Stygobitic invertebrates in groundwater : a review from a hydrogeological perspective

Groundwater-adapted species (known as stygobites) provide an important contribution to biodiversity. Groundwater ecosystems are some of the oldest on earth, and contain many endemic species adapted to live in an environment with no light and limited resources. The controls on stygobite distributions are not yet fully resolved because of the complex interaction between many processes operating at different scales. Many of these processes are geological or hydrogeological in nature and therefore more detailed geological and hydrogeological studies could provide improved understanding of stygobite distributions. Hydrogeologists can assist ecologists by providing expertise on both general geological characteristics of sampling sites, and how groundwater at sampling sites relates to the wider aquifer setting. Geological input would be especially useful in stygobite dispersal studies because dispersal depends upon habitat continuity associated with geological dispersal corridors, and is limited where rocks that do not provide a suitable habitat form geological barriers. Stygobite studies are of benefit to hydrogeology because stygobite distributions can provide information on ground¬water-surface water interaction and aquifer connectivity over a range of spatio-temporal scales. Future studies using DNA analysis of stygobites may provide much more detailed information on hydraulic connectivity within and between aquifers. There is also potential for the development of stygobites as indicators of groundwater quality. The biogeochemical function of stygobites is of interest to both hydrogeologists and ecologists. Studies have demonstrated that stygobites graze biofilms and bacteria but their role in biogeochemical cycles is still not fully understood. Ecosystem services provided by groundwater fauna depend upon their abundance and biomass. Future studies using hydrogeological data (e.g. borehole packer techniques) may provide an improved understanding of where in aquifers stygobites live and how many there are, which would be an important step towards assessing the significance of their role in biogeochemical cycling of nutrients and carbon.