Limnol. Oceanogr., 44(7), 1999, 1693–1701

The phosphate sorption capacity of intertidal vegetated marsh sediments was measured along a salinity gradient in the Cooper River estuary, South Carolina. The phosphate sorption capacity of the surface sediments (0–10 cm) of a freshwater marsh was higher than the sorption capacity of sediments from brackish and saline marshes, and surface sediments had greater sorption capacity than subsurface (10–20 cm) sediments. These trends were opposite that of available phosphorus, which increased downstream and with depth. Freshwater marsh sediments trap phosphorus in a less-bioavailable form as evidenced by the low zero equilibrium phosphorus concentration (ZEPC) of the ambient sediment and low exchangeable phosphorus found there. Soil ZEPC values were similar to the in situ mean pore-water phosphate concentrations, which shows that sorption has a major effect on the spatial distribution of pore-water phosphorus along the estuarine salinity gradient. The magnitude of phosphorus sorption by the freshwater marsh sediments greatly reduced the pore-water phosphate concentration, while the phosphorus sorption properties of brackish and salt marsh sediments maintained in situ equilibrium pore-water phosphorus concentrations at surplus levels (with respect to its availability to plants). These differences in P sorption properties of the sediments can be explained on the basis of their physical and chemical characteristics. For instance, approaching the sea, the surface area of sediments declined, with freshwater marsh sediments (0–10 cm) supporting 8.53 higher surface area than the salt marsh sediments. However, the sorption capacity of freshwater sediments was 333 greater than that of salt marsh sediments, which indicates that other properties such as sediment mineral composition are important. The concentrations of important elements such as Al and Fe in sediments also declined downstream. The results suggest that the differences in phosphorus exchange properties among these marshes are a function of sediment type and sedimentary concentrations of Fe and Al. These in turn are related to the changes in ionic strength and associated parameters (e.g., pH) and physical sorting mechanisms. Phosphorus bioavailability (operationally defined here as the molybdate-reactive phosphorus) in wetlands is controlled by complex in situ biotic and abiotic processes. The latter include removal of dissolved phosphate from pore-water through sorption onto sediment particles and organic aggregates. Sorption–desorption processes within a sediment chemical environment are influenced by the mineral composition of the sediment. For example, P sorption is positively correlated with the amount of free iron oxide in acid sulfate soils (Jugsujinda et al. 1995). Furthermore, organic molecules can form complexes with metal ions such as iron (Fe) and aluminum (Al), which in turn can sorb phosphorus and reduce its bioavailability (Jones et al. 1993). This is consistent with the high phosphorus sorption capacity of mineral-rich, freshwater wetland sediments (Richardson 1985). Calcium carbonate also has a high affinity for phosphate adsorption (de Kanel and Morse 1978). These inorganic complexes are susceptible to changes in pH and redox conditions. In river-dominated tidal estuaries, mixing processes form salinity and pH gradients that dominate the chemistry and