Production and Species Richness. Oxygen is an 

 essential element in the metabolic processes con- 

 trolling life. When wetland flooding (inundation) 

 occurs, the amount of oxygen reaching the sediment 

 surface is reduced. As the available oxygen in the 

 sediments is depleted by benthic organisms, a com- 

 plex series of chemical transformations take place, 

 producing hydrogen sulfide, methane, and other 

 toxic substances (DeLaune et al. 1976). 



One unique feature of the marsh biota is the 

 ability to tolerate these anerobic conditions through 

 the evolution of speciahzed mechanisms. For instance, 

 most marsh grasses have anatomical structures that 

 enable atmospheric oxygen to diffuse through leaves 

 and stems to roots (Armstrong 1975). 



The relationship between plant success and the 

 inundation regime is not known quantitatively. 

 However, hydrological parameters (flooding fre- 

 quency and duration, water depth, and current 

 velocity) determine the sediment-carrying capacity 

 and the level of nutrients flowing into a marsh. 

 The inundation pattern in coastal wetlands varies 

 across habitats. As the tidal influence decreases 

 from salt to fresh marshes, the frequency of wet- 

 land inundation also decreases and the average 

 duration of each flooding increases. However, the 

 total yearly inundation time remains fairly constant 

 across all habitats (part 3.3). 



The high productivity of salt marshes has been 

 attributed to the regular flushing with tidal waters 

 that carry in nutrients and sediments, and carry out 

 wastes (Schelske and Odum 1961). Recent studies in 

 fresh and brackish wetlands, however, show that 

 these irregularly flooded marshes can be as productive 

 as salt marshes (Good et al. 1978). Fresh and brackish 

 marsh plants appear to be adapted to long periods of 

 root exposure to anoxic sediments, and apparently 

 depend less on nutrients and sediments brouglit 

 in by flooding waters and more on recycUng of avail- 

 able nutrients than plants found in regularly flooded 

 salt marshes. 



Hydrology and Sedimentation. The process of 

 sedimentation and the proportion of organic to in- 

 organic material in marsh sediments is largely con- 

 trolled by the hydrologic regime. Marshes act as traps 

 for sediments which are carried by water flowing 

 across them. As the water in adjacent streams rises and 

 floods over the marsh surface, velocities slow and 

 suspended sediments drop out. Knowledge of the 

 sediment-trapping capacity of salt marshes, in par- 

 ticular, has been extensively used in Europe to build 

 land in areas where high tidal energy results in large 

 suspended loads (Zurr 1952,Dalby 1957). 



As early as 1888, Dunbar (Coates 1972) described 

 how harbors of New England filled with silt when the 

 great marshes were drained and leveed. This occurred 

 because the marshes were no longer able to trap silt, 

 and because water currents in tidal passes were reduced 

 as the intertidaJ volume decreased. 



In addition to acting as a sink for suspended 

 sediments (largely inorganic), marshes are also a source 

 of organic detritus which can be incorporated into 

 the marsh assediment. The more vigorous the flooding 

 action, the more organic detritus is exported from the 

 marsh (Gosselink et al. 1977). As a result, the organic- 

 inorganic mix of wetland sediments depends largely 

 upon the hydrologic regime. High energy marshes 

 accumulate inorganic sediments. If current velocities 

 are slow and inundation periods long, as in inter- 

 mediate, brackish, and fresh marshes, little inorganic 

 sediment is brought in, detritus is deposited, and 

 marsh sediments are peaty. 



The rate of peat formation in the Chenier Plain 

 varies greatly. A cross section of sediments across the 

 Chenier Plain shows that the surface peat layer is 

 seldom over 1 .5 m (5 ft) thick and is generally under- 

 lain by silts and clays of the Pleistocene Terrace 

 (Gould and McFarlan 1959). Radiocarbon dating of 

 marsh peats has revealed deposition rates averaging 

 from 0.3 to 1.2mm/yr (0.01 to 0.05 in/yr) (Gould 

 and McFarlan 1959). Most of the Louisiana Chenier 

 Plain coastal marshes are subsiding, yet, the marsh 

 surface stays at the same level relative to local water 

 levels. This suggests that the rate of deposition is as 

 great as the rate of subsidence and that water level 

 and circulation play a vital role in determining what 

 proportion of detritus contributes to peat. If the 

 deposition rate becomes slower than that of subsi- 

 dence, the frequency and duration of inundation 

 increases, resulting in plant death and erosion of wet- 

 lands to open waters. 



Export of Detritus. Recent estimates indicate 

 that about 10% of the litterfall in swamp forests are 

 exported (Butler 1975) compared to 30% of salt marsh 

 production (Hopkinson 1973). If detritus export is 

 assumed to be proportional to the frequency of inunda- 

 tion (a measure of the magnitude of flushing), it is 

 possible to estimate the expected export from other 

 marsh habitats. Figure 4-6 illustrates the hypothesized 

 linear relationship between the frequency of flooding 

 and organic export from wetlands. The coastal region 

 is inundated almost daily by tides, but the more inland 

 marshes, especially fresh marshes, are flooded by less 

 frequent wind tides and during periods of high rain- 

 faU (Hopkinson 1973, Butler 1975, Byrne et al. 1976). 

 As a result, less detritus is exported from these 

 marshes than from salt marshes (table 4.1). Severe 

 stomis are also important in flushing wetlands, but no 

 information is available about the magnitude of storm 

 effects. 



4.2.3 DYNAMICS OF ENERGY FLOW IN WET- 

 LANDS 



Primary production, the conversion of solar to 

 chemical energy by plants, sets an upper limit to the 

 flow of energy through habitats. This chemical energy, 

 fixed as plant tissue, is the energy available for the 

 rest of the food web, so the potential for secondary 

 production of fish, waterfowl, and furbearers is di- 

 rectly related to the magnitude of primary produc- 

 tion. An examination of the energy pathway through 



159 



