05 Jan 2024
 | 05 Jan 2024
Status: this preprint is currently under review for the journal BG.

From iron curtain to green belt: Shift from heterotrophic to autotrophic nitrogen retention in the Elbe River over 35 years of passive restoration

Alexander Wachholz, James W. Jawitz, and Dietrich Borchardt

Abstract. We investigate changes in in-stream nitrogen retention and metabolic processes in the River Elbe between 1978 and 2020. We analyzed multi-decadal time series data and developed a metabolic nitrogen demand model to explain trends in dissolved inorganic nitrogen (DIN) retention, gross primary production (GPP), and ecosystem respiration (ER) during a period of highly dynamic pollution pressures in the Elbe River (Central Europe). Our findings reveal a marked increase in summer DIN retention and a decrease in winter DIN retention, establishing a distinct seasonal pattern. We identified three periods in the Elbe's DIN retention dynamics: dominantly heterotrophic under high pollution pressure (1980–1990), transition (1990–2003), and dominantly autotrophic with lower pollution (2003–2017). We link these changes to reduced industrial pollution, improved wastewater treatment, and a shift in the in-stream balance between heterotrophic and autotrophic processes. During the first period, high ER and heterotrophic growth efficiency contributed to elevated metabolic nitrogen demand, primarily driven by heterotrophic processes. As pollution levels decreased, GPP rates increased, and ER gradually declined, prompting a shift towards an autotrophic-dominated nitrogen retention regime. Our study indicates a tight coupling of nutrient reduction from external sources and dominant processes of natural attenuation in large rivers which needs to be considered for projections of recovery trajectories towards sustainable water quality.

Alexander Wachholz, James W. Jawitz, and Dietrich Borchardt

Status: open (until 07 Mar 2024)

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Alexander Wachholz, James W. Jawitz, and Dietrich Borchardt
Alexander Wachholz, James W. Jawitz, and Dietrich Borchardt


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Short summary
Human activities are rivers' main source of nitrogen, causing eutrophication and other hazards. However, rivers can serve as a natural defense mechanism against this by retaining nitrogen. We show that the Elbe River retains more nitrogen during times of high pollution. With improvements in water quality, less nitrogen is retained. We explain this with changed algal and bacterial activities, which correspond to pollution and have many implications for the river and adjacent ecosystems.