Can shells of freshwater mussels (Unionidae) be used to estimate low summer discharge of rivers and associated droughts?

This paper examines if shell oxygen isotope ratios (delta O-18(ar)) of Unio sp. can be used as a proxy of past discharge of the river Meuse. The proxy was developed from a modern dataset for the reference time interval 1997-2007, which showed a logarithmic relationship between discharge and measured water oxygen isotope ratios (delta O-18(w)). To test this relationship for past time intervals, delta O-18(ar) values were measured in the aragonite of the growth increments of four Unio sp. shells; two from a relatively wet period and two from a very dry time interval (1910-1918 and 1969-1977, respectively). Shell delta O-18(ar) records were converted into delta O-18(w) values using existing water temperature records. Summer delta O-18(w) values, reconstructed from delta O-18(ar) of 1910-1918, showed a similar range as the summer delta O-18(w) values for the reference time interval 1997-2007, whilst summer reconstructed delta O-18(w) values for the time interval 1969-1977 were anomalously high. These high delta O-18(w) values suggest that the river Meuse experienced severe summer droughts during the latter time interval. delta O-18(w) values were then applied to calculate discharge values. It was attempted to estimate discharge from the reconstructed delta O-18(w) values using the logarithmic relationship between delta O-18(w) and discharge. A comparison of the calculated summer discharge results with observed discharge data showed that Meuse low-discharge events below a threshold value of 6 m(3)/s can be detected in the reconstructed delta O-18(w) records, but true quantification remains problematic.