Much of the hydroecological and ecohydrological literature refers to 'overt' interactions in surface ecosystems such as the response of fish to hydraulics in a river (Nestler et al., Chapter 12 in this volume) or the hydroecological variability evident in vegetation distribution in wetlands (Burt et al., Chapter 14 in this volume). It is often easy to see how ecological features such as fish behaviour or water plant distribution are associated with hydrological variables. Less obvious are the 'covert' interactions where groundwater hydraulics play a role in the ecology of surface ecosystems, either through the direct supply of water for so-called groundwater dependent ecosystems (GDEs, Boulton and Hancock, 2005) or by acting as a vector for critical nutrients (Jones et al., 1995) or thermal energy (Power et al., 1999) required by surface biota and processes. However, as many lakes and rivers are largely fed by groundwater (Winter et al., 1998: Wood et al., 2001) the ecohydrological significance of this covert interaction is greater than often appreciated, and this has important ramifications for management of these acquatic ecosystems (Woessner, 2000).