Approach 



We plan to set up replicated 2-ha impoundments (50 ha total; Figure 2) in brackish or salt 

 marsh in the Terrebonne/Timbalier watershed and in a yet to be identified site in fresh or 

 intermediate marsh. We still lack the expected funding for a dredge from Sea Grant, who has been 

 quite cooperative in helping us establish smaller study sites. 



Each replicated suite of impoundments (or semi-impoundments) will have three marshes with 

 three different water levels, one marsh with a natural channel, and one marsh with a natural levee 

 but no channel through it. 



A commercial dredging company will dredge, and individual landholding companies will supervise 

 construction of weirs. It may take as long as 6 months to lay the transit lines, dredge, construct 

 weirs, and do minor post-construction repair work. 



The first impoundment locations are in salt marsh for several reasons. First, salt and brackish 

 marshes account for 62% of the coastal wetlands, and 30-40% of these marshes are already either 

 completely or partially impounded. Second, sediment transport is more of an issue there than in 

 fresh marshes, which are more difficult to work in because of the organic "soup" underlying many 

 of the marshes. Third, the Louisiana Universities Marine Consortium (LUMCON) field research 

 facilities are nearby. Fourth, the landowner has available saltmarsh to use; and fifth, there are 

 relatively few oil and gas fields in the area. 



The following will be examined: 



Soil Properties: Soils respond rapidly to alterations in wetland hydrology because of the relatively 

 quick growth of soil microorganisms and fast chemical reactions. The long-term development of 

 soils can indicate the future of plants, usage, and sediment accumulation. It is, therefore, useful 

 to examine how hydrologic changes affect soil properties and the associated distribution and 

 abundance of plant communities. 



Plant and Sedimentation Rates: In general, salt marsh soils and plants are very sensitive to 

 change in the hydrologic regime (Howes et al. 1981). Mendelssohn et al. (1981, 1982) concluded 

 that the factors inhibiting plant growth in salt marshes are directly related to increased anaerobic 

 soil conditions and factors leading to low Eh. Low soil Eh may indirectly or directly result in 

 nitrogen deficiencies. Related factors include root oxygen deficiencies, toxins produced by soil 

 anaerobic respiration, reduced water movement causing nutrient depletion near the root, decreased 

 root metabolism, and a smaller oxidized rhizosphere which acts to buffer the plant against soil 

 toxins. 



The reduction status of wetland sediments influences the growth of plants, in part, through the 

 plant rhizosphere by affecting the biological availability of the more important plant nutrients, such 

 as nitrogen and phosphorus. Because of the greater likelihood of losses due to denitrification, 

 nitrogen is limiting. Phosphorus is generally more available under flooded conditions (Patrick and 

 DeLaune 1977). Waterlogged soils high in organic matter may become highly reduced, resulting 

 in the reduction of sulfate to sulfide (Gambrell and Patrick 1978). Sulfide is toxic to the biota if 

 it reaches a high concentration. Sulfide leaves the system as hydrogen sulfide gas or can precipitate 

 with ferrous iron to form ferrous sulfide which is not toxic (Patrick and DeLaune 1977). Forms 

 of sulfide can build up in marsh soils, stressing both plants and animals. A study in a Georgia salt 

 marsh showed that where water movement was experimentally increased, plant productivity doubled 

 (Wiegert et al. 1983); where sulfide concentrations were the highest, water movement and plant 



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