Impacts of polyaluminum chloride flocculant on biochemical indicators in freshwater mollusks: a mesocosm approach

Introducing polyaluminum chloride (PAC) flocculant and mollusks (snails and mussels) into eutrophic lakes is a means of restoring the growth and health of submerged macrophyte plants. This study assessed the toxic impacts of PAC application on biochemical and mortality indicators of mollusks, including species-specific sensitivities. A 9 days mesocosm experiment was performed to examine the effects of PAC (20, 40, and 60 mg·L-1) on three mollusk species: Anodonta woodiana mussels, Bellamya aeruginosa snails, and Radix swinhoei snails. At medium and high PAC concentrations, the mortality rates of A. woodiana and R. swinhoei (but not B. aeruginosa) were higher than in PAC-free control groups, with mortality being highest for A. woodiana under all conditions. Polyaluminum chloride flocculant induced oxidative stress in these freshwater mollusks. This manifested as altered SOD and CAT activities, with significant inhibition observed under moderate and high PAC exposure. The magnitude of biochemical responses was closely linked to species-specific physiological stress responses to PAC. All three mollusk species bioaccumulated aluminum in their tissues to a significant degree under medium and high PAC exposure. Tissue Al concentrations in A. woodiana (high PAC exposure) increased consistently over time, whereas in all other groups an increase followed by declining Al was observed during the exposure period. Thus, filter-feeding mussels exhibited greater sensitivity to PAC than scrape-feeding snails. Such species-specific impacts of PAC on mollusks could have broader ecological consequences for aquatic food webs and the biodiversity of eutrophic lakes. Management of eutrophic lakes should minimize the concurrent release of mussels and PAC flocculants, ideally replacing mussels with alternative species.