Vegetation surrounding the experimental plots in both donor and recipient marshes was removed 

 by clipping to within 25 cm of the marsh surface to eliminate the influence of shading on the 

 experimental plants. Vegetation within the treatment and control plots was trimmed to a height 

 of 40 cm in order to eliminate dead material from the previous year. The experiment was 

 replicated five times. 



The field plots were established during May 1986 and monitored over the succeeding months. 

 The experiment was terminated in October 1986. 



Factors Preventing Plant Reestablishment in a Brackish Marsh 



Study Site. The study sites were located in a brackish marsh at Catfish Lake near Golden 

 Meadow, LA. The dieback site was unvegetated and characterized by a soft substrate and standing 

 water. Spartina patens and S. alterniflora were the dominant species in the surrounding vegetated 

 area. Spoil levees originating from the dredging of an oil access canal and adjacent navigation 

 channels circumscribed the dieback site. The control site was located along Bayou Faleau, a 

 natural watercourse within 0.3 km of the dieback site. This area was characterized by unimpeded 

 drainage and 100% vegetative cover composed predominately of S. alterniflora, S. patens, and 

 Distichlis spicata. Salinity levels at control and dieback sites at Catfish Lake were similar and 

 ranged from 11 to 16 ppt during the course of the study. 



Experimental Design Sods of healthy marsh (soil cores 0.10 m 2 surface area and 30 cm deep 

 with intact vegetation) containing either S. alterniflora or S. patens were exhumed from the control 

 site and moved to the dieback site where they were inserted into the substrate at two elevations: 

 equivalent to that of the dieback marsh surface and 10 cm above the marsh surface. 



Disturbed controls in the healthy marshes were removed in exactly the same manner as the 

 transplanted sods but were replaced in their original locations and elevations. Sod disturbance, per 

 se, had no significant effect on the accumulation of aboveground biomass of S. alterniflora 

 (Mendelssohn and McKee 1987). The experiment, which was replicated five times, was initiated 

 in April 1987 and terminated in October 1987. 



Plant and Soil Analyses 



All aboveground material in each experimental plot was clipped at the soil surface and placed 

 into plastic bags. These samples were later analyzed for stem density and live and dead 

 aboveground biomass (after drying at 65 °C). Brightened platinum electrodes were inserted into 

 the soil of each sod at two depths (1 and 15 cm) and allowed to equilibrate for 1 hour prior to 

 measurement of soil redox potential (Eh). Eh was calculated by adding the potential of a standard 

 calomel reference electrode (244 mV) to the millivolt reading. Interstitial water was collected 

 with an in situ water sampler as described in McKee et al. (1988). An aliquot of the water was 

 immediately added to an antioxidant buffer and analyzed for sulfide concentration with a sulfide 

 electrode (Lazar Model IS-146, Lazar Research Laboratories, Los Angeles, CA) (see McKee et 

 al. 1988 for details). The remaining water was used for the measurement of salinity (Fisher 

 conductivity meter), pH, and NH^ concentration (EPA Method 353.2, U.S. Environmental 

 Protection Agency 1979). 



RESULTS 



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



An increase in salinity from approximately 15 ppt to 26 ppt caused a significant reduction in 

 height, biomass, and density of 5. patens in the field (F = 56.38, 93.39, and 80.16, respectively; 



357 



