weighted average of all tributaries is not evaluated, as is the case here, but each North Sea and Baltic Sea tributary must achieve the management target value as a long-term average.

The federal government provides the legal framework to reduce nutrient inputs via ordinances such as the Surface Waters Ordinance, the Fertiliser Ordinance or the Wastewater Ordinance. The federal states implement these ordinances and review their compliance. Measures to reduce nutrient inputs are provided by the Nitrates Directive, the WFD and the MSFD. At present, a major source of riverine nutrient inputs is agriculture. The amendment to the Fertiliser Ordinance aims to reduce these nutrient inputs (see also indicator ‘Agricultural nitrogen surplus‘). In order to achieve the target values, further measures are likely to be required in the agricultural sector.

Methodology

Measuring stations are located in the estuaries at the limnic-marine transition point where the rivers flow into the North and Baltic Sea. The total nitrogen concentration is measured at these stations at least once a month. From the measured concentrations, the flow-weighted average based on each rivers’ individual discharge is used for the indicator. In order to compensate for annual precipitation-induced fluctuations, the indicator is calculated as the 5-year ⁠moving average⁠. Additionally, the maximum and minimum concentrations are calculated as 5-year moving averages.

More detailed information: 'Flusseinträge und direkte Einträge in die Nordsee' and 'Nährstoffeinträge über Flüsse und Direkteinleiter in die Ostsee' (in German only).