Phosphorus recycling in deeply oxygenated sediments in Lake Superior controlled by organic matter mineralization

Remobilization of phosphorus from aquatic sediments has been extensively investigated in systems prone to anoxia, while studies in well-oxygenated systems have been rare. The recycling efficiency of P in the offshore sediments in the Great Lakes, in particular, is still poorly known. We investigated phosphorus cycling at 13 locations (26–318 m water depth) in oligotrophic Lake Superior where oxygen penetrates into sediments by 2–12 cm. Vertical distributions of iron and phosphorus were measured in porewater and solid fractions, and transformation rates and vertical fluxes were calculated. Whereas a significant fraction of P is bound to ferric Fe in surface sediments, P effluxes into the water column (2.5–7.0 μmol m⁻² d⁻¹) are only weakly affected by iron reduction, because Fe : P ratios in surface sediment are high (∼ 40–80 mol : mol), and P sorption capacity is far from its limit. In contrast to organic rich systems where P effluxes are sensitive to redox conditions, phosphate effluxes in organic-poor well-oxygenated Lake Superior are controlled by the rates of organic phosphorus mineralization, similar to marine sediments. The efficiency of P recycling in Lake Superior sediments, however, is substantially lower than in marine sediments due to different P biogeochemistry. Only ∼ 12% of deposited P is returned to the water column. While burial into sediments is the dominant sink for P in the lake, sediments still contribute up to 40% of total water column P inputs. Similar behavior should be expected in other well-oxygenated freshwater systems, such as other large oligotrophic lakes.