Hydrological modifications that impair drainage of water from marshes may promote the 

 accumulation of sulfide to potentially toxic levels. The amount of sulfide accumulation would 

 depend on a number of factors including sulfate input, organic matter content of the soil, available 

 Fe, and flushing rate of the soil substrate. The effect of saltwater intrusion on brackish marsh 

 species such as S. patens may not only be due to osmotic effects of increased salinities, but also 

 to sulfide toxicity. The input of sulfate with more saline water would contribute to the generation 

 of sulfide, which may accumulate in areas where flushing and aeration of the soil is prevented (e.g., 

 in inland subsided marshes or artificially impounded sites). For these reasons, marsh restoration 

 plans that involve changes in drainage patterns, for example, should be carefully evaluated to 

 ensure that conditions conducive to the accumulation of sulfide and other reduced toxic compounds 

 are not created in the process. 



Even though increases in salinity and waterlogging may negatively affect brackish marsh species, 

 this effect does not explain why degraded areas do not become revegetated with more salt- or 

 flood-tolerant species. Recolonization of a deteriorated marsh would not only depend on a source 

 of propagules (seeds or rhizomes), but on conditions suitable for plant survival and growth. 

 Recolonization through seed would require a period in which the marsh surface is exposed or 

 where light requirements for germination are met. In areas where the minimum water level is 

 increased through subsidence or sea level rise to a point above the marsh surface, seed germination 

 would be inhibited because of the continued presence of water over the marsh surface. Even if 

 seed production, dispersal, and germination rates were high, the conditions in the degraded marsh 

 may be too inhibitory for seedling survival. Furthermore, marsh grasses such as S. patens and S. 

 alterniflora spread primarily through vegetative propagation. Thus, if emergent vegetation were 

 suddenly eliminated from an area through a change in salinity regime or inundation levels, erosion 

 and subsidence of the marsh surface may proceed to a point where seedling establishment cannot 

 occur. Vegetative spread may occur too slowly to allow succession to a more flood- or salt-tolerant 

 vegetation type. Furthermore, the presence of high levels of soil phytotoxins, which may 

 accumulate in unvegetated areas, may inhibit vegetative invasion, as well as seedling establishment. 



CONCLUSIONS 



This study has demonstrated how basic research with standard bioassay techniques can be used 

 to evaluate the relative impact of salinity and water level and to identify factors preventing the 

 reestablishment of vegetation in a brackish marsh dieback site. This technique showed that 

 brackish marshes can be negatively affected by both saltwater intrusion and subsidence. However, 

 the ability to restore a deteriorated brackish marsh in Louisiana was not controlled by salinity 

 levels, but rather by the degree of plant inundation and the accumulation of soil phototoxins 

 because of altered hydrology and lowered marsh surface elevation. These data support the 

 hypothesis that marsh restoration can be accomplished by revegetation, sediment addition, or a 

 combination of the two. The data further showed that information about soil-plant interactions 

 would be essential to the proper choice of restoration technique. 



REFERENCES 



Anderson, C.E. 1974. A review of structure in several North Carolina salt marsh plants. Pages 

 307-344 in R.J. Reimold and W.H. Queen, eds. Ecology of halophytes. Academic Press, New 

 York. 



Baumann, R.H., J. W. Day, and C.A Miller. 1984. Mississippi deltaic wetland survival: 

 sedimentation versus coastal submergence. Science 224:1093-1094. 



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