INFLUENCE OF SPARTINA ALTERNIFLORA AND JUNCUS ROEMARIANUS ON SALTMARSH SEDIMENT PORE WATER AND SOLID PHASE GEOCHEMISTRY

The influence of Spartina and Juncus on saltmarsh sediment pore water and solid phase geochemistry was investigated at four sites in a saltmarsh on Sapelo Island, GA during July 2006. Two sites, one mostly unvegetated, and the other densely vegetated by tall Spartina, are adjacent to a tidal creek. Two higher marsh sites are densely vegetated by Juncus and sparsely vegetated by short Spartina. Pore waters collected at 1-2 cm intervals to a depth of 50 cm were analyzed for pH, alkalinity, Mn, Fe(III), Fe(II), ΣH₂S, SO₄^-2, ΣPO₄^-3, ΣNH₄⁺, Ca, Mg and K. Trace element distributions among operationally-defined solid fractions (exchangeable, carbonate, reducible, oxidizable) were measured at 4-5 cm intervals using sequential extractions. The most compressed vertical redox stratification occurs at the short Spartina site, followed by the adjacent Juncus site; sulfidic pore waters are present within cm of the sediment surface. Although sulfate reduction (SR) is likely the dominant organic matter degradation pathway at both sites, O₂, nitrate, Mn(IV) and Fe(III) may be significant terminal electron acceptors in oxic microzones in the rhizosphere and near burrow walls at the Juncus site. The tall Spartina site has the most oxidized sediments, followed by the adjacent unvegetated site. Although SR occurs at these sites, both are dominated by suboxic pore waters. Fe and Mn reduction are significant, especially in near surface sediments, and likely also in the rhizosphere of the tall Spartina site and near burrows at both sites. Radial O₂ loss at the tall Spartina and Juncus sites creates more acidic and oxidized pore waters, with greater reoxidation of reduced solutes than at the less vegetated sites. In suboxic sediments, Fe, Mn, Cr, Ni, Co and Zn occur mostly in association with Fe and Mn hydroxides. In anoxic sediments, Fe, Mn and Co form mostly sulfides, Cr is mostly bound to organic matter, and Ni, Zn, Cu and Pb are found in association with both sulfides and organic matter. The depth- and site- dependent trace element distributions are in excellent agreement with variations in redox geochemistry inferred from pore water data, demonstrating that sequential extractions can yield useful insights into metal speciation, especially when measured in conjunction with pore water geochemistry.