Phosphate sorption in superficial intertidal sediments

The influence of sorption on phosphorus cycling in intertidal sediments was examined by measuring sorption characteristics (isotherms, kinetics) and sediment content of exchangeable PO4 (extracted with 0.1 M NaOH). The latter contents were highly correlated with Fe-oxides and showed a seasonal pattern with highest values in autumn and winter, and lowest in spring and early summer. Part of the PO4 produced through mineralization in late summer and autumn is stored through sorption in the oxic sediment layers and becomes available to e.g. primary producers in the next season. Sorption experiments showed linear isotherms with adsorption coefficients between 6.9 and 22.7 ml•g−1. Slow PO4 release due to mineralization of organic matter was visible after c. 50 h of incubation. We applied a dynamic model to correct the sorption data for mineralization and to estimate the sorption kinetics. The first order sorption rates obtained were between 0.7 and 5 ml•g−1•h−1. When applying the film model for the upper 5 mm of the sediments, measured fluxes and PO4 gradients could only be reproduced with unrealistically high diffusion coefficients. Diffusion coefficients close to molecular diffusion were obtained by using a simple reaction-diffusion model including the data on sorption kinetics. This model indicates that the transfer of PO4 at the sediment-water interface is enhanced by the sorption reaction and that high fluxes can be maintained at low pore water concentrations in the upper millimeters of the sediment. We conclude that the sorption kinetics together with diffusion control the PO4 concentration in the superficial intertidal sediments.