Exploring controls on solute export mechanisms in anthropogenically impacted catchments in Southern Germany in climatically different periods

Abstract. Global warming is assumed to impact the mobilization, transport, transformation, and storage of nutrients, impacting the health and functionality of riverine ecosystems. To enhance future water quality management, it is essential to understand potentially changing solute export mechanisms (SEM) in response to climate change. This study examines SEM for nutrients (NO3-N, NH4-N, soluble reactive phosphorus, total phosphorus, and total organic carbon) and geogenic minerals (Ca2+ and Mg2+) across 40 anthropogenically impacted catchments in southern Germany under changing climatic conditions. The findings reveal seasonal and climate-driven differences in SEM. We identify explanatory controls impacted by alterations in climate by comparing an earlier time period (Period 1: prior to 1 January 2012) with a more recent one (Period 2: after 1 January 2012). Moreover, the comparison of SEM and their controlling factors across both climatically distinct periods provides a valuable proxy for anticipating future SEM responses to climate change. Our results indicate an increase in enrichment behaviour for nutrients, while geogenic solutes exhibit a slight but insignificant increase in dilution pattern. Changes in climate conditions between period 1 and period 2 have altered solute source distribution and hydrological connectivity, depending on catchment characteristics such as land cover, climate conditions, soil properties, and geology. Rising temperatures, prolonged heatwaves, and sporadic but intense 1 d precipitation events have led to greater internal nutrient accumulation and decreased hydrological connectivity. Consequently, solute transport is primarily intensified at near-surface pathways that are only active sporadically during summer and during rising groundwater levels in autumn and winter. Further, nutrient dilution mechanisms are increasingly overprinted by enrichment-driven mobilization processes. Looking ahead, solute peak concentrations may more frequently exceed regulatory benchmarks for water quality, posing risks to riverine ecosystems and drinking water supplies. These findings should be integrated into future catchment management strategies to mitigate the intensification of enrichment export mechanisms and safeguard water resources.