Reaction-transport modeling of centennial-scale phosphorus accumulation and internal loading in a human-impacted boreal lake

Legacy anthropogenic phosphorus (P) accumulated in sediments influences nutrient cycling in eutrophic lakes through so-called internal loading. However, due to the complexity of processes influencing P mobility in the sediment column, the temporal response of internal loading to reduction of external P inputs is difficut to predict. In this study, we use a comprehensive set of porewater and sediment geochemical data to constrain a reaction-transport model simulating long-term anthropogenic inputs and processing of P in Lake Hiidenvesi, a eutrophic lake in southern Finland. The 180 cm sediment core used to train the model encompasses over 800 years of accumulation, including the transitions into and out of the Little Ice Age when land use in the region changed considerably. By defining top 6 cm of sedimentary P as "freshly-deposited" (within the past 10 years) and deeper layers as "legacy P", we find that at any given point in time, the freshly deposited material contributes the majority of regenerated P in porewaters, with an additional contribution from legacy P. A set of linear regressions between P deposition and diffusion rates indicate that internal P loading is primarily controlled by particulate P deposition of organic-P and Fe-P, which may be directly derived from catchment exports or autochthonously produced through in-lake biogeochemical processes. The Little Ice Age is shown by the model to be a period of relatively lower external P inputs an consequently also lower internal loading rates. However, the overall retention of P in sediments is sufficient to suggest that sediment P content can be used as an indicator for historical anthropogenic impacts in catchment areas of lakes in the boreal region.