DECIPHERING SHIFTS IN THE SEASONAL ORIGIN OF PRECIPITATION IN STREAM WATER OVER THE PAST TWO CENTURIES: INSIGHTS FROM OXYGEN ISOTOPES OF FRESHWATER PEARL MUSSEL SHELLS FROM SWEDEN

Oxygen isotopes in stream water are a powerful natural tracer of variations in the seasonal water budget. For example, a combined seasonally resolved record of δ¹⁸O in precipitation and stream water can be used to determine the relative contributions of winter vs summer precipitation to stream flow. If related data were available well before the instrumental era, it would open new vistas to better understand catchment responses to climate change. Although machine learning has improved the ability to model δ¹⁸O values in precipitation in the last century, the absence of long-term (multidecadal to centennial) stream water δ¹⁸O records poses a challenge to determine past changes in the origin of moisture from which stream water formed. Shells of long-lived (>200 years) freshwater pearl mussels (Margaritifera margaritifera) can fill that gap and provide temporally well-constrained stream water δ¹⁸O chronicles, because bivalves can encode the isotope composition of the water in which they lived in their shells in chronological order.

Here, we present a shell-based, monthly resolved stream water δ¹⁸O chronology from a creek in northern Sweden covering the time interval from 1814 to 2023. This dataset comprises more than 1,600 isotope data representing stream water conditions during the main growing season of the mussels (June to August). As shown by comparison of reconstructed stream water δ¹⁸O data with modeled volume-weighted precipitation δ¹⁸O values, until the mid-20^th century, the contribution of winter and summer precipitation to runoff was relatively balanced. However, from 1960 onward a significant trend toward summer precipitation in stream water was observed. This trend coincided with an increase in the interannual variability of the origin of runoff. These observations indicate clear shifts in the way precipitation inputs are dispersed in the catchment, helping to assess the sensitivity of the watershed to climate change and potentially develop mitigation strategies to stabilize ecohydrological conditions.