This essay highlights the advantages and challenges of integrating the scientific disciplines of ecology and hydrogeology in the study of groundwater-dependent ecosystems (GDEs). We define ecohydrogeology as “a unifying, synthetic field of study integrating the approaches from the ecological and hydrogeological sciences in the study of groundwater (GW)-related ecosystems, habitats, and organisms to advance science, stewardship, and policy”. Specific case studies are selected to illustrate first how hydrogeological approaches can favour in-depth understanding and modelling of springs and crenobiontic (spring-dependent) species distribution, assemblage composition and organization. Second, how taxa and assemblages serve as bioassays and ecosystem indicators to infer hydrogeological aspects of GW flow and discharge, as well as quantitative and qualitative human impacts. We define both types of features and parameters as ecohydrogeological indicators. The presented examples span from topics related to the impacts of variation in springs and other GDE geomorphological types and classification, GW quality influences on crenobiont distribution, phreatophyte ecophysiology in relation to water table depth, and flow variability in karstic systems, to nutrient dynamics in relation to dinoflagellate blooms in GDE montane lakes. Conceptual approaches that integrate ecology with hydrogeology include the investigation of GDE distribution and ecology, groundwater-surface water (GW-SW) interactions, and the development of the discipline of ecohydrology. Despite widespread applications, we are still far from a complete and effective integration of groundwater hydrogeology with ecology, ecophysiology, and environmental biology. Springs are aquatic-wetland-riparian habitats that link shallow subsurface-surface processes and assemblages, often functioning as biodiversity hotspots, ecotones, keystone, and refugial ecosystems, for which coordination between hydrogeology and ecology are both obvious and essential. Over the past century, springs ecosystem ecology has been largely ignored by hydrologists, and, conversely, hydrogeology has been under-emphasized by ecologists. Recent global recognition of the extraordinary biodiversity and socio-cultural significance of springs, coupled with their universally highly threatened conservation status, stimulated this inquiry into how to better integrate hydrogeology with springs ecology. Given the highly threatened status of springs ecosystems around the world, it is time to integrate and invigorate the union of these two disciplines into ecohydrogeology, the study of groundwater- dependent organisms, habitats, and ecosystems.
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