Reconstructing the history of flowing waters and stream water isotopes from freshwater mussels

With the intensification of the hydrological cycle, the identification and assessment of factors controlling catchment climate resilience are key. A major obstacle to the design and implementation of precautionary measures against ‘once in a lifetime’ flood events is the still very limited understanding of the hydrological mechanisms involved. Along similar lines, the clustering of extreme events remains elusive until this day.Stable isotopes of O and H in streams and precipitation are cardinal tools for investigating questions related to water source, flowpaths and transit times. However, their spatial and temporal variability remain largely unknown – essentially due to the limited availability of long historical time series of O-H isotope signatures in stream water, as opposed to the multi-decadal records in precipitation.Based on their quality as natural archives of in-stream environmental conditions, freshwater mussels have been recently used for complementing stream water δ18O isotope records. Their potential is far from being exhausted, with nearly 1200 freshwater bivalve species inhabiting a large variety of river systems and lakes around the globe. Their average life span is ca. 10 years, even though many species can live much longer (up to 200 years for the freshwater pearl mussel). Here, we introduce an innovative avenue for pushing the boundaries in hydrological time series reconstruction even further. Our proof-of-concept work from the Our River (Luxembourg) is geared towards widening the portfolio of analysed proxies in shells, eventually extending the high-resolution (seasonally to annually resolved) reconstruction from stream water δ18O to river discharge.Note that the newly gained knowledge on multi-decadal and centennial changes in streamflow generation is of direct relevance for freshwater mussel population dynamics – an issue that is at the heart of all past and ongoing projects for the protection of freshwater molluscs.