<p>Eco-enzymes, promoted as natural solutions for water purification, have been applied in tropical lakes due to it is reported success as a biofertilizer, antiseptic, and water treatment agent. However, their use has been based on limited scientific evaluation. This study evaluates the effects of eco-enzyme application through laboratory-scale experiments on the water chemistry and ecological dynamics of Lake Batur, Bali, focusing on phosphate speciation, pH changes, and phytoplankton responses. Surface water and demineralized water were treated with various eco-enzyme formulations under controlled conditions. Results indicated that eco-enzyme addition caused a pH decrease and a significant increase in soluble phosphate concentrations. Geochemical modeling revealed that acidic eco-enzymes shift mineral equilibrium from calcium-bound phosphate (hydroxyapatite) to available phosphate forms. The shift in pH also enhanced the availability of dissolved carbon dioxide, which, together with elevated phosphate, temporarily stimulated phytoplankton growth. A short-term increase in phytoplankton abundance occurred within three days, followed by a sharp decline, suggesting transient eutrophication followed by nutrient depletion. These findings indicate that eco-enzyme applications can substantially alter lake biogeochemistry, where both increased phosphate bioavailability and pH-driven carbon dynamics may trigger phytoplankton proliferation, leading to unintended eutrophication effects if not carefully managed.</p> Graphical Abstract <p></p>