Quantifying phosphorus levels in water columns equilibrated with sediment particles in shallow lakes: From algae/cyanobacteria-available phosphorus pools to pH response

Sediment phosphorus (P) release in shallow eutrophic lakes is a major contributor of P to algal blooms. This research proposes an innovative notion in which the P diffusive fluxes at the sediment-water interface (SWI) of shallow lakes are controlled by the P adsorption-desorption equilibria, with pH as the major regulating factor. The P equilibrium concentration (C_e) at SWI was conceptualized into a dependent variable responding to two factor-dependent variables, the algae/cyanobacteria-available P pools of the SWI and the pH in the water column, resulting in the empirical equation C_e(pH) = C_m/[1 + e^-k(pH-pH1/2)]. C_m is the maximum P equilibrium concentration when all algae/cyanobacteria-available P in sediments is released, and the value relies on the thickness of the oxygen and pH transition layer that contains iron/aluminium (hydr)oxide-adsorbed P. The parameters in the empirical equation are accessible from P desorption tests conducted on a set of sediment samples with different P pollution levels. This research provides a quantitative approach for determining the sediment P criteria of shallow lakes, with sediment iron/aluminium (hydr)oxide-adsorbed P and water depth as two main indicators with ecological implications. A decrease in water depth would proportionally increase the P concentration at a similar sediment P releasing flux and increase algae/cyanobacteria-available P pools that are ready to equilibrate with the water column by increasing hydrodynamic disturbance of the SWI.