Food availability impacts oxygen isotope composition in freshwater snail shells

Freshwater mollusk shells serve as valuable archives for the reconstruction of past environmental conditions as they record water temperatures in their stable isotopic compositions during biomineralization. Yet, the impact of food availability on these isotopic records has never been fully explored. This study addresses that gap by investigating the effects of both temperature and food regime on the stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions in the shells of the freshwater gastropod Lymnaea stagnalis. Under controlled laboratory conditions, snails were reared at six distinct temperatures and two food regimes (limited and unlimited). The results reveal strong but significantly different negative relationships between the δ¹⁸O fractionation factor and water temperature per food regime. The relationship for the limited food regime is expressed as: T (°C) = (19.99 ± 0.16) − (4.30 ± 0.16) × (δ¹⁸O_a−w) + (0.20 ± 0.09) × (δ¹⁸O_a−w)². For the unlimited food regime as follows: T (°C) = (21.60 ± 0.24) − (4.33 ± 0.14) × (δ¹⁸O_a−w) + (0.20 ± 0.09) × (δ¹⁸O_a−w)². This study identified a 1.63°C ± 0.03°C offset in predicted temperatures between the two food regimes likely due to metabolically induced changes to carbonate δ¹⁸O fractionation. This offset introduces a previously unrecognized source of uncertainty in paleotemperature reconstructions and underscores the need to account for food availability when interpreting δ¹⁸O data. In addition, δ¹³C values exhibit high variability and no significant correlation with temperature for both food regimes, probably because the effect of temperature is overprinted by that of metabolism.