The overall response of wetland species from coastal Louisiana to saline water treatment is a 

 relatively long-lasting reduction in net carbon assimilation rates (Table 1). This response agrees 

 with studies reporting the effects of salt stress on coastal plants from other wetland regions 

 (Longstreth and Strain 1977; Drake and Gallagher 1984; Pearcy and Austin 1984). The response 

 ranges from decline in gas exchange rates under low salt concentrations to complete tissue death 

 under high salinity, especially in freshwater marsh species. The overall effect of decline in daily 

 rates of gas exchange is reduced carbon fixation leading to inhibition of growth and productivity. 



The reported studies of selected wetland species from the Louisiana gulf coast shows that 

 saltwater intrusion causes reduction of net photosynthesis in wetland plants. The effect is 

 dependent on vegetation type and concentration of salt. The observed impacts are at salt levels 

 similar to those currently occurring in coastal Louisiana because of saltwater intrusion and brine 

 discharge from oil recovery operations (Salinas et al. 1986). Results suggest that many of these 

 species continue to encounter an increasingly greater level of stress as a result of saltwater 

 intrusion. The stress-induced reduction in photosynthetic rates adversely affects carbon fixation and 

 net primary productivity of brackish species which are experiencing saltwater intrusion. Freshwater 

 habitats including freshwater marshes and bottomland forests will also be affected as saltwater 

 intrusion continues inland. 



Reduction in productivity of wetland species will affect estuarine carbon cycling as well as organic 

 matter pools, the important governing factors for vertical marsh accretion (DeLaune et al. 1978). 

 Organic carbon accumulation is essential in maintaining intertidal marshes. Smith et al. (1983) 

 reported that approximately 292 g C/m a are accumulated through processes of vertical marsh 

 accretion in brackish marshes in Louisiana. The source of this organic carbon is mainly from 

 primary production of marsh macrophytes. Reduction in this source will adversely affect the ability 

 of these marshes to remain intertidal, causing further marsh degradation. Similar effects on net 

 primary productivity, carbon allocation, and nutrient cycling are expected in lowland coastal forests 

 where the effect of these stresses are beginning to appear. Further research is needed to study 

 the long-term responses of other important wetland species from the gulf coast region to gradual 

 increases in flooding and salinity under field conditions and to examine the effects on survival, 

 productivity, and nutrient cycling of major ecosystems. 



MANAGEMENT IMPLICATIONS 



The general consequences of hydrologic changes resulting from canalization, subsidence, and 

 sediment deprivation are a net movement of marine water northward into Louisiana's coastal 

 wetlands. This is characterized by an overall increase in reducing conditions, increased salinities, 

 and increased hydrogen sulfide production in marsh soils. Many emergent marsh communities 

 have apparently disappeared due to the detrimental effects of these stress factors. Increased open 

 water exacerbates the problem because this causes greater edge exposure and erosion. 



There is a general consensus that the continuing loss of this wetland resource base is 

 unacceptable. Responsible management must be directed to the extent possible toward finding 

 cost-effective means to reverse or slow down the effects of marine water encroachment, and aid 

 impacted wetland communities to adapt to these changes. 



To reduce the amount of marine water in our wetland systems, additional freshwater must be 

 introduced or marine water must be physically prevented from entering the system. The freshwater 

 diversion project at Caernarvon is an excellent example of management designed to add freshwater 



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