EFFECTS OF HISTORIC TIDAL RESTRICTIONS ON SALT-MARSH SEDIMENT CHEMISTRY

The effects of tidal restrictions by diking on salt marsh biogeochemis try were interpreted by comparing the hydrology, porewater chemistry a nd solid phase composition of both seasonally flooded and drained dike d marshes with adjacent natural salt marshes on Cape God, Massachusett s. Flooding periods were greatest in natural and least in drained mars hes. Differences between the chemistry of the natural and diked marshe s depended upon the depth of the water table and the supply of sulfate for anaerobic metabolism. Drained marsh sediments were highly acidic (pH < 4) with porewaters rich in dissolved Fe; the natural and diked f looded marshes had pH 6-7.5 and Fe orders of magnitude lower. Porewate r nutrients, sulfides and alkalinity were much lower in both flooded a nd drained diked marshes than in the natural marsh. Sediments of the d rained marsh had subsided 90 cm relative to the natural site due to or ganic matter decomposition and compaction. However, despite the loss o f organic matter, much of the P and N was retained, with NH4 likely pr otected from nitrification by low pH and PO4 adsorbed on Fe and Al oxi des. Iron, and to a lesser degree sulfur, had also been well retained by the sediment. Despite eight decades of diking, substantial amounts of reduced S, representing potential acidity, persisted near the top o f the water table. In contrast, the surface of the seasonally flooded marsh was only 15 cm below the natural marsh. Accretion since diking a mounted to 25 cm and involved proportionally less mineral matter. The restoration of seawater flow to both seasonally flooded and drained di ked marshes will likely extend flooding depth and duration, lower redo x, increase cation exchange, and thereby increase NH4, Fe(II), and PO4 mobilization. Increased porewater nutrients could benefit recolonizin g halophytes but may also degrade surface water quality.