accumulate decaying vegetation and elevate to high marsh, outstripping sea 

 water rise and sediment compaction. Redfield (1972) reported that the marsh 

 surface on which cordgrass became colonized rose 1 foot (30 cm) in 6 years due 

 to sedimentation but gradually slowed as the marsh surface approached high 

 water. The deposition of sediments changes inundation patterns and salinity 

 regimes and may induce successional patterns in the marsh vegetation as other 

 higher marsh plants, such as salt hay and black rush, invade the area. 



In higher energy situations salt marshes and protective barrier structures may 

 transgress inland as high energy waves breach the protective sand structures 

 pushing the marsh inland (figure 5-42). Two geological processes are 

 responsible for transgression; the actual rise in sea level (eustatic rise) 

 caused by melting ice caps and the compaction of sediments, which causes the 

 near shore land surface to subside gradually. 



From a geological point of view salt marshes are very recent formations and 

 undergo change very rapidly. For example, the peat in the Barnstable Marsh is 

 25 feet deep due to the relative rise of the sea compared to the land 

 (Redfield 1972). Radiocarbon dating showed that the oldest peats 25 feet 

 below the present sea level were approximately 4000 years old (Post 

 Quaternary). Unlike geological features such as mountains or rocky headlands, 

 salt marshes are modified through accretion, erosion, and transgression over a 

 period of a few years rather than hundreds or thousands of years. Thus, they 

 are relatively fragile geological entities. 



The daily ebb and flow of the tide is the prime regulatory factor in coastal 

 salt marsh ecosystems (Miller and Egler 1950). Salts and nutrients are 

 introduced to and removed from the marsh with the twice-daily tides. Salt 

 marsh floristic zonation is dependent upon the frequency and duration of tidal 

 inundation (Adams 1963). Tidal inundation affects all the physical regulatory 

 parameters cited by Penfound (1952) as influencing plant distribution in 

 estuarine emergent wetlands. 



The relationship between the vertical range of the tide and the distribution 

 of the three dominant emergent species in a Maine salt marsh has been 

 determined by Linthurst (1977; table 5-l4). These vertical ranges become 

 meaningful when compared with inundation patterns derived by Chapman (1940a) 

 for a Massachusetts salt marsh. Inundation patterns as a function of height 

 above mean low water are similar in Massachusetts and Maine (table 5-13). 



The lower limit of growth for cordgrass was determined to be 7.6 feet (2.3 m) 

 above mean low water (MLW) in a Maine salt marsh. This translates into 

 approximately 690 inundations/year (about 2/day) with a submergence time of 6 

 hours/day. No other emergent species can compete with cordgrass and tolerate 

 the physical and chemical stresses associated with the submergence pattern. 



Salt hay begins to intergrade at 9 feet (2.7 m) above MLW. Salt hay in Maine 

 is characterized by an average of 200 inundations per year, with a submergence 

 time of about 2 hours/day. The vertical range of distribution for salt hay is 

 quite restricted, <1 foot (30 cm). 



5-110 



