PHOSPHATE SORPTION IN SUPERFICIAL INTERTIDAL SEDIMENTS

The influence of sorption on phosphorus cycling in intertidal sediment s was examined by measuring sorption characteristics (isotherms, kinet ics) and sediment content of exchangeable PO4 (extracted with 0.1 M Na OH). The latter contents were highly correlated with Fe-oxides and sho wed a seasonal pattern with highest values in autumn and winter, and l owest in spring and early summer. Part of the PO4 produced through min eralization in late summer and autumn is stored through sorption in th e 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 relea se due to mineralization of organic matter was visible after c. 50 h o f incubation. We applied a dynamic model to correct the sorption data for mineralization and to estimate the sorption kinetics. The first or der sorption rates obtained were between 0.7 and 5 ml.g(-1).h(-1). Whe n applying the film model for the upper 5 mm of the sediments, measure d fluxes and PO4 gradients could only be reproduced with unrealistical ly high diffusion coefficients. Diffusion coefficients close to molecu lar diffusion were obtained by using a simple reaction-diffusion model including the data on sorption kinetics. This model indicates that th e transfer of PO4 at the sediment-water interface is enhanced by the s orption reaction and that high fluxes can be maintained at low pore wa ter concentrations in the upper millimeters of the sediment. We conclu de that the sorption kinetics together with diffusion control the PO4 concentration in the superficial intertidal sediments.