A possible nitrogen limitation ahead? Low discharges fuel nitrogen retention in the Elbe estuary

<p>Eutrophication of surface water bodies is an important factor that impairs the chemical quality of surface waters. In consequence, legislation and management efforts have been made over the past decades to reduce the agricultural nutrient input and meet the goals set by the EU Water Framework Directive and, more recently, by the UN Sustainability Goals.&#160;&#160;</p><p>In this study, we evaluate trends in nitrate concentration and isotope composition at the entrance of the Elbe Estuary, Northern Germany.&#160; We find a distinct seasonality of nitrate isotope composition and nitrate concentration, with high isotope values in summer, pointing towards assimilation and denitrification in the Elbe River and catchment.</p><p>Our data indicate that low discharge conditions intensify biological nitrate retention and nitrogen uptake during the growing season, leading to more intense nitrate isotope enrichments and low the water column concentrations. This suggests that recent reduction in Elbe River nutrient loads do not result from successful nutrient management but from a long-lasting drought in the catchment. In consideration of climate change predictions, we anticipate more frequent and extensive periods of low discharges, possibly even leading to a future nitrogen limitation in the lower Elbe River. &#160;</p><p>&#160;</p><p>&#160;</p><p>This study was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany&#8216;s Excellence Strategy &#8211; EXC 2037 &#8220;CLICCS - Climate, Climatic Change, and Society&#8221; &#8211; Project Number: 390683824, contribution to the Center for Earth System Research and Sustainability (CEN) of Universit&#228;t Hamburg.</p>