Drivers of multi-decadal in-stream nitrate regime shifts in a large European catchment

Long-term monitoring shows evidence of persistent changes in the magnitude and timing of the seasonal pattern and C-Q relations of nitrate concentrations in rivers, with possibly grave effects on aquatic ecosystems. Seasonal patterns of riverine nutrient concentrations are driven by a complex interplay of inputs, transport, and in-stream processing. Over multi-decadal periods, each of these factors may change due to socio-economic factors such as consumption patterns, governance regimes, or technological control measures. Here we test the hypothesis that observed multi-decadal changes in stream nitrate seasonality could be explained by changes in the relative importance of catchment nutrient sources over time. We analyze 66 years of shifting nitrate seasonality in a large, central-European river (Elbe) during a period of significant socio-political changes (1954 to 2019), with correspondingly significant changes in the sources of anthropogenic nitrate emissions. We show that the in-stream nitrate seasonality of the River Elbe changed significantly from a weak seasonal pattern (chemostatic) with peak concentrations during summer in the 1950s to a strong seasonal pattern (chemodynamic) with peak concentrations during winter in the 1990s. We link these shifts to a succession of technical/ political developments which influence the contribution of point and diffuse sources over time. Such shifts in seasonal concentration patterns can significantly impact the macronutrient (carbon, nitrogen, phosphorus) ratios in rivers, which in turn highly affect the health of aquatic ecosystems.