Trends in Instream Nitrogen-to-Phosphorus Ratios: A 30-Year Study of Ecological Relevance Across 750 Catchments

Excess nutrients in aquatic ecosystems cause toxic algal blooms, deoxygenation, fish kills and health risks for humans, resulting in high costs for the environment and society. Next to absolute nitrogen (N) and phosphorus (P) concentrations, the stoichiometric ratio of N to P affects biological activity and thus on potential nutrient retention and the occurrence of adverse eutrophication effects. P inputs to streams have been largely reduced by improved wastewater treatment since the 1980s/1990s. In contrast, N inputs, mainly stemming from diffuse sources, have been reduced more recently and with slower rates. Moreover, diffuse inputs travel from the catchments’ surface along different pathways until reaching the streams with time lags up to decades from application to export, while wastewater is usually disposed directly into streams. These asynchronous changes in N and P inputs and distinct pathways suggest widespread increases in instream N:P ratios. However, little is known about the spatial and temporal variability of these shifts across catchments, their ecological implications, and whether recent improvements in N control and fading P reductions have reversed this trend. 

To fill this knowledge gap, we analyze instream N:P trajectories in 767 catchments in France, Germany and Denmark in the period from 1990 to 2019. We classify the catchments based on ecologically relevant classes of molar N:P ratios and their decadal changes. For classification, we consider N-depletion for N:P < 20, P-depletion for N:P > 50 and N&P co-depletion in between, following Guildford and Hecky (2000), both for annual and summer periods.

We found 

(1) a widespread increase of N:P in 1990-2010 that is levelling off in the last decade and partly even reversing, primarily controlled by trends in P. 

(2) that 40% of catchments experienced at least one shift in depletion class over the decades.

(3) summer N:P ratios to be lower with 50% of the catchments remaining N- or N&P co-depleted in the last decade, despite overall positive N:P trends.

This indicates that, although shifts in nutrient management have intensified P depletion, notable spatial and temporal variations remain, which we intend to investigate in more detail. This study provides the first comprehensive analysis of N:P trajectories in Western European streams, offering new insights into stream water quality and its ecological implications for aquatic ecosystems.

Guildford, S. J., & Hecky, R. E. (2000). Total nitrogen, total phosphorus, and nutrient limitation in lakes and oceans: Is there a common relationship? Limnology and Oceanography, 45(6), 1213-1223. https://doi.org/10.4319/lo.2000.45.6.1213