IRON-MANGANESE-PHOSPHORUS CYCLING IN SHALLOW FRESHWATER SYSTEMS UNDER ICE: THE CASE FOR PRIMING THE P PUMP
Under-ice conditions are also thought to be an integral part of the physical and chemical conditions antecedent to seasonal blooms as redox conditions ‘prime’ these systems with more labile phosphorus attached to nanoparticulate iron oxides. We demonstrate that a balance of hydrodynamic, geochemical, and biological drivers are at play under ice cover, and that differences in iron v. manganese levels in the water column can provide clues to dominant processes that impact phosphorus buildup in the uppermost sediments. Ice as a diffusional barrier to oxygen to the water column affects the redox conditions in a way that enhances mobilization of phosphorus from deeper sediment to the SWI via iron oxide reduction, with manganese oxide reduction occurring as a key precursor. Melt and precipitation events affect river discharge and input of external sources of nutrient and metals, except manganese. Buildup of significant phosphorus at the SWI may increase the amount of nutrients that can be accessed once blooms have started in the summer and can affect redox conditions on their own, creating a positive feedback loop of reducing conditions enhancing nutrient flux.