Phosphorus Mobility and Speciation Under Dynamic Redox Conditions in Shallow Eutrophic Freshwater Systems

Internal loading of phosphorus (P) from lake sediments can delay the recovery of lakes from eutrophication for years to decades following decreases in external nutrient inputs. While internal P loading is a pervasive problem in freshwater systems, molecular speciation of P in benthic sediments of these systems remains poorly characterized. As different P species will exhibit different responses to changing sediment-water interface (SWI) geochemistry, quantifying P speciation in sediments is a critical step in understanding P dynamics in sediment-water systems. Here, various synchrotron-based techniques were employed to directly probe the bonding environments of P and iron (Fe) in natural and experimentally manipulated lake sediments in order to link chemical speciation to chemical behavior and to identify the geochemical drivers that mediate this linkage. We manipulated SWI redox conditions in mesocosm experiments to investigate the impacts of prolonged anoxia and redox oscillations on P mobility and speciation in sediments. Mesocosm experiments demonstrate that oscillating redox conditions near the SWI may drive accelerated P release from sediments relative to uninterrupted reducing conditions. Sediment P is found to be predominantly associated with Fe oxyhydroxides, calcium carbonate, and apatite minerals in three shallow hyper/eutrophic lakes in northern Vermont. In Missisquoi Bay and Lake Carmi, Fe redox cycling controls P mobility via precipitation and dissolution of Fe oxyhydroxides. In the hypereutrophic Shelburne Pond, the presence of Fe sulfides precludes redox-driven P cycling and P mobility is instead dominated by organic matter mineralization. Our results demonstrate that internal P loading can manifest differently in similar shallow lake systems due to differences in lake configuration, sediment P and Fe speciation, and organic content of sediments. This work demonstrates the potential utility, as well as the limitations, of P K- edge X-ray absorption near edge structure spectroscopy in determining sediment P speciation in freshwater lakes