Karst springs, groundwater dependent ecosystems and global changes: a few examples from France

International audience

English. In addition to the habitat they provide to microbiological fauna (Griebler and Avramov 2015), groundwaters contribute to numerous ecosystem services, including the production and distribution of high-quality water and the feeding of groundwater-dependent aquatic ecosystems such as rivers, lakes and wetlands. This role is particularly important during dry periods, when the water stored in aquifers supplies significant quantities to surface waters that are at low flow.Springs are the interface between aquifers and surface water, making them ideal sites for monitoring groundwater and its ability to provide services to downstream ecosystems. Among these, karst springs are particularly noteworthy for the quantity of water they convey to a specific point. Karst accounts for 21.6% of Europe's surface area (Chen et al. 2017) and some of the ecosystem services it provides have been already described by Goldsheider (2012). However, the supply of water to aquatic ecosystems remains a largely undervalued and understudied feature.In this presentation, we highlight the role of karstic springs in various ecohydrological contexts of France (Fig. 1) and the risks that dependant ecosystems will face in a context of global changes.The Argens is a river that flows through the Triassic limestones of Provence Verte. With a constant temperature throughout the year, the bouillidoux karst springs help to regulate the river's temperature by decreasing it in summer and increasing it in winter. The section of river downstream from the spring is a thermal refugee that is home to significant biodiversity, like other thermal refugees in the south of France. However, these are vulnerable to climate change, which will alter the way they function, as well as the respective inflows of groundwater and surface water, and the temperatures of these waters.The Vise Spring is a submarine karst spring located in the Thau Lagoon in southern France, on the Mediterranean coast. Its karst hydrosystem regularly supplies freshwater to the lagoon, contributing to its quality and ecological balance. Shellfish farming is a major activity here, but other uses of the groundwater also coexist: thermal water for the Balaruc Spa and drinking water supply for neighbouring villages. This spring is the site of regular flow reversals, which cause salt water intrusion from the lagoon into the karst aquifer, disrupting the hydro-ecological balance of the lagoon (Ladouche et al. 2023). The chronic drop in the water level in the aquifer, linked to repeated droughts and the increasing use of groundwater, is leading to an acceleration in the number of flow reversal phenomena. In addition, the particular geometry of the site makes it vulnerable to long-term or even permanent flow reversal and salt water intrusion.The karst springs of the Loue and Lison Rivers are among the largest in France in terms of flow. They emerge in the Jura Mountains and feed the Loue River, which draws most of its flow from the surrounding limestone plateaux. For more than a decade, numerous contaminations of the rivers have led to fish kills, linked in particular to major inputs of nutrients flowing through the karst aquifers. Farming practices linked to cattle rearing for cheese production need to be improved to reduce inputs in the upstream basins.These karst springs, which are highly sensitive to climate change and modifications in land use in their catchment areas, constitute sentinels of global change. As vectors of groundwater, they link upstream and downstream ecosystems and guarantee major ecosystem services. Monitoring them is vital for gaining a better understanding of their ecological functions, which depend on their hydrogeological functions. In this context, a new research project called SentinelSpringS is starting within the Water4All program, with colleagues from Portugal, Italy, Poland, Denmark and Malta, in order to propose a better framework for managing groundwater resources and associated springs.