Urban groundwater fauna - natural or anthropogenically influenced?

Climate change and anthropogenic activities cause multiple changes in groundwater systems. Particularly, these processes lead to an increase in groundwater temperature under densely populated urban areas. While physico-chemical effects have been widely studied, the consequences for groundwater ecosystems are scarcely understood. However, a thorough understanding of how this sensitive ecosystem responds to various stressors, such as temperature, in urban environments is critical for a sustainable resource management.

Thus, the aim of this study is to provide an assessment of the groundwater fauna in and around the city of Karlsruhe, Germany. We examine the ecological status of an urban aquifer by analysing fauna and physico-chemical parameters in 39 groundwater monitoring wells between 2011 and 2022. For classification, we apply the groundwater ecosystem status index (GESI), in which a threshold of more than 70% of crustaceans and less than 20% of oligochaetes serves as an indication for very good and good ecological conditions. Our study reveals that only 35% of the wells in the residential, commercial and industrial areas, and 50% of wells in a natural forest area fulfil these criteria and can be classified as natural and unstressed groundwater habitats in 2011-2014. In 2022, however, all wells in the forest area show a very good or good ecological status, irrespective of changes in diversity and number of individuals. Repeated measurements in 2022 also show significant changes in groundwater fauna with only slight changes in the physico-chemical parameters over time. A significantly decreasing number of individuals per well together with a decreasing biodiversity both in the forest and urban areas is observed.

Overall, no clear spatial patterns in the ecological status are found with respect to land use and other anthropogenic impacts, such as groundwater temperatures. Nevertheless, we observe noticeable differences in the spatial distribution of groundwater species in combination with abiotic groundwater characteristics, such as groundwater temperature and geological settings. Monitoring wells in forest areas are characterized by lower groundwater temperatures, lower nitrate concentrations, and higher dissolved oxygen concentrations, indicating a link between abiotic groundwater characteristics and land use. In addition, these monitoring wells contain larger numbers of amphipods, which are considered as indicators of healthy ecosystems.

The results of our study reveal heterogeneous and time-varying conditions in urban and natural groundwater as a habitat, which do not allow a clear assessment of the ecological status with existing assessment approaches. In the future, additional indicators, such as groundwater temperature, local geology, presence of indicator species, delineation of site-specific characteristics and natural reference conditions, should be therefore also considered in ecological groundwater assessments.