Where springs save rivers: Karst springs as sentinels and refuges in a warming world

Karst aquifers play a fundamental role in sustaining groundwater-dependent ecosystems (GDEs) through their capacity to deliver large flows and highly buffered thermal regimes via springs. In Mediterranean environments, where hydrological variability and climate-driven extremes are intensifying, these groundwater contributions act as ecological stabilizers that support biodiversity and enhance ecosystem resilience. Drawing on two complementary case studies, this contribution examines how karst groundwater controls thermal and ecological dynamics in surface ecosystems: (i) the Argens River at Châteauvert (France), monitored within the ESTHER project, where the Bouillidoux spring system generates persistent cold-water refuges during summer low flows; and (ii) the Lez karst spring system near Montpellier (France), investigated under the SentinelSprings project initiative as a representative “sentinel” of long-term environmental trends, where the abstraction of water for drinking water supply competes with the river baseflow necessary to sustain aquatic ecosystems.

In both the Argens and Lez study sites, high-frequency monitoring networks were installed, including continuous temperature, electrical conductivity, and dissolved oxygen probes deployed in springs and river reaches. These datasets enable detailed thermal budgets to be established, revealing the mechanisms by which groundwater inflows generate thermal refuges and regulate stream metabolic conditions. The combined analysis of hydrological and thermal signals also supports the development of refined conceptual models that can be used to simulate groundwater–surface water interactions and quantify the potential impacts of climate change on spring-fed thermal regimes and river corridor resilience. These groundwater inputs sustain critical habitats, modulate the sensitivity of river corridors to warming, and provide natural buffering capacities that constitute important nature-based solutions for climate-change adaptation. By linking high-resolution monitoring with hydrogeological conceptualisation, our study advances the understanding of feedback processes between karst groundwater and surface ecosystems, highlighting springs as essential indicators and protectors of ecological stability. This work contributes to improving the identification, assessment and long-term management of GDEs.