114 



BIOLOGICAL REPORT 31 



Fig. 6.8. Upland retreat rates for Massachusetts 

 coastal communities, expressed as the percentage 

 of total upland lost per century at the present rate of 

 relative sea-level rise Note several barrier beaches 

 and sand spits are not shaded and are not included in 

 the calculations of upland loss for their respective 

 towns From Giese and Aubrey ( 1 987) 



altering the flooding frequency and duration of the 

 wetland plant communities. This necessitates a bal- 

 ance between the rate of sea-level rise and sedi- 

 ment accretion, however. At present rates of sea- 

 level rise. Buzzard Bay wetlands appear to be able 

 to maintain their elevation, and marsh drowning and 

 the conversion of vegetated marsh to open water is 

 not occurring. Given that almost all of the vertical 

 accretion is self-generated (organic matter produced 

 by the plants) rather than trapped imported inor- 

 ganic matter, however, it is likely that at the highest 

 rates of relative sea-level rise some of the wetland 

 area will be converted to open water. This conver- 

 sion will have an associated loss of wetland 



2 4 6 8 10 



Tidal range (ft) 



Fig. 6.9. Vertical range of Spartina alterniflora in rela- 

 tion to the range of the tide Open circles: positions 

 between Massachusetts and Florida, solid circles: 

 positions on Cape Cod The slope of the line is 0.7. 

 From Redfield( 1972) 



functions within the estuarine system, such as nutri- 

 ent transformations, spawning and nursery grounds 

 for fish and shellfish, etc. The rate of sea-level rise 

 that results in the conversion of wetlands to open 

 water is currently the subject of intense study. 



Even if the wetlands can "keep up," changes will 

 occur, and in the Buzzards Bay system the salt marsh 

 area is likely to diminish. As stated, as relative sea- 

 level rises, the upland retreats. Because wetlands 

 are composed of relatively unconsolidated sedi- 

 ments, they persist only in lower wave energy envi- 

 ronments, and therefore they will be eroded back 

 with the land margin on an open shore. The more 

 general case for Buzzards Bay is the development 

 of salt marsh behind a barrier dune complex (Fig. 

 6. 1 0). As sea level rises, storms occasionally erode 

 the dune barrier and wash the sand back onto the 

 marsh in a process called overwash (Fig. 6. 1 1 A). 

 With successive overwash events and continually 

 rising sea level, the dune complex is reestablished 

 inland of the initial location over some of the present 

 marsh, and old marsh surface can be exposed on 

 the new shore (Figs. 6. 1 and 6. 1 1 B) where it is 

 quickly eroded by the direct wave action from Buz- 

 zards Bay. With the now-higher sea level, however. 



