Biodiversity and water conservation challenges in urban blue–green infrastructure under climate extremes

Urban blue–green infrastructure (UBGI) is essential to addressing urbanization challenges. However, its potential to mitigate climate extremes remains unclear. This study assessed biodiversity and water conservation challenges in UBGI using testate amoebae (TA) as indicators of ecosystem health. The studied UBGI consists of 0.67 km² of forested hills and 3 668 m² of ponds, located in the 55 800-km² city cluster in the Pearl River Delta, South China. The analysis incorporated a two-year (June 2021 and June 2022) dataset, comprising TA records from 27 soil samples, 30 pond water samples, and 30 pond sediment samples; 27 microspatial factors, including five factors representing weather conditions (WC), eight factors for air quality (AQ), and 14 factors for water quality (WQ); and four climate extreme scenarios (heatwaves, droughts, typhoons, and floods). Biodiversity and water quality concerns linked to interactions between urban emissions and aquatic ecosystems within UBGI were illuminated in the following three key findings: (1) biotope connectivity enabled redistributions of soil-specific TA (six out of 42 species) along hillslopes (moisture gradient) and pond lengths (hydrological gradient); (2) TA showed strong biotope adaptation, with stochastic processes explaining 69.3% of community variations in water and 78.8% in sediment; and (3) UBGI showed limited effectiveness in mitigating urban emissions, such as CO and NH₃, particularly when TA were adversely impacted by WC driven by climate extremes and when WQ was adversely influenced by AQ. The findings suggest that TA are reliable bioindicators, informing UBGI performance and supporting climate-resilient interventions to monitor cross-border pollution and its effects at the biotope level.