| Nutrient, iron and sulfur cycling were compared in adjacent natural and altered Cape Cod (Massachusetts) salt marshes to describe the effects of diking and to predict the effects of tidal restoration on sediment chemistry. Changes caused by the historic blockage of seawater flow, subsequent flushing of salt, and drainage were interpreted by comparing the modern hydrology, pore water chemistry and solid phase composition of both seasonally flooded and drained diked marshes with natural salt marshes. Flooding periods were greater in natural than in flooded than in drained marshes. Pore water alkalinity, sulfide, ammonium and orthophosphate were much lower in the flooded than in the natural marsh, where sulfate reduction predominates. Pore water of the drained marsh was acidic (pH {dollar}<{dollar} 4) and contained more than 1000 times the ferric and ferrous iron of the natural marsh, as a result of pyrite oxidation. Despite many decades of drainage, substantial quantities of ammonium, phosphorus, sulfur and iron are retained in the sediment. Wetland subsidence of 80 cm in the drained marsh is due to the aerobic decomposition of organic matter.; In greenhouse microcosm experiments, I examined the effects of the restoration of seawater flooding to diked marshes. The addition of seawater to cores from the flooded marsh accelerated organic decomposition by sulfate reduction, evident in pore water changes, sediment subsidence, and significant losses of organic solids. Addition of seawater to the drained peat increased pore water pH, alkalinity, orthophosphate and ferrous iron, attributed to renewed sulfate reduction and lower Eh; ferrous iron and ammonium were mobilized by increased cation exchange.; In additional microcosm experiments, I studied the short-term effects on sediment chemistry and plant production of freshening and draining modern salt marsh peat. Net Spartina alterniflora production was significantly greater in drained and in fresh waterlogged than in natural treatments after one growing season. Sulfide accumulation in waterlogged cores resulted in reduced plant growth. Aeration invigorated growth in drained cores, but oxidized sulfide minerals and lowered pH.; Seawater restoration should proceed cautiously in diked salt marshes where the mobilization of nutrients may stimulate eutrophication in adjacent surface waters. |
