The phosphorus cycle in coastal marine sediments

Abstract

The concentration of dissolved phosphate in sediment box-cores from the Laurentian Trough in the Gulf of St. Lawrence increased sharply across the sediment-water interface from 2.0 µmol/PO₄/1 in the bottom water to 6 ± 3 /mot PO₄/1 in the top cm, remained almost constant at this value down to 5–15 cm depth, and then increased rapidly with further depth. In the region of constant concentration, phosphate is buffered by sorption equilibria with the sediment. The production rate of phosphate, the sorption capacity of the sediment, and the thickness of the diffusive boundary layer at the sediment-water interface appear to control the shape of the pore water profile. Even though the buffering places an upper limit on the concentration gradient across the sediment—water interface, and hence on the flux, the phosphate flux to the overlying water is controlled by the production rate of phosphate within the sediment. A model is proposed to relate sediment chemistry to phosphorus fluxes.