Assessing the sensitivity of urban aquatic nature-based solutions to hydroclimate variability using stable water isotopes.

Natural and engineered water features, or blue infrastructure are increasingly implemented in cities as a form of water-related nature-based solutions (aquaNBS), to address ecological and hydrological challenges that threaten urban biodiversity and water security. Nevertheless, the combination of impacts from climate change, multi-faceted consequences of past management, current urban expansion, population growth, and overall urban ecosystem complexity makes it challenging to evaluate the hydrological function of these aquaNBS, and their sensitivity to hydroclimatic and other environmental changes. To enhance adaptation capacity of aquaNBS towards multiple urban and climatic stressors, it is crucial to understand the main hydrologic processes, as well as hydroclimate influences, that determine the functioning of aquaNBS. Stable water isotopes have proven to be a valuable tool in providing integrated understanding of hydrologic functioning over extended spatial scales. While higher frequency isotope data is usually most informative, even limited isotopic data can aid hydrological characterization. We conducted seasonal sampling over the period of one year in 2023/2024, across a major hydroclimate gradient across four European cities (Poznań, Berlin, Antwerp, Lisbon). The goal was to identify the dominant physical processes (in terms of water sources, dominant flow paths, and age proxies) linked to the main hydroclimate factors along a continental climate gradient. Comparative analyses of local stable water isotope signatures from different aquaNBS types (i.e., streams, ponds) revealed the strong influence of local hydroclimate, as well as varying water source contributions and mixing processes. The application of transit time proxies, such as tracer damping and young water fraction estimations, suggests ponds to be more sensitive to hydroclimate changes, as evidenced by the strong seasonality in evaporative enrichment and high fractions of young water contributions. In contrast, most streams indicated greater mixing of water sources and longer transit times, suggesting greater resilience to hydroclimate variability. In addition, a comparison between seasonally sampled data and monthly sampling for selected locations in Berlin showed that even relatively coarse temporal data collection, but with more extensive spatial coverage, can be sufficient and still insightful for broader hydrologic characterizations of aquaNBS at larger scales.