IRON-MANGANESE-PHOSPHORUS CYCLING IN SHALLOW FRESHWATER SYSTEMS UNDER ICE: THE CASE FOR PRIMING THE P PUMP

Phosphate is strongly sorbed to nanoparticulate iron oxides in freshwater lake sediments, often controlling the availability of inorganic and organic phosphorus to microorganisms in the water column that constitute cyanobacterial species responsible for Harmful Algal Blooms. In a shallow freshwater system in northern Lake Champlain, we have established that these oxide minerals are sensitive to changing redox chemistry near the sediment water interface (SWI) over both seasonal and diel time frames (Smith et al., 2011; Pearce et al., 2013). Mesocosm experiments have shown that changes to the redox environment at the SWI over very short time intervals (minutes to hours) can change the degree and direction of phosphate flux across the SWI. Field evidence and modeling has additionally shown that cyanobacterial blooms themselves stimulate the most reducing conditions at the SWI, affecting the flux of phosphorus from the SWI to the water column.

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.