Exploring Controls on Solute Export Mechanisms for Major Nutrients in Anthropogenically Impacted Catchments in Southern Germany under Climate Change

Abstract. Global warming is assumed to impact the mobilization, transport, transformation, and storage of major 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 major nutrients (NO₃-N, NH₄-N, SRP, and TP), total organic carbon (TOC), and geogenic minerals (Ca²⁺ and Mg²⁺) across 40 anthropogenically impacted catchments in southern Germany under global warming conditions. The findings reveal seasonal and climate-driven differences in SEM. We identify explanatory controls impacted by climate change by comparing an earlier time period (Period 1: prior to January 1, 2012) with a more recent one (Period 2: after January 1, 2012). Our results indicate an increase in enrichment behaviour for major nutrients and TOC, while geogenic solutes exhibit slightly increase in diluting export mechanisms. Climate change has altered solute source distribution and hydrological connectivity, depending on catchment characteristics such as land cover, climate conditions, hydrological indices, soil properties, and geology. Rising temperatures, prolonged heatwaves, and sporadic but intense one-day 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 with 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.