Tracing ecohydrology and biodiversity in aquatic, urban nature-based solutions integrating water stable isotopes, water chemistry and eDNA

The combined effects of rapid urbanization and climate change challenge ecohydrology and water quality in urban systems. Water related nature-based (aquaNBS) solutions such as stormwater ponds and streams are being widely implemented in cities to address ecological and hydrological challenges that threaten urban biodiversity and water security. However, there is still a lack of process-based evidence of ecohydrological interactions in urban aquaNBS, and their relationship to water quality and quantity at the ecosystem level. As part of a pan-European project aimed at understanding ecohydrological functioning and future resilience of aquaNBS, we applied a novel, integrative multi-tracer approach using stable water isotopes, hydrochemistry and environmental DNA to disentangle the effects of urbanization and hydroclimate on ecohydrological dynamics in urban aquaNBS. Insights from stable isotopes and microbial data show a strong influence of urban water sources (i.e. treated effluent, urban surface runoff) across stream NBS. This highlights potential limitations of aquaNBS contributions on water quality and biodiversity, as microbial signatures appear more biased towards potentially pathogenic bacteria in these streams, compared to non-effluent impacted systems. Urban ponds appear more sensitive to hydroclimate perturbations, causing increased microbial turnover and lower microbial diversity than expected. Within the European dataset, diatom richness revealed an overarching influence of urbanization and urban water sources, as well as the presence of unique species in more naturalized sites. This demonstrates the need to adequately consider nutrient variability as well as aquatic organisms in planned restoration projects, particularly those implemented in densely urbanized ecosystems. Our findings highlight the use of novel integrated tracer approaches to explore the interface between ecology and hydrology, and provide insights into the ecohydrologic functioning of aquaNBS and their potential limitations. We illustrate the benefit of coupling ecological and hydrological perspectives through multiple environmental tracers, and hope to support future aquaNBS design and applications that consider the interactions between water and the ecosystem more effectively.