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 (δ18Op) is a powerful tool for tracking P sources and cycling in diverse natural ecosystems. Here, we use δ18Op distribution in a small freshwater body (a lagoon–lake system) with high biological activity. We report δ18Op values seasonally along the water flow path in lagoon–lake system adjacent to Lake Biwa, Japan. The δ18Op values of inflowing water originating as agricultural runoff were constant throughout the study period at +16.3‰ ± 0.2‰. The δ18Op values in the system were generally offset from temperature-dependent isotopic equilibrium with the surrounding water, ranging from +11.1‰ to +17.8‰. The δ18Op 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 δ18Op 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 δ18Op values to deviate toward lower values relative to both equilibrium and agricultural source δ18Op 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 δ18Op for understanding P dynamics in shallow lake ecosystems.