GEOCHEMICAL CYCLING OF PHOSPHATE AND IRON IN SALT MARSH SEDIMENTS

Salt marshes are highly productive wetlands which represent an interface between freshwater/terrestrial ecosystems and marine systems. The availability of nutrients in a salt marsh is directly related to salt marsh biological productivity. To establish baseline nutrient concentrations in sediment porewaters, sediment cores were taken from Dixie Plantation, located on the Stono River and characterized by lowland salt marshes and upland maritime forest. Dixie Plantation is a pristine 800 acre tract of land south of Charleston; this location is an ideal control site for a region experiencing rapid land use change. In two locations, two piston cores were collected: one from the edge of the tidal creek in the low marsh, and one from the upland border. The porewaters were extracted, and the resulting pore water samples were analyzed for dissolved inorganic phosphate and iron (II) concentrations using a UV-VIS spectrophotometer.

In general, porewater phosphate followed similar trends with depth at all sites. Porewater phosphate profiles for both low marsh sites were more variable than the upland upland border sites. Concentrations were lowest at the sediment surface, likely due to diffusive flux of phosphate from the sediment to the water column. Concentrations reach a maximum around 10-20 cm depth, due to release of phosphate during organic matter decomposition.

The Fe (II) concentrations for all sites decreased with depth, but the overall concentrations were very different for the low marsh and transitional sites. At the sediment surface, the low marsh site iron concentrations ranged from 220 µmol/L and 194 µmol/L, while the upland border sites ranged from 7 µmol/L and 4 µmol/L. Iron decreased to 0 µmol/L at 5 cm for the upland border cores, and at 13 cm for the low marsh cores.

Overall, the upland border zones revealed lower concentrations of Fe (II) and phosphate in the pore waters than the low marsh. These results suggest that tidal creeks and vegetation play a dynamic role in biogeochemical nutrient cycling on salt marshes.