that live in the mat make it appear to 

 be rooted in the soil, and the floating 

 plant community is dense enough to sup- 

 port a human, but there is no mistaking 

 it for normal marsh when one is walking 

 on it. There are also numerous holes in 

 the mat created by nutria, muskrats, and 

 alligators. The depth of water under the 

 mat averages about 1 m throughout the 

 year. Beneath the flotant, peat accumu- 

 lates over a clay sediment base, as a 

 result of particles dropping through the 

 water column from the overlying root mat 

 (Sasser et al. 1981). Flotant marshes in 

 the MDPR are usually composed primarily 

 of Panicum hemitomom and Sagittaria com- 

 munities, which also include other plant 

 species such as alligatorweed, water- 

 primrose, and spikerush. During periods 

 of high water or severe storms, sections 

 of the mat can break off and float into 

 surrounding water bodies. 



Emergent fresh marsh includes a di- 

 verse macrophyte community made up of 

 the same species that comprise flotant 

 marshes. The primary difference is that 

 the plants are rooted in the sediment. 



The most important factor influ- 

 encing the distribution of marsh types 

 and vegetative communities is hydrology. 

 The fresh marsh in the MDPR, like the 

 other marsh habitats, is characterized 

 by net community production, which im- 

 plies peat production and export of 

 organic matter to adjacent systems. The 

 hydrology of the marsh is a function of 

 inputs from rainfall and terrestrial 

 runoff, evapotranspiration from the 

 marsh surface, groundwater exchanges, 

 and surface runoff. Runoff is deter- 

 mined by the slope of the land, marsh 

 friction or resistance to flow, which is 

 determined by the vegetation, and sedi- 

 ment characteristics. The fresh marshes 

 are bordered upstream by swamps and up- 

 land habitats, and downstream by inter- 

 mediate and brackish marshes. Exchanges 

 of water, organic matter, nutrients, and 

 sediments occur primarily within these 

 habitats (Stone et al. 1978), at least 

 in the pristine situation. In the MDPR, 

 canals often border fresh marsh habitats 

 (see the hydrologic model section in the 

 technical report). 



Fresh marshes are inhabited by a 

 variety of animals. Nutria are the most 

 abundant large herbivores, in contrast 

 to salt marsh habitats, in which musk- 

 rats are more numerous (Bahr and Hebrard 

 1976). Deer, rabbits, raccoons, mice, 

 and other upland mammals may also use 

 these areas periodically as feeding 

 grounds. Many species of birds, in- 

 cluding waterfowl and wading birds, move 

 in and out of the fresh marshes daily as 

 well as during seasonal migrations. The 

 fresh marsh habitat contains the highest 

 density of alligators, and the majority 

 of the alligator harvest comes from 

 these areas (McNease and Joanen 1978) . 

 Other reptiles found in the fresh marsh 

 include snakes, turtles, and lizards. 



Detritivores appear to be the func- 

 tionally dominant fresh marsh consumers. 

 As is the case in the saline and brack- 

 ish marshes, little (about 11%) of the 

 net primary production is directly 

 grazed (Gosselink et al. 1979). About 

 60% of the remaining plant production is 

 consumed by detritivores, including 

 small Crustacea (amphipods and mysids) 

 and microbial decomposers. 



One of the cultural practices in 

 the fresh marsh habitat is an annual 

 marsh burning in the spring. This 

 practice is common in many areas of the 

 MDPR to encourage the production of 

 vegetation favored by furbearing mam- 

 mals, especially nutria. Burning proba- 

 bly stimulates short term primary pro- 

 duction by remineralizing nutrients. It 

 also reduces plant species richness and 

 rough vegetation. It results in a net 

 loss of minerals that blow away with the 

 smoke. When a dry marsh is burned, a 

 "deep burn" occurs that oxidizes much of 

 the stored organic matter in the sedi- 

 ment, which can result in marsh loss. 

 Data are inconclusive as to the overall 

 influence of this practice. 



FRESH OPEN WATER (9) 



Inland freshwater bodies (Figure 

 26) include lakes, ponds, and impounded 

 freshwater areas. These lakes are typ- 

 ically shallow, have depths less than 



54 



