Salt marsh diking and restoration: Biogeochemical implications of altered wetland hydrology

In salt marshes, most biomass plus large reserves of biologically important N, P, Fe, and S are sequestered below ground under saline, waterlogged, an d anaerobic conditions. Thus, hydrologic alterations such as diking and dit ch drainage that reduce salinity and increase peat aeration can cause radic al changes in the composition of salt marsh soils. Experimental short-term desalination and drainage of salt marsh cores in gr eenhouse microcosms caused Spartina production to increase after one growin g season, reflecting decreased salt stress and sulfide toxicity. However, p roduction thereafter declined, likely due to pyrite oxidation and acidifica tion in drained treatments and sulfide accumulation in waterlogged treatmen ts. A survey of longer-term (decadal) effects of diking on pear composition of Cape God, Massachusetts, USA, marshes revealed acidification, Fe(II) mobili zation, and decreased organic content in drained sites. Despite the aerobic decomposition of organic matter, abundant nutrients remained as sorbed NH4 and mineral-bound PO4. In diked, seasonally waterlogged sites, porewater a lkalinity, sulfide, ammonium and orthophosphate were much lower, and organi c solids higher, than in adjacent natural marsh. Seawater was added to cores from diked marshes to study the effects of tida l restoration. Salination of the drained peat increased porewater pH, alkal inity ammonium, orthophosphate, Fe, and Al; copious ammonium N, and Fe(II) for sulfide precipitation favored Spartina growth. Salination of diked-wate rlogged peat increased sulfate reduction and caused 6-8 cm of sediment subs idence. The resulting increase in porewater sulfides and waterlogging decre ased vigor of transplanted Spartina alterniflora. Results indicate that sea water restoration should proceed cautiously to avoid nutrient loading of su rface waters in drained sites or sulfide toxicity in diked-waterlogged mars hes.