accretion deficit and spoil bank placement. Although no documentation exists for stable marsh in 

 the park more than 30 years ago, Williamson et al. (1984) reported that fresh marsh north of Lake 

 Salvador converted from stable marsh to flotant during the last three decades. The flotant 

 formation characteristics in Jean Lafitte National Park are consistent with other observed flotant 

 patterns: a sediment deficit, hydrologic alteration, prolonged flooding and mat detachment, and 

 successional stages developing into shrub-scrub species. 



The following steps constitute our hypothesis for the development of floating marsh in Jean 

 Lafitte National Park: 



(1) The park experiences a sediment accretion deficit and levees (spoil banks) inhibit sheet 

 flow. Because local sedimentation is less than apparent water level rise, there is an 

 increasing sediment accretion deficit, especially in the interior parts of the park distant 

 from sediment sources. 



(2) Low bulk density substrate roots combined with anaerobic conditions and methane gas 

 formation beneath cause the marsh mat to detach from the bottom and float (O'Neil 1949; 

 Cypert 1972; Hogg and Wein 1988). 



(3) Sagittaria lancifolia succeeds to Panicum spp. which succeeds to Myrica cerifa in fresh 

 areas. 



(4) Small salinity increases are accompanied by encroachment of Spartina patens. 



Jean Lafitte National Park is located in an interdistributary basin which, because of flood control 

 levees, no longer receives direct sediments from the Mississippi River. Spoil banks and altered 

 hydrology reduce input of resuspended sediments. The area is experiencing a sediment accretion 

 deficit with respect to local apparent water level rise. Soil characteristics and field observations 

 indicate that sites with little or no sediment input and hydrologic alterations have low bulk density, 

 high organic content, and high water content, characteristics of floating marshes. 



Canal spoil banks in the park inhibit sheet flow and cause water to flood the marsh for longer 

 periods of time. These conditions tend to accelerate the formation of a floating marsh. Prolonged 

 flooding of fresh marsh results in anaerobic conditions which enhance methane gas formation 

 beneath the vegetative mat. Upward gaseous pressure beneath the mat, in combination with the 

 low bulk density of vegetation roots, causes the mat to detach from the bottom and float up like 

 a cork (O'Neil 1949; Cypert 1972; Hogg and Wein 1988). After detachment from the solid 

 substrate, the top of the vegetation mat floats a few centimeters above the water level and thus 

 no longer floods. The floating mat oscillates freely up and down in phase with the water layer 

 beneath it (Swarzenski 1987). Field observations of several wax myrtle stands in the central area 

 of the park (stations 10 and 15) indicate that these sites are freely floating. The response of this 

 system to the conditions discussed above has been to succeed to a floating wetland ecosystem. 



Hydrological alteration has led to the formation of floating marsh in several regions. In northern 

 Wisconsin, floating bog formed behind a sand sill adjacent to the lake (R.P. Novitski, U.S. 

 Geological Survey, Ithaca, NY 14850; pers. comm.). A Canadian floating Typha spp. marsh 

 emerged in a diked freshwater impoundment (Hogg and Wein 1988). In Louisiana, Bahr et al. 

 (1983) noted that 4,000 ha of scrub-shrub developed in the Barataria and Verret Basins since the 

 1950's as a result of human modifications to basin hydrology. Much of this newly emerged scrub- 

 ,shrub in Louisiana is floating (R. Chabreck, School of Wildlife, Forestry, and Fisheries, Louisiana 

 State University, Baton Rouge, LA 70803; pers. comm.). The emergence of the floating scrub- 

 shrub in the park and areas of the Verret and Barataria Basins both occurred during the same time 

 and under the same circumstances: post 1950's canal construction activities. 



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