The phosphorus cycle in coastal marine sediments

Abstract

Approximately half of the sedimentation flux of particulate phosphorus in the Laurentian Trough in the Gulf of St. Lawrence is mobilized within the sediment and returned to the water column. In the oxidizing surface sediment, a major portion of the sedimentation flux of organic phosphorus is mineralized, and the released phosphate is partitioned between the pore water and surface adsorption sites. Surface-adsorbed phosphate is released to the pore water as needed to replace dissolved phosphate that escapes to the overlying water. Most of the phosphate is released deeper in the sediment column from iron oxides undergoing reduction. The nonmobilized phosphorus, which is buried with the accumulating sediment, appears to consist mostly of stable minerals such as apatite.The concentration of dissolved phosphate in sediment pore waters increases sharp!y across the sediment-water interface from 2 pmol PO, liter-l in the bottom water to 6+3 pmol PO, liter-' in the top centimeter, remains almost constant at this value down to 5-l 5-cm depth, and then increases rapidly with further depth. In the region of constant concentration, phosphate is buffered by adsorption-desorption equilibria with the sediment. The production rate of phosphate, the buffering capacity of the sediment, and the thickness of the diffusive boundary layer at the sediment-water interface control the shape of the pore-water profile.In the aquatic environment, dissolved phosphate is consumed during the growth of phytoplankton and is regenerated during bacterial decomposition of organic matter. Much of the regeneration takes place in the water, but in relatively shallow environAcknowledgments

Statistical Modeling of Variability in Sediment-Water Nutrient and Oxygen Fluxes

“…The phosphate release from iron and manganese oxides in anoxic conditions (Sundby et al, 1992) is also confirmed by higher phosphate pore water concentrations in cohesive-muddy stations (A and B) compared to sandy permeable stations (D and F; Figure 7). Pore water profiles also showed that phosphate concentrations remained constant down to 10 cm sediment depth in sandy stations D and F (Figure 7) indicating the lack of iron and manganese reductions within these depths.…”

“…Pore water profiles also showed that phosphate concentrations remained constant down to 10 cm sediment depth in sandy stations D and F (Figure 7) indicating the lack of iron and manganese reductions within these depths. Conversely, muddy stations A and B showed an increase of phosphate with sediment depth until 5-6 cm depth and remained constant below these horizons indicating that iron and manganese reduction dominated the organic matter mineralisation pathways in the upper sediment layers in these stations (Fenchel and Jørgensen, 1977;Froelich et al, 1979;Sundby et al, 1992).…”

“…In this study, we estimated a mean net phosphate flux of −0.015 ± 0.09 mmol m −2 d −1; Figure 6E) implying a net uptake by the sediment. Suspended sediments have long been known to influence concentrations of dissolved phosphorus: the high turbidity we recorded in winter signified the occurrence of resuspension events causing sediment phosphate release (e.g., Søndergaard et al, 1992;Sundby et al, 1992;Holmroos et al, 2009). However, its effect is complicated.…”

“…Re-suspended particles can in fact release, absorb or have no effect on soluble reactive phosphorus depending on water and particle phosphate concentrations (Søndergaard et al, 1992) and water pH, with release driven by high pH values (Koski-Vähälä and Hartikainen, 2001). In oxic conditions, the sediment is generally a sink of phosphate which co-precipitates with iron and manganese oxides; the occurrence of anoxic conditions, often present in semi-enclosed stratified waters in late summer and in deep sediment horizons, can determine sediment phosphate release (Carman and Wulff, 1989;Sundby et al, 1992). The regression model explained a low percentage of the deviance because it described only the winter release of phosphate, however, once again we could be underestimating the phosphate flux due to the lack of a stirring system that simulates the effect of resuspension in incubation chambers (Almroth et al, 2009).…”

“…However, its effect is complicated. Adsorption on oxidized surface sediment is an important factor affecting phosphate availability, and can lead to net uptake of phosphate by sediments (Sundby et al, 1992). Re-suspended particles can in fact release, absorb or have no effect on soluble reactive phosphorus depending on water and particle phosphate concentrations (Søndergaard et al, 1992) and water pH, with release driven by high pH values (Koski-Vähälä and Hartikainen, 2001).…”