Reducing excess phosphorus (P) loads that cause eutrophication in aquatic systems is essential for meeting water quality standards. The oxygen isotopic composition of phosphate (δ 18 O p ) is a powerful tool for tracking P sources and cycling in diverse natural ecosystems. Here, we use δ 18 O p distribution in a small freshwater body (a lagoon–lake system) with high biological activity. We report δ 18 O p values seasonally alo...