BIOGEOCHEMICAL EFFECTS OF SEAWATER RESTORATION TO DIKED SALT MARSHES

We conducted greenhouse microcosm experiments to examine the biogeoche mical effects of restoring seawater to historically diked Cape Cod sal t marshes. Feat cores from both seasonally flooded and drained diked m arshes were waterlogged with seawater, and porewater chemistry was sub sequently monitored for 21 mo. The addition of seawater to highly orga nic, seasonally flooded peat caused the death of freshwater wetland pl ants, 6-8 cm of sediment subsidence, and increased N and P mineralizat ion. Also, sulfides and alkalinity increased 10-fold, suggesting accel erated decomposition by sulfate reduction. Addition of seawater to the low-organic-content acidic peat from the drained marsh increased pore water pH, alkalinity, PO4-P, and Fe(II), which we attribute to the ree stablishment of SO4 and Fe(III) mineral reduction. Increased cation ex change contributed to 6-fold increases in dissolved Fe(II) and Al and 60-fold increases in NH4-N within 6 mo of salination. Seawater reintro ductions to seasonally flooded diked marshes will cause porewater sulf ides to increase, likely reducing the success of revegetation efforts. Sulfide toxicity is of less concern in resalinated drained peats beca use of the abundance of Fe(II) to precipitate sulfides, and of NH4-N t o offset sulfide inhibition of N uptake. Restoration of either seasona lly flooded or drained diked marshes could stimulate potentially large nutrient and Fe(II) releases, which could in turn increase primary pr oduction and lower oxygen in receiving waters. These findings suggest that tidal restoration be gradual and carefully monitored.