Abstract
Legacy anthropogenic phosphorus (P) accumulated in sediments drives internal P loading in eutrophic lakes. However, predicting the temporal response of internal P loading to the external loading reduction is difficult due to the complexity of sedimentary P cycling. In this study, we use geochemical profiles obtained from a 180‐cm sediment core to constrain a reaction‐transport model and investigate the centennial‐scale response of internal P loading to the catchment development in Lake Hiidenvesi, a eutrophic boreal lake. This includes the transitions into and out of the Little Ice Age (LIA) when land use in the region changed considerably. A set of linear regressions between particulate depositional fluxes and diffusion rates indicates that internal P loading is primarily controlled by the deposition of organic‐P and iron (Fe) bound‐P, which may be directly linked with external loading from the catchment. We also show the relatively lower external inputs and consequently lower internal P loading during the LIA. The closed connection between external and internal P loading suggests that lake mitigation would be achieved if the reductions of external loading and sedimentary deposition were maintained. Additionally, the burial of sedimentary humic‐bound P (or total P dominated by humic‐bound P) is sufficient to reflect the historical anthropogenic impacts in boreal lakes.