Phosphate oxygen isotope insights into the coupled distribution of phosphorus, iron, and manganese in lake sediments

Understanding phosphorus (P) dynamics in lake and marine sediments is critical for predicting P re-release into water bodies and its stable accumulation through burial. The distribution of P in sediments is influenced by inputs from rivers and groundwater, as well as post-depositional redistribution processes, such as sink switching and geochemical focusing. Although P, iron (Fe), and manganese (Mn) dynamics are expected to be coupled under reductive conditions, the effects of terrestrial inputs and these redistribution processes on their behavior remain unclear. To assess P, Fe and Mn transport and accumulation in lake sediments, this study applied phosphate oxygen isotope (S18OPO4) analysis to track P sources and redistribution, along with sequential extractions of sedimentary P, Fe, and Mn. The study was conducted in Lake Biwa, Japan's largest lake. We collected the samples of potential P sources (groundwater and river water) and sediments from nearshore to offshore areas. The S18OPO4 values of Fe-bound P and concentrations of Fe-bound P, and reactive Fe and Mn in the sediments increased from nearshore to offshore. Such spatial trends can be explained by groundwater discharge with a high S18OPO4 value and/or geochemical focusing with biological recycling (P uptake and release by organisms). Sedimentary P, Fe, and Mn concentrations were significantly correlated to organic matter concentrations in the sediments, suggesting that organic matter is crucial in their coupled dynamics. This study highlights the utility of S18OPO4 for evaluating P sources and redistribution processes and the importance of the factors contributing to spatial heterogeneity for understanding lake biogeochemistry.