Restoring the waters while polluting the food web – the unintended consequences of aluminum use in lake management

Aluminum-based coagulants are widely used to immobilize phosphorus during lake restoration, yet their transfer through aquatic food webs and potential effects on elemental homeostasis remain poorly resolved. We conducted microcosm experiments with the submerged macrophyte Myriophyllum spicatum and the freshwater snail Lymnaea stagnalis to test whether polyaluminum chloride (PAC), applied at restoration-relevant doses of 12.9 mg Al dm-3 and 24.9 mg Al dm-3, results in aluminum accumulation and associated stoichiometric disturbances. We explicitly separated two exposure routes: assessing trophic transfer (dietary exposure) by feeding snails macrophytes previously treated with PAC, and quantifying environmental exposure by exposing snails directly to PAC in the water column. Our findings reveal that Al treatments significantly altered short-term growth and the elemental composition of macrophytes, implying potential long-term consequences for nutrient cycling and ecosystem functioning. Moreover, both direct and dietary exposure routes lead to Al accumulation in snails, accompanied by imbalanced nutrient uptake, even in the absence of significant acidification during the initial phase of coagulant application, which is considered environmentally safe. These findings raise important questions about the presumed ecological neutrality of chemical restoration techniques and highlight the need for a more critical evaluation of their widespread application. By revealing the often-overlooked environmental costs of a widely implemented geoengineering practice, this study underscores the imperative for comprehensive, ecosystem-wide risk assessments in the management of aquatic environments.