marsh surface turf (Redfield 1972). Pond holes in the middle of high (salt 

 hay) marshes are characteristic of newly formed high marsh and, in older high 

 marshes, may exist in several developmental forms. Pond holes that have been 

 drained as the result of erosion or ditching are invaded by stands of 

 cordgrass . 



Little data are available on the size, floristic composition, and frequency of 

 salt pannes in Maine. Gore (1965) and Rasar (1968) discuss the occurrence in 

 Maine and function of widgeon grass pools ( Ruppia maritima) , which are flooded 

 forms of salt pannes. 



Ecological succession may be defined as "...an orderly process of community 

 development that involves changes in species structure and community processes 

 with time" (Odum 1971). In the intertidal salt marsh, successsion culminates 

 in change in the biotic structure of the flora (figure 5-45). However, the 

 floristic changes are triggered most often by (1) changes in the height of the 

 marsh surface relative to mean sea level; (2) changes in tidal range and 

 inundation; (3) changes in the drainage pattern across the marsh; and (4) 

 change of the salinity regime in the marsh. 



Changes in the height of the marsh surface are most often the result of 

 accumulation of sediment in the marsh (sedimentation) . Human activities can 

 also influence marsh elevation (e.g., ditching and filling). As salt marsh 

 grasses slow the velocity of tidal flow, particles in the water are more apt 

 to become deposited on the marsh. As the height on the marsh floor increases, 

 the inundation patterns change and species that require less inundation become 

 established. Primary succession in a New England salt marsh with increasing 

 elevation proceeds with cordgrass giving way to salt hay (figure 5-42). 



Retrogressive succession, in which high marsh becomes progressively wetter due 

 to subsidence of the marsh floor or a rise in the level of sea water causes 

 salt hay to be replaced by cordgrass. Any natural or artificial disturbance 

 (e.g., ditching or canals) of drainage across the marsh, tidal regime, 

 sedimentation, or salinity can accelerate or reverse salt marsh succession. 



Keser and coworkers (1978) studied the effect of removing the Cowseagan 

 Narrows Causeway (Montsweag Bay, region 2) on the intertidal community of 

 cordgrass. The causeway had for many years acted as a restraint to tidal 

 waters flowing up and down the bay. Its removal initiated retrogressive 

 succession in the neighboring causeway salt marsh. The physical parameters of 

 the upper bay changed overnight: 



"After removal of the causeway in the fall of 1974, the 

 average tidal range increased by 0.34 m reflecting greater 

 water flow in and out of Montsweag Bay. The average high 

 tide was 0.06 m higher and the average was 0.27 m lower. 

 The resultant average tidal range, 2.88 m, exposed an 

 additional 109 to 168 ha (269 to 415 acres) of mud flat 

 during each tide" (Anonymous, in Keser et al. 1978). 



5-119 



10-80 



