Eutrophication persists in many freshwater systems despite extensive efforts to control nutrient emissions from point and diffuse sources. While intensely studied at local or regional scales, the joint response of benthic and pelagic algae to nutrient loading across entire river networks remains poorly understood. Here, we assessed spatial patterns of pelagic and benthic algal biomass in response to point source and diffuse phosphorus loading in the Elbe River Basin, a temperate, transboundary river network, based on extensive monitoring data and with the parsimonious hydro-ecological model CnANDY (Coupled Complex Algal-Nutrient Dynamics). We referenced our simulations to median river discharge data and phosphorus inputs from point (1,900 wastewater treatment plants) and diffuse sources, determined with the MoRE model and CORINE land cover analysis. We found distinct spatial eutrophication patterns across the river network and complex responses to local and cumulative anthropogenic nutrient emissions. Lower stream orders, particularly those in urban and agricultural areas, showed the highest dissolved phosphorus concentration and benthic algae density. Conversely, pelagic algae dominated higher stream orders, influenced by nutrient transport from lower-order streams to downstream reaches. The validated CnANDY model effectively identified eutrophication hotspots, enabling prioritized nutrient and eutrophication management. Although extensive monitoring data were available, systematic gaps in established monitoring schemes limited the model calibration and validation. Therefore, we advocate for a revision and propose model-aided eutrophication monitoring at the river basin scale with representative coverage of all stream orders from up to downstream and the algal biomass in the benthic and pelagic compartments.

We analyzed a multidecadal time series of nitrogen in the Elbe River between 1978 and 2020 and developed a metabolic demand model to explain trends in dissolved inorganic nitrogen (DIN) retention, gross primary production (GPP), and ecosystem respiration (ER) before and after the break down of the Iron Curtain. 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 distinct periods in the Elbe’s DIN retention dynamics: dominantly heterotrophic under high organic and inorganic 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 metabolism. During the first period, high ER caused elevated nitrogen turnover, primarily driven by heterotrophic processes. As pollution from industrial and wastewater emissions decreased, GPP rates increased and ER gradually declined, prompting a shift towards an autotrophic dominated nitrogen retention regime. Our study indicates tight coupling of nutrient reduction from external sources and dominant processes of natural attenuation in large rivers.

Groundwater is a vital freshwater resource for domestic use, agriculture production, and diverse ecosystems services in southwestern Europe. Despite increasing concerns about water scarcity in the region due to climate change and human activity, our study surprisingly shows that groundwater levels in Portugal, Spain, France, and Italy have remained stable overall in the past decades. Multi-decadal groundwater level data from over 12,000 wells revealed that 20% of wells showed rising levels, 68% were stable, and only 12% were declining. Rising levels are often due to improved groundwater management and higher precipitation in some areas, while declines are linked to agricultural use and urbanization. Our findings highlight the importance of long-term and systematic groundwater monitoring to understand these trends and to manage this vital resource sustainably.

Too many nutrients are entering the water of many shallow lakes, leading to their over-fertilization, a phenomenon known as eutrophication. We now demonstrate a clear connection between the amount of nutrients in the water and the amount of Chlorophyll a – an indicator of algal biomass. However, this connection only becomes apparent when the ratio and concentration of both nitrogen and phosphorus are taken into account. Additionally, the relationship between the nutrients and Chlorophyll a significantly improves when data over a longer period of 5 years is analyzed. According to our analysis, the remaining differences in the fluctuation of Chlorophyll a between the lakes are due to other environmental variables, not nitrogen and phosphorus. Shallow lakes constitute around 89% of all lakes worldwide, and our findings clearly indicate that targeted, long-term control of the ratio and concentrations of nitrogen and phosphorus can be a sustainable strategy for the management of these lakes.