to dilute marine water with freshwater. Given the estimated cost of these projects (25 million 

 dollars for the Caernarvon project alone) and the limited number of suitable sites for such projects, 

 this approach is not generally feasible. 



Currently proposed methods to prevent saltwater intrusion rely on physical barriers such as weirs 

 and levees to control water exchange rates. Strenuous objections to this approach have been raised 

 on the grounds that the altered hydrology may result in diminished overall estuarine productivity, 

 increased marsh break-up, and limited sediment input to the enclosed area. On the other hand, 

 proponents of this approach counter that these problems are preferable to complete loss of the 

 resource and that for the most part there are not enough sediments available for input to 

 compensate for the amount that will be lost as a result of the wholesale marsh erosion that follows 

 the death of the emergent vegetation. Research is now in progress that we hope will shed some 

 light on this controversial issue. 



The question of facilitating the adjustment of our wetland communities to the above-mentioned 

 changes also needs to be seriously investigated. In the case of forested wetlands, the work 

 mentioned dealing with cypress, tupelo gum, and green ash indicated that unless freshwater 

 conditions are maintained, these species will not survive. There are no known tree species that 

 will tolerate prolonged flooding and high salinities at Louisiana latitudes. Louisiana is on the 

 northern fringe of the range of black mangrove, Avicennia germinans, and it is only represented 

 by underdeveloped individuals in the southernmost part of the State. A cold-hardy variety would 

 have application to this problem. 



In the case of herbaceous marsh communities, the situation is not as bleak as in the case of 

 the forested wetlands. There are several species capable of tolerating various degrees of flooding 

 and salinity stress. Among those tolerant of saline marsh conditions are Juncus roemerianus, 

 Distichlis spicata, Spartina alterniflora, Batis maritima, and Salicomia spp. Those tolerant of 

 brackish conditions include Spartina patens, Scirpus olneyi, and Scirpus robustus. By far the most 

 important of these species in terms of management potential is Spartina alterniflora because of its 

 unique ability to withstand extreme levels of flooding and salinity. 



Spartina alterniflora marshes are traditionally restricted to the coastal areas characterized by 

 greater concentrations of mineral soil than the brackish and intermediate marshes interior to them 

 (Chabreck 1972). Thus, as marine water moves farther northward, edaphic conditions may prove 

 unsatisfactory for large scale colonization by Spartina alterniflora. Data regarding this question are 

 inconclusive to date, and further research is needed. For instance (Figure 3), it has been 

 documented that salt marsh soil with bulk densities below 0.20 g/cm 3 will not support growth of 

 Spartina alterniflora (DeLaune et al. 1979; DeLaune 1988). Mineral sediment associated with 

 higher bulk densities is a source of iron and manganese which can precipitate sulfide formed from 

 the reduction of sulfate in seawater which inundates the salt marsh near the coast. Pezeshki et 

 al. (1988b) demonstrated that sulfide can reduce photosynthetic activity of Louisiana wetland plants 

 under certain soil conditions. Much of Louisiana brackish marsh soil contains soil bulk densities 

 of 0.20/g cm 3 or less (Hatton et al. 1983; DeLaune 1988). Thus, it is questionable if Spartina 

 alterniflora can colonize lower density brackish marsh due to the above-mentioned soil-plant 

 chemistry relationships. It would be possible only if the transition is gradual and accompanied by 

 sufficient sediment deposition to increase the soil bulk density to the level which will support 

 growth of Spartina alterniflora. If Spartina alterniflora will successfully reproduce in selected areas 

 and it is infeasible to sufficiently reduce the effects of marine water, then the management 

 objective of erosion prevention would probably best be fulfilled by introducing this species into 

 those areas which possess the characteristics to promote its regeneration. 



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