Influence of Spartina and Juncus on Saltmarsh Sediments. I. Pore Water Geochemistry

The influence of Spartina alterniflora and Juncus roemarianus on saltmarsh sediment pore water geochemistry was investigated during summer at four sites in a saltmarsh on Sapelo Island, GA, USA. Pore waters were collected from each site at 1–2 cm intervals, to a depth of 50 cm, and analyzed for pH, alkalinity, dissolved manganese, ferric iron, ferrous iron, total sulfide, sulfate, phosphate, ammonium, calcium, magnesium and potassium. The most compressed vertical redox stratification occurs at a short Spartina site, followed closely by an adjacent Juncus site. Both sites have shallow oxic and suboxic zones, with sulfidic conditions only a few centimeters or less from the sediment water interface. The densely vegetated Juncus site is inferred to have greater primary productivity and organic matter turnover compared to the short Spartina site. More radial oxygen loss is postulated to occur in the subsurface of the Juncus site, leading to reoxidation of reduced species, more acidic conditions and less accumulation of dissolved sulfide, ammonium and oxidizable-Fe in the solid phase compared to the adjacent short Spartina site. A creekside site vegetated by tall Spartina has the most oxidized sediments, followed by an adjacent unvegetated site. Both of these sites are dominated by suboxic pore waters in most of the upper 50 cm. Subsurface injection of oxygen via roots at the densely vegetated tall Spartina site is inferred to create more acidic pore waters with significantly less accumulation of reduced solutes, including ammonium and alkalinity, compared to the adjacent unvegetated creekside site. Fe and Mn reduction are expected to be significant processes in the bulk near-surface sediments of the tall Spartina and unvegetated sites and within rhizosphere sediments at the tall Spartina and Juncus sites. This study demonstrates the significant influence of Juncus roemarianus and Spartina alterniflora on saltmarsh sediment pore water geochemistry, with important implications for nutrient and trace metal mobility and bioavailability. Future work is needed to explore differences in organic matter concentration and especially lability in the subsurface of saltmarsh sites with varying types and densities of vegetation.