Geochemical controls on rare earth element and yttrium bioaccumulation in Unio elongatulus from contrasting subalpine lakes

Rare earth elements and yttrium (REY) are increasingly recognized as elements of emerging environmental interest in aquatic ecosystems, yet the factors governing their accumulation in freshwater organisms remain poorly constrained. Here, we investigated REY bioaccumulation and fractionation in soft tissues and shells of the freshwater mussel Unio elongatulus from two subalpine Italian lakes with contrasting geochemical settings. REY concentrations were determined by ICP–MS/MS and normalized to European Shale in order to distinguish natural geogenic signatures from potential anthropogenic inputs. Soft tissues accumulated up to 28-fold higher ΣREY than shells, reflecting short-term versus long-term integration, and were consistently enriched in light REE (67–79% of ΣREY). Shale-normalized patterns showed [La/Yb] _SN > 1 in both matrices, together with moderate positive La, Gd, and Eu anomalies and weak negative Ce anomalies. These features indicate that geogenic processes, including feldspar weathering and redox-controlled Ce behaviour, are the dominant drivers of REY bioavailability. Shells from Lake Maggiore displayed slightly higher ΣREY than those from Lake Orta, in agreement with greater sediment inputs and natural REY supply. Overall, U. elongatulus integrates ambient REY signatures across different biological compartments and shows potential as a biomonitor of geochemical gradients in freshwater ecosystems. Our findings indicate that REY accumulation in this species is primarily governed by natural geogenic factors rather than anthropogenic pollution, with implications for the environmental assessment of trace elements in freshwater environments.