Factors controlling long-term phosphorus efflux from lake sediments: Exploratory reactive-transport modeling

We perform, for the first time, a global sensitivity analysis on a generic diagenetic reaction-transport model (RTM) to elucidate the effects of environmental, thermodynamic, and kinetic factors on the magnitudes of phosphorus release fluxes from aquatic sediments under a range of conditions typically found in lake sediments. Our model includes the processes that describe redox-sensitive phosphorus releases (the so-called classical model) and processes by which phosphorus mobilization is affected by porewater sulfate. On decadal and longer time scales, sediment phosphorus effluxes are primarily determined by: sedimentation flux of reactive organic matter, sedimentation flux of iron oxyhydroxides, concentrations of dissolved oxygen and sulfate at the sediment-water interface, and the rate at which phosphate is immobilized in reduced sediment. We show that the effects of these factors on phosphorus effluxes are interdependent and discuss the mechanisms of such interactions. The dominant pathways by which dissolved sulfate increases phosphorus efflux in iron-rich hypoxic sediments are discussed in detail. In contrast to short-term phosphorus releases, such as described by the classical model, long-term phosphorus retention is controlled by phosphorus removal to deep, reduced, sediment, rather than processes at the sediment-water interface. Hence, the results of short-term laboratory or in-situ studies (such as sediment incubations experiments) cannot be unequivocally extended to long time scales. Our results provide explanations to the reports that lake restoration measures such as restricting phosphorus inputs to a lake or oxygenating the lake's hypolimneon (or both) in the long-term often fail to decrease sediment phosphorus effluxes. The re-deposition of sediment substances after their release into the water column (the feedback often overlooked in sediment RTMs) can critically affect the magnitude and dynamics of phosphorus efflux from sediments. Depending on sediment history, the same set of external (boundary) conditions can generate diagenetic regimes with either high or low phosphorus effluxes (bistability).