Using oxygen isotopes in phosphate to assess biological phosphorus cycling in a small and shallow freshwater lake system

Abstract 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.