Syuhei Ban

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 along the water flow path in lagoon–lake system adjacent to Lake Biwa, Japan. The δ 18 O p values of inflowing water originating as agricultural runoff were constant throughout the study period at +16.3‰ ± 0.2‰. The δ 18 O p values in the system were generally offset from temperature‐dependent isotopic equilibrium with the surrounding water, ranging from +11.1‰ to +17.8‰. The δ 18 O p values of the lake water approached equilibrium values in July and October, when dissolved inorganic P (DIP) retention rates were high, consistent with extensive biologically mediated phosphate cycling. A δ 18 O p two end‐member mixing model, involving inflowing P and biologically recycled P, suggests that P turnover rates in the lagoon–lake system were high during the productive seasons. In contrast, the longer lake water residence time in the non‐irrigation season (winter) allowed δ 18 O p values to deviate toward lower values relative to both equilibrium and agricultural source δ 18 O p values, suggesting that P metabolism was dominated by extracellular/ecto‐enzymatic hydrolysis of dissolved organic P under low DIP concentrations. This work highlighted the utility of δ 18 O p for understanding P dynamics in shallow lake ecosystems.