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.

中文翻译：

---

使用磷酸盐中的氧同位素评估小型浅水淡水湖系统中的生物磷循环

减少导致水生系统富营养化的过量磷 (P) 负荷对于满足水质标准至关重要。磷酸盐的氧同位素组成（δ18氧p）是追踪不同自然生态系统中磷源和循环的强大工具。这里我们用δ18氧p分布在具有高生物活性的小型淡水体（泻湖-湖泊系统）中。我们报告 δ18氧p日本琵琶湖附近泻湖-湖泊系统沿水流路径的季节性价值。 δ18氧p在整个研究期间，源自农业径流的流入水量恒定在+16.3‰±0.2‰。 δ18氧p系统中的值通常偏离与周围水的温度依赖性同位素平衡，范围从+11.1‰到+17.8‰。 δ18氧p七月和十月湖水值接近平衡值，此时溶解性无机磷（DIP）保留率很高，这与广泛的生物介导的磷酸盐循环一致。 δ18氧p涉及流入磷和生物循环磷的双端元混合模型表明，泻湖-湖泊系统中的磷周转率在生产季节很高。相比之下，非灌溉季节（冬季）湖水停留时间较长，允许δ18氧p相对于平衡和农业来源 δ 值偏离较低值18氧p值，表明磷代谢主要是在低 DIP 浓度下溶解有机磷的细胞外/酶外酶水解。这项工作强调了 δ 的实用性18氧p用于了解浅湖生态系统中的磷动态。