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ABSTRACT. The sulfur stable isotopic composition of Spartina dlterniflora in a low salinity tidal creek system was related to differences in porewater sulfur chemistry determined by salinity, hydrodynamics, and season. The extent of porewater sulfide accumulation, the reoxidation of sulfide minerals, and sulfate limitation of sulfate reduction were important processes controlling the tj3'S of the sulfur available for plant uptake. The influence of sedimentary sulfate reduction rates on S. alterniflora 634S was demonstrated in the comparison of 2 sites with s~milar sulfate supply but differing hydrology; plant tj3'S values were heavier where sediments were more oxidized relative to plant 634S values from a fl-equently flooded marsh with more reduced sediments. The role of sulfate supply in determining S alterniflora P 4 S values was apparent in the comparison of 3 sites with similar hydrology but differing salinitv; shoot P 4 S decreased with increasing salinity. In low salinity marshes, oxidation of stored sulfide minerals in winter and spring led to lighter shoot 6"s values, while sulfate depletion in late summer was associated with isotopically heavier shoot sulfur. Variability on spatial and temporal scales in sulfur stable isotopic composition of S. alterniflora has implicat~ons for the use of its P 4 S values in studies of trophic dynamics in estuarine marshes. The sulfur chem~stry of the marsh sediments and the sampling season may both influence the stable isotopic signature of this important prlrnary producer.

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