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BIOLOGICAL REPORT 31 



for Buzzards Bay. however, will be the loss of wet- 

 land hectares. Much of the current upland/wetland 

 border is currently or likely to be armored or graded 

 to protect inland areas from flooding by coastal 

 storms. 



The result of these alterations to the upland-marsh 

 boundary is that as the bay edge of the marsh mi- 

 grates inland, the marshes will compress into the 

 upland embankments decreasing their areal extent 

 (Fig. 6. 1 0). This potential mechanism for loss of 

 wetlands is a management issue that increases in 

 importance as coastal development continues in the 

 face of an increasing rate of relative sea-level rise. 

 Ecological impacts of wetland loss extend out into 

 the bay and are possibly multiplied in that the nutri- 

 ent-buffering capacity provided by wetlands may 

 be decreasing just as the loading from the water- 

 shed is increasing. 



Marsh Dieback. Alteration of the flooding fre- 

 quency and duration of wetlands, as a result of 

 changing relative sea level or in response to human 

 alteration of the hydrodynamic regime, can lead to 

 rapid, local large-scale declines of salt marsh plant 

 communities through a process called "dieback." 

 This response is similar to the initial stages of wet- 

 land conversion to open water such as what occurs 

 when sea level is rising faster than the marsh sur- 

 face can accrete. Salt marsh dieback is a poorly 

 understood phenomenon where large stands of tidal 

 wetland plants simply die. Major dieback events 

 have occurred in North Carolina, Louisiana, parts 

 of the mid-Atlantic coast, and Great Britain 

 (Goodman and Williams 1961; Smith 1970; Sears 

 and Parker 1981). The first documented case of 

 marsh dieback in New England occurred at Nonquitt 

 Marsh, South Dartmouth, beginning in the mid- 

 1 970's. By September 1 980 over 60% of the for- 

 merly healthy stands of Spurt ina alterniflora had 

 become denuded. The marsh lies along the south- 

 western shore of Buzzards Bay and is bordered on 

 three sides by hardwood and pine upland and by a 

 barrier beach and road running parallel to the shore. 

 Tidal exchange with the bay is through a culvert run- 

 ning under the road, which operated for over four 

 decades prior to the dieback event. Investigations 

 conducted into the potential cause of this dieback 



(Sears and Parker 1 98 1 ) ruled out domestic and 

 chemical pollutants. 



Although there are many hypothetical causes for 

 various dieback events, the one at Nonquitt Marsh 

 is thought to be due to restriction of tidal exchange. 

 The adverse impact of impeded circulation within 

 this type of system is extended soil waterlogging, 

 which results in oxygen deficiency that can alter the 

 physiology and growth ofSpartina ( Mendelssohn 

 and Seneca 1980; Howes et al. 1986). Between 

 60% and 80% of the tidal volume remains within 

 Nonquitt Marsh between tidal cycles, as compared 

 to other healthy systems like Barnstable Marsh. 

 Massachusetts, where only 1 0% of the volume re- 

 mains in the confines of the marsh system at low 

 tide (Redfield 1 972). In Nonquitt, the marsh was 

 apparently able to tolerate restricted circulation for 

 several decades until storms appear to have caused 

 excessive clogging of the culvert, triggering the die- 

 back event. 



Although dieback occurs rapidly, recovery oc- 

 curs over longer periods, even after adequate tidal 

 exchange is restored. In Nonquitt, initial regrowth 

 along the denuded edges began immediately, with 

 naturally invading plants having somewhat more 

 success in colonization than transplants. Four years 

 later most of the barren areas remained wet or wa- 

 terlogged with little new growth, except for sparse 

 occurrences of rapidly colonizing Salicornia spe- 

 cies. The remaining large denuded areas are most 

 likely due to the highly reducing conditions resulting 

 from extended soil waterlogging, as well as salinity 

 elevation in shallow pools and sediments on the 

 marsh surface resulting from evaporative losses. 

 Concentrations of up to 55 ppt chloride were found 

 in the top 2 cm of sediment in denuded areas (three- 

 fold higher than Buzzards Bay waters); however, 

 tidal water salinities did not appear significantly el- 

 evated (D. Goehringer, unpublished data). It ap- 

 pears that as plants recolonize the edges of this 

 marsh, sediment characteristics become more fa- 

 vorable for growth with increased oxidation of the 

 sediments in the presence of plants (Howes et al. 

 1 986). Without restriction to circulation, much of 

 the wetland probably will recover to predieoff con- 

 ditions. On the other hand, the settling of peat and 



