Elevated internal phosphorus loading from shallow areas of eutrophic boreal lakes: Insights from porewater geochemistry

Internal phosphorus (P) loading is widely recognized as a major cause of lake eutrophication. One conventional paradigm states that the magnitude of internal loading through P diffusion is constrained by the presence of iron (Fe) oxides in surface sediments under oxic conditions near the sediment-water interface (SWI). However, biogeochemical P dynamics in Fe-rich sedimentary systems are still not fully understood, especially in eutrophic lakes where intensively coupled organic matter (OM) remineralization and reductive dissolution of Fe-bound P (Fe-P) exist concurrently. Here, we assess the diagenetic processes that govern sedimentary P cycling in two eutrophic Fe-rich lakes in southern Finland, Lake Hiidenvesi and Lake Kytäjärvi, using a combination of porewater and solid-phase analyses. Coupled reductive dissolution of Fe-P and OM remineralization controlled P regeneration in both lakes, with Fe-P acting as the dominant source for porewater P. Vivianite formation likely immobilized sedimentary P in the deepest basin of Hiidenvesi. Elevated P diffusion rates were observed at shallow sites under oxic bottom water conditions in summer in both lakes, stimulated by enhanced remineralization of both freshly- (mostly phytoplankton-origin) and earlier-deposited OM under elevated temperatures. Areas overlain by oxic bottom water contributed more benthic P fluxes to the water column compared to anoxic/hypoxic areas in both lakes during all sampling seasons. Our study suggests that in shallow eutrophic settings with high OM deposition and elevated temperatures, remineralization in upper sediments regenerates P efficiently enough to support a significant amount of P release to the water column even under sedimentary molar Fe/P ratios >20. We also discuss the implication of our findings for lake restoration strategies.