110 



BIOLOGICAL REPORT 31 



over the past several thousand years: subsidence 

 of the land and changing oceanic or eustatic sea 

 level. 



The subsidence of the watershed of Buzzards 

 Bay (and the region) is due to the after effects of 

 the same ice sheets that led to its formation. As the 

 Laurentide Ice Sheet, with its Buzzards Bay, Cape 

 Cod Bay, and South Channel lobes, formed and 

 expanded, it imposed an overburden on the earth's 

 surface. This overburden resulted in a "sinking" of 

 the land surface under the load and a rise around 

 the margin of the ice sheet. The rise around the 

 margin is called the peripheral bulge, which during 

 the last ice age extended along much of the Atlantic 

 coast with a high centered near Cape Hatteras (cf. 

 Emery and Aubrey 1 99 1 ). As the ice sheet melted, 

 the weight was released and the land surface re- 

 bounded with a concomitant subsidence of the pe- 

 ripheral bulge. The initial rise in sea level into 

 Buzzards Bay resulted from this subsidence and the 

 return to the oceans of the vast amount of water 

 that had been held in the ice sheets. 



The fraction of relative sea-level rise resulting 

 from eustatic (oceanic) sea-level rise is of present 

 concern in that the rate of rise may be accelerating 

 because of climate change. Predictions of eustatic 

 sea-level rise over the next century are driven pre- 

 dominately by attempts to assess the extent of ther- 

 mal expansion of ocean water and the volume of 

 "new" water entering from the current glacial stocks, 

 both factors related to hypothesized global wann- 

 ing trends. 



Because the Buzzards Bay lobe extended south 

 of the bay and a terminal moraine extended south 

 to Long Island, recent sea-level rise in Buzzards 

 Bay can be evaluated within this regional context. 

 The contours generated from many spatially sepa- 

 rated tide gauges measuring relative sea level in- 

 crease tend to parallel the historic margin of the 

 Laurentide Ice Sheet with Buzzards Bay at the pe- 

 ripheral bulge (Fig. 6.5). Recent rates of relative 

 sea-level rise in southeastern New England and 

 Buzzards Bay have been determined from tide gauge 

 records, generally from within the past 70 years (Tig. 

 6.5 A and B). Although there is significant year-to- 



year variation in the records, all of the tide gauges 

 in the region indicate increasing relative sea levels, 

 although the long-term rates vary. In the Buzzards 

 Bay watershed over the past century, relative sea 

 level has been rising at about 0.24 m/1 00 years (2 

 mm/year) with about 0.09 m from global sea level 

 change and 0. 1 5 m from sinking of the land (Emery 

 and Aubrey 1 99 1 ). Whatever the cause of the rise 

 and regardless of the debate over acceleration in 

 the current eustatic component, the relative sea level 

 will continue to rise at least at current rates, and the 

 modification this produces to Buzzards Bay shores 

 will continue far into the future. 



Effects on Upland Area. As the sea rises in 

 this region, it covers (floods) the historic upland 

 surface. The degree of encroachment on the land is 

 directly related to the amount of land at each new 

 flooding elevation and to the erosional retreat of the 

 upland face, headlands, and scarps. However, in 

 Buzzards Bay most of the upland is protected from 

 ocean waves, and the flooding over or passive re- 

 treat of the upland edge is the major contributor to 

 land loss (Geise 1 989). It is not possible, at present, 

 to ascertain the incremental retreat at each point 

 along the Buzzards Bay coastline. However, be- 

 cause sea level rises over long periods and the land- 

 scape in most areas decreases in elevation, it is pos- 

 sible to predict the long-term rate of passive retreat 

 of the upland edge from hypsometric curves of the 

 upland topography. An upland hypsometric curve 

 is the cumulative percent of the area of interest (e.g., 

 a town) below each measured elevation relative to 

 current sea level (Fig. 6.6). The curve indicates the 

 area of land lost as the rising sea floods the lower 

 elevations. When used to determine rates of loss 

 over large areas and long periods (25- 1 00 years) 

 in watersheds like Buzzards Bay, the technique has 

 useful application, especially as a management and 

 educational tool. 



Hypsometric curves for representing the ex- 

 tremes found in the Buzzards Bay watershed. New 

 Bedford (western shore), and Wareham (head of 

 the bay) demonstrate the low relief of the land sur- 

 face. The elevational distribution of each town also 

 indicates its different susceptibility to upland loss in 



