To make a long story short, the freshwater mussels that grew in the monitoring experiment for over a year, indeed nicely recorded the environment. In this case that means their shell oxygen isotopic composition (δ18O values) reflected the δ18O values and temperature of the water they were living in:
water δ18O + temperature → shell δ18O
The next thing to find out was if this could be used to reconstruct floods or droughts in a river. In the river Meuse, there is a logarithmic relationship between discharge and water δ18O values.
By analysing shells from a wet time interval (1912-1918) and an extremely dry time interval (1969-1977) we found out that droughts with a very low river discharge are readily recorded by the shells. This is logical if you look at the graph. The water δ18O composition does not change very much between average and high discharges, but values become much higher when discharges (Q) are very low (at the left side of the graph).
This means that when we are going to analyse old shells, from archaeological finds or palaeogeographical samples in the Meuse River area, we will be able to recognise droughts.
If you’re interested in reading the full story, it can be found here (open access – free of charge).