Freshwater pearl mussels as stream water stable isotope recorders (MUSES)

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Isotope records of streamflow are pivotal for improving our understanding and modelling of hydrological, ecological, biogeochemical and atmospheric processes – yet the use of their full potential is hindered by short and truncated time series. Here we propose an innovative contribution to the problem of stream isotope record limitations: the use of freshwater bivalves as long-term stream water delta18O recorders. We have done pilot work by analysing mollusc shell delta18O data (compiled from 10 studies), spanning a latitudinal sequence of 18 sampling sites on 16 streams around the globe. We found strong links between isotope signatures in precipitation, stream water and freshwater molluscs – both stream water and mollusc showing a strong damping of the precipitation signal. Our exploratory work on freshwater mussels has blazed a trail that we now intend to follow for multi-decadal stream water delta18O reconstructions. First, we will test the conjecture that stream water temperature – as a prerequisite for reconstructing stream water delta18O from shell delta18O – can be inferred from (a) shell growth rate, (b) shell microstructural properties and/or (c) element chemical properties of the mussel shell. Second, we will benchmark IRMS and SIMS and test their full compatibility and complementarity in terms of mollusc shell delta18O measurement accuracy. While IRMS is a standard tool for measuring delta18O in mollusc shells, SIMS may extend – with high measurement accuracy – delta18O data to later ontogenetic shell portions. Third, we will rely on measured multi-decadal precipitation delta18O data and the reconstructed delta18O signals in stream water from three study catchments in Luxembourg, Sweden and Germany for testing our hypothesis that bedrock geology controls the long-term sensitivity of a catchment’s hydrological response to climate change. For testing our hypothesis, we will rely on the mechanistic relationship between the fraction of young water (Fyw) increasing with discharge – indicating an increase in the proportional contribution of rapid flow paths towards higher flows. In our study catchments with distinct physiographic characteristics and climates, we hypothesise the discharge sensitivity of Fyw (i.e., the linear slope of the relationship between Fyw and discharge) to be unresponsive to a potential inter-decadal variability in precipitation and subsequent stream water delta18O signals.