Disentangling urban and hydrogeological influences on groundwater fauna in Halle (Saale)

Groundwater fauna play an important role in subterranean aquatic ecosystems. In urban areas, their habitats are shaped not only by hydrogeological conditions but also by stressors such as elevated groundwater temperatures and oxygen depletion. However, the combined effects of urbanisation and geological factors on groundwater fauna remain poorly understood.

To characterise urban influences and contrast them with hydrogeological controls, abiotic and faunal data were collected from 91 groundwater monitoring wells in Halle (Saale) over the course of one year, comprising five measurement campaigns. Both the urban area and the surrounding rural region were investigated. The hydrogeological setting of the city is highly variable due to diverse near-surface geological formations, resulting in multiple aquifer types across several hydrostratigraphic units.

The urban gradient was characterised by elevated temperatures (>12 °C) in the city centre, while differences in dissolved oxygen (DO) and dissolved organic carbon (DOC) reflected both urban and hydrogeological influences. Spatial patterns were evident in the regional variation of faunal community composition. However, these patterns did not clearly correspond to contrasts between urban and rural areas or to specific aquifer types. Instead, fauna in near-surface aquifers were more strongly influenced by hydraulic conductivity and groundwater depth. Crustaceans were primarily found at wells with groundwater levels shallower than 6 m, whereas worms (Oligochaeta, Polychaeta, Platyhelminthes) dominated at wells with deeper groundwater levels.

The abundance of stygofauna and the number of taxonomic groups showed significant, albeit weak, correlations with redox-relevant parameters (DO, Eh, NH₄⁺, NO₃⁻ and DOC), with higher DO concentrations generally being associated with higher abundance and diversity. We also observed a weak negative correlation with temperature, which was particularly pronounced in combination with low DO concentrations.

These findings demonstrate the necessity for an integrative approach to assessing complex urban groundwater ecosystems, taking into account the interactions between abiotic and biotic factors within the context of the respective hydrogeological setting.