Disentangling multiple stressors in rivers using multivariate biogeochemical spaces

Multiple stressors in aquatic systems interact across temporal and spatial scales, which complicates the evaluation of their individual and conjoint effects on water quality and ecosystem functioning. This is particularly complex when gradual changes (e.g. from multi-year droughts to decadal warming) add to the impact of short-termed extreme events (e.g. a heatwave or the sudden disruption of flow). At the same time, focusing on the individual dynamics of water quality/composition indicators as proxies of ecosystem functioning may lead to the underestimation of system-wide sensitivities. Here, we define the ‘biogeochemical space’ of a river system as the realized, two-dimensional configuration of water composition dynamics as captured by non-metric multidimensional scaling of spatially discrete and temporally-resolved monitoring data.

We applied this concept to explore the combined expression of hydrological, meteorological, and anthropic stress in the Lower Oder River, which flows along the German-Polish border, and where an unprecedented harmful algal bloom caused a major environmental disaster in the summer of 2022. Using 20 years of monthly physicochemical data over a 200-km river reach, and in combination with long-term temperature and discharge records, we reconstructed a progressive shift toward increasingly concentrated (ion-enriched) water composition states. This displacement was, on one side, strongly associated with multi-year anomalies in water temperature and in discharge (> 2°C and –40%) caused by drier conditions in the catchment since 2016. On another side, conservative ions showed monotonic increases that could not be explained by short- nor medium-term changes in discharge alone, which confirmed the increased pressure from industry and mining-related salt inputs that are significant in this region. Finally, we further illustrate how the biogeochemical space framework can be used to diagnose diverse responses in other river systems at regional and global scales, and characterize their sensitivities to multiple impacts.