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

Reducing excess phosphorus (P) loads that cause eutrophication in aquatic systems is essential for meeting water quality standards. The oxygen isotopic composition of phosphate (delta 18Op) is a powerful tool for tracking P sources and cycling in diverse natural ecosystems. Here, we use delta 18Op distribution in a small freshwater body (a lagoon-lake system) with high biological activity. We report delta 18Op values seasonally along the water flow path in lagoon-lake system adjacent to Lake Biwa, Japan. The delta 18Op values of inflowing water originating as agricultural runoff were constant throughout the study period at +16.3 parts per thousand +/- 0.2 parts per thousand. The delta 18Op values in the system were generally offset from temperature-dependent isotopic equilibrium with the surrounding water, ranging from +11.1 parts per thousand to +17.8 parts per thousand. The delta 18Op 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 delta 18Op 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 delta 18Op values to deviate toward lower values relative to both equilibrium and agricultural source delta 18Op 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 delta 18Op for understanding P dynamics in shallow lake ecosystems.