EGUsphere

Copernicus Publications

Göttingen, Germany

10.5194/egusphere-2026-830

Seasonal dynamics of eutrophication in human-impacted rivers

Hubig

Alexander

https://orcid.org/0009-0001-1101-7604

¹ Ebeling

Pia

https://orcid.org/0000-0002-5080-5275

¹ Graeber

Daniel

https://orcid.org/0000-0001-8331-9639

² Scharfenberger

Ulrike

² Musolff

Andreas

https://orcid.org/0000-0002-0115-1359

Department of Hydrogeology, Helmholtz-Centre for Environmental Research - UFZ, Leipzig, Germany

Department of Aquatic Ecosystem Analysis, Helmholtz-Centre for Environmental Research - UFZ, Magedeburg, Germany

28 04 2026

2026 1 27

2026

This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/

This article is available from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-830/

The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-830/egusphere-2026-830.pdf

Eutrophication – i.e., biomass overproduction due to nutrient enrichment – persists as a threat to riverine ecosystems despite achievements in lowering phosphorus (P) concentrations. This study explores seasonal patterns and drivers of chlorophyll a (Chl-a) linked with P availability across a selection of human-impacted rivers in Germany. We analyzed Chl-a and total phosphorus (TP) concentration measurements from 133 river sites and quantified their relationship using the degree of realized eutrophication (αrealized) – the ratio of measured to maximum Chl-a at a given TP. By applying k-means clustering on seasonal αrealized cycles, we identified five archetypal patterns. To understand the drivers of these patterns, we examined the seasonal dynamics of total nitrogen (TN), the TN:TP ratio, and fractions of reactive P and N. We further conducted a correlation analysis of αrealized and photosynthetically active radiation (PAR), water temperature, and discharge. In addition, we compared static river network and catchment characteristics between the clusters. We found that (1) constantly high αrealized was associated with close upstream lakes, along with nutrient concentrations suggesting co-control of Chl-a by P and N. (2) High αrealized in mostly lake-free rivers throughout spring and summer were associated with light control, as indicated by a high correlation with PAR. (3) Rivers with spring peaks and low summer αrealized were explained by summer Chl-a losses through grazing. Here, differences in spring peak timing and intensity could be related to differences in land use, hinting to riparian shading as a modulator of phytoplankton growth. Therefore, we find especially high risk of phytoplankton blooms downstream of lakes throughout the vegetation period, in long rivers without effective grazer control from mid-spring to early autumn, and in rivers with a lack of riparian shading during spring. Effective management may comprise dual management of P and N, especially for locations prone to summer blooms, and targeted riparian shading as an additional measure.