Management Implications of Saltwater Intrusion 



Saltwater intrusion has historically been identified as one of the principal causes of wetland loss, 

 so controlling it--mostly with structures-has been one of the principal goals of wetland management 

 in Louisiana. Yet considerable literature suggests that increased submergence and sediment deficits 

 may be as important in causing vegetation diebacks in the coastal zone. 



Submergence results in poorer drainage and increased waterlogging of wetland soils (Day et al. 

 1987). The deleterious effects of waterlogging on plants has been amply reported in the literature. 

 For example, responses of plant shoots to water logging may include reduced stem elongation, 

 chlorosis, senescence, abscission of lower leaves, wilting, hypertrophy, epinasty, leaf curling, and a 

 decline in relative growth rate (Drew 1983; Jackson and Drew 1984). Apparent water level rise 

 leads to submergence and waterlogging stress. 



Williamson et al. (1984) determined that inundation, not salinity, was the principal cause of the 

 decline of floating wax myrtle stands in the Lake Salvador management area. In addition, 

 Mendelssohn and McKee (1988) reported that in a laboratory study, sudden submergence of 10 

 cm had a significant negative effect on the biomass productivity of salt, brackish, and fresh marsh 

 species. Increased salinity had a negative effect on fresh and brackish marsh species, the extent 

 of which was dependent on salinity level and duration and abruptness of the stress. Analysis of 

 freshwater and brackish species tolerances to salinity and inundation reveals that many of these 

 species have rather wide ranges of salinity tolerance and only limited inundation tolerances with 

 respect to environmental conditions in the park. Future research is needed on the relative 

 importance of submergence and salinity as the cause of vegetation die-offs in the coastal zone. 



Two alternate approaches could be used to deal with problems of salinity intrusion. The first 

 approach is site specific where intrusion of saltwater to a specific area is limited by a combination 

 of structural devices such as levees and weirs. There is evidence that this method has had limited 

 success in the Louisiana coastal zone (Cowan et al. 1986). An alternate approach is basin wide 

 management of freshwater resources and hydrology. The rate of saltwater input to the southern 

 coastal region, via canals such as the Barataria Waterway, could be slowed by locks, gates, and 

 canals closures. Long term effects of increased salinity (Figure 8) strongly suggest that the 

 enlargement of the Barataria Bay waterway resulted in an abrupt increase in salinity in the mid- 

 basin. In the upper basin, diversion of Mississippi River waters and management for retention 

 of freshwater could be used to both increase sediment sources to wetland and buffer and dilute 

 saltwater flow within the wetlands. Since most of the upper Barataria Basin is channelized, 

 freshwater runoff through the coastal zone proceeds very quickly. Therefore the goals would be 

 to slow both freshwater runoff and saltwater input to the wetlands. 



The rate of saltwater intrusion into a healthy fresh marsh is very important. If saltwater 

 intrusion into a fresh area is rapid, sudden, and includes a significant salinity increase, then salinity 

 can be a problem. But this is not the case in Jean Lafitte National Park; salinity increase has been 

 slow enough that the plants and soils have been able to adapt to the changes. 



The Soil Conservation Service proposed a management plan for the park which advocated the 

 use of plugs along all the oil slips intersecting the Segnette Waterway (Figure 3; SCS 1985). In 

 light of the results of this study, which show small salinity increases and healthy robust vegetative 

 communities where the salinity increase has occurred, it seems more important to practice 

 management which enhances open access to the sediment sources derived from the lake and 

 surrounding bayous and canals, than to deter waterflows with weirs and plugs at the current salinity 

 levels. Plugs and weirs could lead to further waterlogging and more rapid flotant formation. It 



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