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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. [ABSTRACT FROM AUTHOR]

Groundwater ecosystems remain poorly understood yet may provide ecosystem services, make a unique contribution to biodiversity and contain useful bio-indicators of water quality. Little is known about ecosystem variability, the distribution of invertebrates within aquifers, or how representative boreholes are of aquifers. We addressed these issues using borehole imaging and single borehole dilution tests to identify three potential aquifer habitats (fractures, fissures or conduits) intercepted by two Chalk boreholes at different depths beneath the surface (34 to 98 m). These habitats were characterised by sampling the invertebrates, microbiology and hydrochemistry using a packer system to isolate them. Samples were taken with progressively increasing pumped volume to assess differences between borehole and aquifer communities. The study provides a new conceptual framework to infer the origin of water, invertebrates and microbes sampled from boreholes. It demonstrates that pumping 5 m3 at 0.4–1.8 l/sec was sufficient to entrain invertebrates from five to tens of metres into the aquifer during these packer tests. Invertebrates and bacteria were more abundant in the boreholes than in the aquifer, with associated water chemistry variations indicating that boreholes act as sites of enhanced biogeochemical cycling. There was some variability in invertebrate abundance and bacterial community structure between habitats, indicating ecological heterogeneity within the aquifer. However, invertebrates were captured in all aquifer samples, and bacterial abundance, major ion chemistry and dissolved oxygen remained similar. Therefore the study demonstrates that in the Chalk, ecosystems comprising bacteria and invertebrates extend from around the water table to 70 m below it. Hydrogeological techniques provide excellent scope for tackling outstanding questions in groundwater ecology, provided an appropriate conceptual hydrogeological understanding is applied. [ABSTRACT FROM AUTHOR]