Taken from the official translation of the German Sustainable Development Strategy
The pollution of rivers with phosphorus is measured by the Länder as part of their monitoring under the EU Water Framework Directive. The data used for the indicator are taken from the surveillance network, which comprises some 250 monitoring sites. In most cases, the monitoring sites were established in the main flows of the large rivers and at the mouths of important tributaries. The data are compiled by the Federal Environment Agency on the basis of information from the German Working Group on Water Issues of the Länder and the Federal Government (LAWA).
Each of the figures used to calculate the indicator value shows whether the annual average reading from a particular monitoring site adhered to or fell below the benchmark value but not the extent to which the threshold was exceeded. The information from the individual monitoring sites is presented in aggregated form. Accordingly, the value of the indicator depends on the number of monitoring sites and how representative their distribution is. Lakes and other bodies of water are not covered by the indicator.
Since the different bodies of water react with differing levels of sensitivity to nutrients such as phosphorous, the precise benchmark values vary. The vast majority of flowing waters use the benchmark value of 0.1 mg/l of phosphorous. In organic substrate-dominated rivers, the benchmark value is 0.15 mg/l, for marshland streams 0.3 mg/l and for transitional waters influenced by tidal movement 0.045 mg/l.
The indicators of phosphorous and nitrate levels (6.1.a and 6.1.b) cover two key aspects of water quality. However, there are other, additional components such as the existence of natural habitats around water bodies and the exposure to pollutants (such as pesticides, metals, medicines), all of which are also relevant to water quality. Phosphorous generally enters a body of water through the input of phosphates.
In 2018, the annual average of values measured was below the benchmark value at 44% of the monitoring points at rivers. 50% of the monitoring points showed average concentrations of up to twice the benchmark value, while 4% of the monitoring points were in the range of up to four times the benchmark value (not shown in the chart). The remaining 2% showed even higher concentrations.
When viewed over time, the proportion of monitoring points not exceeding the benchmark value has continuously increased and has doubled since 1990. However, the percentage rate of monitoring points with concentrations of up to twice the benchmark value tripled during the same period. Conversely, the share of monitoring points with even higher values has fallen significantly since the early 1990s. The level of pollution has been reduced significantly thanks in particular to the introduction of phosphate-free detergents and the specification of threshold values for the discharge of treated waste water.
Considering the average trend of the last five years, the indicator has developed slightly positively. The goal of not exceeding the specified threshold value at all monitoring points was still clearly missed.