The objectives of this study were to (1) determine under field conditions to what extent saltwater 

 intrusion and subsidence-induced plant submergence are responsible for the deterioration of 

 brackish marshes in Louisiana, and (2) determine the relative importance of these two factors in 

 preventing plant establishment in brackish marshes that are in a state of degradation. It should 

 be noted that the factors that initially caused the degradation of a wetland may not be the same 

 factors preventing subsequent plant colonization. 



MATERIALS AND METHODS 



Effect of Increased Salinity and Submergence on Plant Biomass in a Brackish Marsh 



Study Site. The study was conducted in part in a brackish marsh site dominated by Spartina 

 patens. This site was located at the northern end of Bayou Mink near Leeville, LA. Sods with 

 intact vegetation were transported from this site (salinity range = 11 to 15 ppt) to a higher salinity 

 marsh (salinity range = 21-28 ppt) located 30 km south near Airplane Lake. The second site was 

 dominated by S. alterniflora. 



Experimental Design. The following design was used for the simulation of subsidence, saltwater 

 intrusion, or a combination of the two. 



Simulation of subsidence. Water level was increased by removing sections of marsh (soil cores 

 0.10 m 2 surface area and 30 cm deep with intact vegetation) and replacing them in their original 

 locations, but at a lower elevation (-10 cm) (Figure 1). Disturbed controls, which were located 

 adjacent to the subsidence treatment sods, were removed in exactly the same manner as the 

 treatment sods, but were replaced in their original locations and elevations. Undisturbed controls 

 were established by marking off areas of the same size but were not disturbed in any way. Sod 

 disturbance had no effect on the accumulation of aboveground biomass of S. patens (Mendelssohn 

 and McKee 1987). 



Simulation of saltwater intrusion. A consistent and homogeneous increase in soil salinity over 

 a long period of time by additions of NaCl or saltwater would be difficult and expensive to achieve 

 in the field. Instead, saltwater intrusion was simulated by removing sections of vegetated marsh 

 (same as above) from the original donor marsh and transplanted to a recipient marsh, where the 

 salinity was higher (Figure 1). In this way, a continuous input of water at the desired salinity level 

 (within a specific range) was assured for the duration of the study. We recognize the nutrient and 

 sulfide concentrations may be different between donor and recipient marshes and may affect plant 

 response such that the result may not be exactly the same as what would occur during saltwater 

 intrusion. Treatment sods were removed with the intact vegetation and transported by boat to a 

 recipient marsh where they were inserted into the substrate at three different elevations (Figure 

 1)- 



Salinity-subsidence interaction. Sods from the donor marshes were placed into the recipient 

 marshes at three elevations: equal to the marsh surface (simulated saltwater intrusion only), 10 cm 

 below the marsh surface (simulated saltwater intrusion and subsidence together), and 10 cm above 

 marsh surface (simulated saltwater intrusion but reduced the effect of submergence) (Figure 1). 

 The latter treatment was established primarily to ensure that there was a salinity effect without an 

 increased submergence effect (due to the difference in surface elevation between the donor and 

 recipient marshes). It also allowed the examination of plant response to increased salinity at 

 reduced levels of flooding. 



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