Regional-scale drivers of groundwater faunal distributions

Freshwater aquifers are a major source of drinking water. They also possess unique assemblages of organisms. However, little is known about the distributional drivers of obligate groundwater organisms at the regional scale. We examined the distribution and composition of stygobiont assemblages in a complex geological setting and explored the relationship between groundwater fauna, hydrogeology, and water chemistry. In the study area, we grouped similar geologies into 5 hydrogeological formations (hydro-units [HUs]) within which habitats for groundwater fauna were broadly similar. The occurrence of stygobionts differed significantly between HUs. Stygobionts were significantly less likely to be recorded in mudstone/siltstone and sandstone aquifers than in carbonate or igneous/metamorphic rocks. Variance partitioning indicated that HU explained a greater proportion of the variance (7.52%) in the groundwater community than water chemistry (5.02%). However, much of the variation remained unexplained. The macrofaunal stygobiont species in our study area formed 3 groups: 1) Niphargus glenniei was recorded in a range of HUs but only in the western part of the study area. 2) Niphargus kochianus, Niphargus fontanus, Proasellus cavaticus, and Crangonyx subterraneus were predominantly recorded in carbonate aquifers in the eastern part of the study area. 3) Niphargus aquilex and Microniphargus leruthi were found throughout the study area and in a range of HUs. We hypothesize that physical barriers prevent some stygobiont taxa from colonizing apparently suitable geologies. The low-permeability deposits dividing the western and eastern parts of the study area may partly explain the observed distributions.