Chloride Concentrations in German Rivers and Their Impact on the Potential Distribution of the Golden Algae Prymnesium parvum

Freshwater salinization poses a threat to river ecosystems, with anthropogenic influences playing a crucial role. A low discharge rate can further increase river salinity. The destructive impact of this issue became evident in Germany and Poland in August 2022, when elevated chloride levels in the Oder River facilitated the bloom of Prymnesium parvum, a toxic brackish-water alga. The release of its toxin led to an ecological disaster with a massive fish kill, highlighting the urgent need for analysis and preventive measures to avoid similar incidents in the future.

Using data from 1,628 stream water monitoring stations in Germany, this study examines which rivers are particularly affected by chloride concentrations above critical thresholds. By quantifying the concentration-discharge (C-Q) relationship for chloride at 250 stations, we assessed whether chloride concentrations can be reliably predicted from discharge data. Correlation analyses with catchment characteristics allowed the discussion of chloride input pathways and their influence on the C-Q model parameters. Finally, station-specific discharge values were determined, at which critical chloride thresholds are exceeded, thereby impeding the achievement of a good ecological status and promoting the spread of the alga P. parvum.

We found that more than 70% of all stations reach or exceed 50 mg/l chloride at least once. For a threshold value of 200 mg/l it was 16%, with 8% of these showing near-permanent exceedances. Around 9% of the monitoring stations surpass a critical value of 300 mg/l. Distinct spatial patterns of elevated chloride levels are particularly noticeable in the Weser River network, the Saale, the Oder River near the Polish border, and the Ems, as well as in or in proximity to major cities such as Berlin and Frankfurt. Our results further indicate that the C-Q relationship varies significantly across river systems. While more than half the stations (64.8%) exhibit a dilution pattern between discharge and chloride concentration, stations showing chemostatic behavior suggest more complex input pathways. The correlation analysis revealed that chloride concentrations are controlled by hydroclimatic characteristics, land use and the input of wastewater. Surprisingly, lithological and hydrogeological factors have a comparatively minor impact on surface water chloride levels.

These results illustrate the complex, region-specific dynamics of chloride pollution in rivers, and underscore the need for targeted management strategies that account for hydrological variability. Refined predictive models that consider both temporal and spatial variability of chloride sources and the dilution potential of rivers are essential for developing such management strategies.