Section introduction: groundwater sciences in limnology

Groundwater is quantitatively the most important source of freshwater and sustains surface terrestrial and aquatic ecosystems and human beings across the globe. Groundwater has for millennia been used as a drinking water source, for recreation and therapy (e.g. thermal springs), as well as for irrigation. Scientific disciplines targeting abiotic aspects of groundwater (e.g. hydrology, hydrogeology, geology, geochemistry) have a long history. However, research on the biotic and ecological aspects of groundwater, particularly subterranean ecosystems started comparatively late, about 100years ago, first looking into the biodiversity of fauna and later into microbial communities. By definition, groundwater of the terrestrial subsurface is inland water, making groundwater ecology a sub-discipline of limnology. Despite this, standard textbooks in limnology largely ignored groundwater and aquifers as limnic ecosystems (Welch, 1952; Ruttner, 1962; Lampert and Sommer, 2007; Schwoerbel, 1999; Wetzel, 2001). Only recently, limnology textbooks have paid attention to subsurface aquatic habitats (Schönborn, 2003; Schwoerbel and Brendelberger, 2013; Dodds and Whiles, 2020). There is a handful of books focusing on groundwater microbiology and biogeochemistry (e.g. Chapelle, 2000; Cullimore, 2007; Ferris et al., 2020). The first book solely dedicated to groundwater ecology appeared in 1994 edited by Janine Gibert, Dan Danielopol, and Jack Stanford. This year (2022) a completely reworked 2nd edition will be released (Malard et al., 2022). While there are a few other books on groundwater fauna (Spandl, 1926; Chappuis, 1927; Botosaneanu, 1986) and subterranean ecosystems including caves (Vandel, 1965; Wilkens et al., 2000; Culver and Pipan, 2014), groundwater ecology textbooks have never been published, with the exception of the German textbook by Griebler and Mösslacher (2003). This second edition of the Encyclopedia of Inland Waters is a unique opportunity to have key groundwater sciences topics introduced as part of limnology.