THE INFLUENCE OF MACROPHYTES AND MACROFAUNA ON SALTMARSH SEDIMENT REDOX GEOCHEMISTRY (SAPELO ISLAND, GA, USA)

Saltmarsh sediment geochemistry depends on complex interactions between abiotic geochemical processes and the activities of microorganisms and macroorganisms. Here, sites at a saltmarsh on Sapelo Island, GA are used to investigate the influence of the cordgrass Spartina alterniflora and bioturbating marofauna on pore water redox geochemistry. pH, alkalinity, Fe(II)/Fe(III), Mn(II), sulfate, sulfide, ammonium and phosphate concentrations in pore waters are assessed at 1-2cm intervals from the sediment surface to a depth of ~50cm at sites forming a transect away from a tidal creek. Sites include two creek bank sites, one unvegetated and one recently colonized by tall Spartina, a levee with dense, tall Spartina and a ponded marsh vegetated by sparser, short Spartina. Creek bank and levee sites are densely inhabitated by polychaete worms, fiddler and mud crabs and, at the creek bank, shrimps; the ponded marsh site has much lower macrofaunal populations. A large suboxic zone, with high concentrations of dissolved Fe(II) and no detectable sulfide dominates the unvegetated creek bank site during all seasons; at the ponded marsh site, a thin suboxic zone is always underlain by a large sulfidic zone with high concentrations of dissolved sulfide and very little Fe(II). Pore water redox geochemistry at the levee site is intermediate between these. The two creek bank sites, located <2m apart, demonstrate the net influence of Spartina on pore wate redox geochemistry. Pore waters in the vegetated sediments have higher alkalinity, Mn(II), Fe(II), and sulfide concentrations than adjacent unvegetated sediments, suggesting that colonization stimulates microbial sulfate reduction, probably by increasing the availability of labile organic carbon. The influence of macrofauna on sediment redox is investigated using a stochastic model of enhanced solute transport via bioirrigation (Koretsky et al., 2002, Geochem. Trans.) 3D burrow networks are stimulated and used to quantify bioirrigation mass transfer coefficients. Model results confirm that bioirrigation mass transfer coefficients are larger at the creek bank and levee sites. The field and computational data demonstrate that changes in pore water redox at these sites reflect variations in the activity of macrophytes and macrofauna.