Hidden effects of aluminum coagulant on freshwater mussels: Responses of Anodonta anatina and Unio tumidus to lake restoration

Methods of chemical lake restoration, such as dosage with aluminum (Al)-based coagulants, are widely employed due to their rapid and effective action. However, the ecological implications of these methods, especially their potential adverse effects on aquatic biota, remain poorly understood. In this study, we investigated the impact of a commonly used coagulant, polyaluminum chloride (PAC), on two freshwater mussels: Anodonta anatina Linnaeus, 1758 and Unio tumidus (Philipsson, 1788), at restoration relevant doses (2 - 20 g Al m^-2). We analyzed physicochemical properties of water, filtration behavior, elemental composition of muscle tissue, and oxidative stress parameters. The PAC application significantly impaired the filtration activity in both species in a clear dose-dependent relationship. Concentrations exceeding 10 g Al m^-2 induced acute, nearly complete inhibition that persisted up to 72 h. Tissue chemistry changed significantly, with strong dose-dependent Al bioaccumulation. After 15 days of exposure at a dose of 20 g Al m^-2, Al concentrations in U. tumidus muscle tissue increased from below the detection limit to 7000 mg kg^-1 dry weight (d.w.), whereas in A. anatina they exceeded 10,000 mg kg^-1 d.w. In contrast, no indicators of oxidative stress were detected. Overall, exposure to Al-based coagulants, particularly at elevated doses, elicited multiple adverse physiological and biochemical effects in freshwater mussels, highlighting ecotoxicological risks associated with their use.