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Urban Aquatic Nature-Based Solutions in the Context of Global Change: Uncovering the Social-ecological-technological Framework
Diversification of macrophytes within aquatic nature-based solutions (NBS) developing under urban environmental conditions across European cities
Understanding ecohydrology and biodiversity in aquatic nature-based solutions in urban streams and ponds through an integrative multi-tracer approach
Abstract. Rapid urbanization and climate change affect ecohydrology, biodiversity, and water quality in urban freshwaters. Aquatic nature-based solutions (aquaNBSs) are being widely implemented to address some of the 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 aquaNBSs and their relationship to water quality and quantity issues at the ecosystem level. Through a novel, integrative multi-tracer approach using stable water isotopes, hydrochemistry, and environmental DNA we sought to disentangle the effects of urbanization and hydroclimate on ecohydrological dynamics in urban aquaNBSs and understand ecohydrological functioning and the future resilience of urban freshwaters. Stable isotopes and microbial data reflected a strong influence of urban water sources (i.e., treated effluent, urban surface runoff) across stream NBSs. The results show potential limitations of aquaNBS impacts on water quality and biodiversity in effluent-impacted streams, as microbial signatures are biased towards potentially pathogenic bacteria. Urban ponds appear to be more sensitive to hydroclimate perturbations, resulting in increased microbial turnover and lower microbial diversity than expected. Furthermore, assessment of macrophytes revealed low diversity and richness of aquatic plants in both urban streams and ponds, further challenging the effectiveness of NBSs in contributing to aquatic diversity. This also demonstrates the need to adequately consider aquatic organisms in planned restoration projects, particularly those implemented in urban ecosystems, in terms of habitat requirements. Our findings emphasize the utility of integrated tracer approaches to explore the interface between ecology and hydrology and provide insights into the ecohydrologic functioning of aquaNBSs and their potential limitations. We illustrate the benefit of coupling ecological and hydrological perspectives to support future NBS design and applications that consider the interactions between water and the ecosystem more effectively.
Assessing the sensitivity of urban aquatic nature-based solutions to hydroclimate variability using stable water isotopes
Abstract 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.