Life below the City – Impacts of urbanization and subsurface heat islands on groundwater fauna in the city of Vienna

Groundwater is one of our most important and heavily utilized resources. However, it is also home to a variety of microbes and fauna, that have adapted to a dark, cold, and typically energy-poor environment for thousands of years. Therefore, groundwater is a habitat with high numbers of endemic and cryptic species as well as hidden biodiversity hotspots. These highly specialised animals, while assumed to provide vital ecosystem services including water purification, are susceptible to intermediate-term (years to decades) changes in environmental conditions. In urban areas, a multitude of pressures like increased temperatures, extensive surface sealing and pollution are impacting groundwater ecosystems with deteriorating effects on biodiversity and groundwater quality. Aiming to reveal key factors of spatio-temporal biodiversity patterns, fauna community composition and links between subsurface urban heat islands, species richness and water quality, 150 groundwater wells in the city of Vienna were sampled in autumn 2021 and in spring 2022 to include seasonal variability. Focusing on subsurface heat accumulation as a main driver, a large set of biotic and abiotic variables was analysed as part of the project “Heat below the City”. The results show that the mean groundwater temperature of 14°C in Vienna is about 2 K above the natural background, with anthropogenic heat sources having an impact on the degree of warming and groundwater fauna composition. The absence of dissolved oxygen (DO) and NO_3- as well as the presence of dissolved Fe²⁺, HS^- and CH₄ hint at zones with reducing conditions correlated with low faunal biodiversity. The application and comparison of several groundwater ecosystem health indices as well as the development of a robust habitat suitability assessment for groundwater fauna contribute to the establishment of an integrative groundwater management strategy, combining groundwater quality aspects, its sustainable use as source for drinking water, heating and cooling, and conservation strategies for groundwater biodiversity in the future.