Phosphorus sorption characteristics of intertidal marsh sediments along an estuarine salinity gradient

Abstract

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 fresh‐water 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 8.5× higher surface area than the salt marsh sediments. However, the sorption capacity of freshwater sediments was 33× 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.