| Summary: | Groundwater is an important resource for drinking water, agriculture, and industry, but it also plays an essential role in supporting the functioning of freshwater ecosystems and providing habitat for a number of rare species. However, despite its importance, groundwater ecology often receives little attention in environmental legislation or research. This study aims to improve our understanding of the organisms living in groundwater-dependent habitats and the influence of environmental conditions on their distribution. Invertebrate communities occurring in the benthic, hyporheic and phreatic habitats were surveyed at twelve sites over four years across the Stour Chalk Block, a lowland catchment in southern England. A diverse range of stygoxenes, stygophiles and stygobionts, including the first record of Gammarus fossarum in the British Isles, were identified using morphological and molecular techniques. The results indicate that under normal conditions, each habitat provided differing environmental conditions which supported a distinctive invertebrate community. While the community recorded in the benthic habitat was characterised by a diverse assemblage of surface water species typical of Chalk streams, the phreatic community comprised a small number of exclusively crustacean stygofauna (such as Niphargus kochianus and Crangonyx subterraneus) and the hyporheic habitat supported a mixture of surface and groundwater species. Surprisingly, the results indicate that some species, such as Agapetus fuscipes (normally considered a surface water taxon), move into the hyporheic habitat in a predictable, seasonal pattern, potentially in response to grazing opportunities. However, the results collected during the high and low flow events which occurred during this study also show the widespread movement of multiple species (such as Gammarus pulex and Niphargus fontanus) between habitats in response to environmental disturbance. Collectively, these results reflect the movement of fauna longitudinally, laterally and vertically over time throughout the catchment, as though along a continuum rather than between three separate habitats. This suggests that our conceptualisation of lotic functioning should be expanded to better integrate the contribution from groundwater. The approach taken by this study provides a greater understanding of the full diversity of aquatic invertebrates within this catchment and the way in which their distribution fluctuates across habitats. This study is one of the first to concurrently assess invertebrate distribution across the benthic, hyporheic and phreatic habitats; in addition, the relatively frequent and long-term sampling approach also facilitated a more detailed temporal assessment of these communities. A greater understanding of the distribution and requirements of the fauna inhabiting groundwater-dependent habitats, and their response to environmental change is essential for the conservation of these species and management of lotic ecosystems.
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