Biodiversity in mountain groundwater: The Mercantour National Park (France) as a European hotspot

Mercantour National Park (France) is recognized as a highly heterogeneous region with extremely varied geology, geomorphology and climatology, resulting in an exceptional biodiversity. From a hydrogeological point of view, it is also an area organized into small and discontinuous aquifers, the obligate groundwater fauna of which (stygobionts) remains absolutely unknown. This work explores the species richness of groundwaters in the Mercantour National Park, using a sampling design at the catchment (six major valleys) and aquifer (aquifers in consolidated rocks and unconsolidated sediments) scales. A major finding of this study is the discovery of 44 species restricted to groundwater, of which 43 are new to the Park and ten are new to Science. Although a relatively small number of sites were sampled (53), the area may be considered as a new hotspot of groundwater biodiversity at the European level. The particular structure of the groundwater network, the high environmental heterogeneity of the region and its Mediterranean position may explain such a high biodiversity. The species rarefaction curve showed that many species have yet to be discovered in groundwater of the Mercantour National Park. With more than 78% of species collected in the hyporheic zone, this study also highlights the importance of porous aquifers in sustaining the groundwater biodiversity of mountainous regions.