Freshwater pearl mussels as a stream water stable isotope recorder

For several decades, stable isotopes have been a commonly used and effective tool
for flow path analysis, stream water source apportionment, and transit time analysis.
The Global Network of Isotopes in Precipitation repository now has monthly precipitation
isotope time series extending over several years and even decades in some
settings. However, stream water isotope composition time series remain rather short
with only very few data sets spanning over more than a few years. A critical challenge
in this respect is the collection of stream water isotope data sets across a wide variety
of headwater streams and for long durations. We rely on a new approach for stream
signal reconstruction based on freshwater mussels, specifically the freshwater pearl
mussel Margaritifera margaritifera. We use secondary ion mass spectrometry (SIMS)
to quantify oxygen isotope ratios in pearl mussel shell growth bands. In our study
area, the observed seasonal variability in precipitation δ18O values ranges between
−15‰ and −3‰. This input signal is strongly damped in stream water, where
observed values of δ18O range between −10‰ and −6.5‰. These values are consistent
with our measured average shell‐derived stream water δ18O of −7.19‰. Along
successive growth bands, SIMS‐based stream water δ18Ow values varied within a seasonal
range of −9‰ to −5‰. The proposed SIMS‐based shell analysis technique is
obviously well suited for analysing isotopic signatures of O in shell material—especially
from the perspective of reconstructing historical series of in‐stream isotope
signatures.