Long-term trajectories of Nitrogen and Phosphorous point sources to German river systems

Concurrent excesses of nitrogen (N) and phosphorous (P) compounds in the environment are causing the eutrophication of water bodies worldwide, including the North Sea and Baltic Sea that border Germany. The N:P ratio is a crucial measure of the nature of nutrient limitation and of the state of aquatic ecosystems. While many studies focus on the diffuse sources of N and P, point sources from urban and industrial wastewater can substantially contribute to in-stream N and P levels. Yet, systematic studies of the co-development of N and P point sources that span different river basins and a long time period are greatly lacking at a national scale like Germany.

To this end, we provide a comprehensive investigation of the long-term trajectories of N and P point sources in German catchments over the last seven decades (1950-2019). We construct a novel gridded dataset of N and P point sources for Germany, adapting the methodology proposed by Morée et al. (2013) and using country-specific datasets on population counts, protein supply, food wastes, and population connection to sewer and wastewater treatment plants. In addition, we estimate the consumption of P detergents combining datasets of household detergent phosphate and phosphonate sales, household ownership of automatic dishwashers, and professional detergent use. Our reconstruction approach accounts for the uncertainty in coefficients (e.g. efficiency of wastewater treatment, N content in proteins). We create an ensemble of plausible combinations of coefficient values that are constrained by the contemporary observed N and P loading from urban wastewater treatment plants (2012-2016 values; Büttner 2020).

From the newly constructed dataset, we analyze the trajectories of N and P loading, the N:P ratio and the relative importance of diffuse and point sources across major German river basins. In particular, N and P loadings show large differences between West and East Germany. In addition, P loading exhibits a stronger decrease in the 1980s and early 1990s than N loading because of the introduction of phosphate-free laundry detergents. Overall, N and P trajectories have large temporal and spatial variations, in particular due to differences in treatment efficiency, in population density, and in regulations that limit the maximum phosphate content in detergents.

Büttner, O.: DE-WWTP - data collection of wastewater treatment plants of Germany (status 2015, metadata), https://doi.org/10.4211/hs.712c1df62aca4ef29688242eeab7940c, 2020.

Morée, A. L., Beusen, A. H. W., Bouwman, A. F., and Willems, W. J.: Exploring global nitrogen and phosphorus flows in urban wastes during the twentieth century, Global Biogeochemical Cycles, 27, 836–846, https://doi.org/10.1002/gbc.20072, 2013.