oyster communities, noting in a number of 

 cases, nothing but silt-covered, dead oys- 

 ter shells remained of once-productive 

 oyster reefs. This historical decline in 

 the welfare of the intertidal oyster com- 

 munity is further supported by the most 

 recent survey of Harris (1980), Total 

 acreage of the intertidal oysters has 

 decreased dramatically from approximately 

 688 ha (1,700 acres) in 1889 to less than 

 121 ha (300 acres) in 1977 (Harris 1980). 

 Large areas of dead oyster shell were also 

 reported in the 1977 survey. Harris re- 

 lated the steady decline of the Georgia 

 commercial oystering industry to the de- 

 crease of total oyster acreage. In addi- 

 tion, there is reason to believe that the 

 acreage figures reported by Harris (1980) 

 are somewhat exaggerated, perhaps because 

 they were partly based on aerial imagery 

 that did not permit easy distinction be- 

 tween living reefs and dead shells. For 

 example, Harris reported a total reef area 

 of 9,632 m^ in the Duplin River; Bahr 

 (1974) reported 6,040 m^ of living oyster 

 reefs in the same river based on a ground- 

 level survey. 



Intertidal oyster populations in 

 South Carolina have apparently also de- 

 clined during the same period. We are un- 

 able at present to attribute this decline 

 to any specific factor. It may be the 

 result of a slow shifting of ecological 

 conditions that reflect a natural succes- 

 sional pattern in the marsh-estuarine eco- 

 system (e.g., sea level change). Puffer 

 and Emerson (1953) cited natural cyclic 

 changes in environmental conditions — pri- 

 marily temperature and salinity--as the 

 cause of oyster reef death and subsequent 

 repopulation in Aransas Bay, Texas. Alter- 

 natively, this decline may be the result 

 of a man-induced perturbation of the 

 marsh-estuarine ecosystem, such as dredg- 

 ing, waterway construction, pollution, or 

 overharvesting. 



It is easy to explain a decline in 

 oyster reefs near population and indus- 

 trial centers such as Savannah, Georgia, 

 but it is much more difficult to account 

 for a decline of reef area in the more 

 pristine part of the Georgia coast near 

 Sapelo Island. 



The salinity of the Duplin River at 

 Sapelo Island, Georgia, appears to have 



increased recently (B. J. Kjerfve, Univer- 

 sity of South Carolina, Columbia; pers. 

 comm. ). This salinity increase could be 

 caused by a reduction in ground water 

 inputs due to consumptive losses resulting 

 from pumping for agricultural irrigation. 

 This change could partly explain the grad- 

 ual decline in viable oyster reef area in 

 the Duplin River and in other parts of the 

 study area, although, it is not clear how 

 a salinity increase up to 25 /oo or 30 /oo 

 would affect the reef community. 



With respect to local changes in reef 

 distribution, it is possible to find exam- 

 ples of reef area increases in some spe- 

 cific portions of the Georgia coast. For 

 example, in Altamaha Sound, Georgia, oys- 

 ter reefs have developed in areas farther 

 inland in the lower sound than they oc- 

 curred in 1889 (Figure 21). Associated 

 with this shift in reef distribution is 

 the accretion of marsh islands in south- 

 ern Altamaha Sound. The accretion of 

 marsh and marsh islands may relate to the 

 sediment-trapping capacity of intertidal 

 oyster reefs (Grave 1905; Wiedemann 1972; 

 Stephens et al. 1976). The growth of 

 intertidal oyster reefs farther inland of 

 the lower sound may relate to shifting 

 salinity conditions in Altamaha Sound. 



In summary, reef distribution along 

 the Georgia coast surprisingly has changed 

 little over the last 90 years. Oyster 

 reefs occur (in general) today in approx- 

 imately the same locations where they 

 occurred in 1889 (see Figure 21). The 

 living oyster reef area, however, signifi- 

 cantly has declined in the same period. 



4.3 THE PHYSICAL EFFECTS OF OYSTER REEFS 

 ON THE MARSH-ESTUARINE ECOSYSTEM 



Hypothetical ly, reefs affect the 

 physiography and hydrologic regime of salt 

 marsh estuaries three ways: by modifying 

 current velocities, both positively and 

 negatively; by passively changing sedimen- 

 tation patterns; and by actively augment- 

 ing sedimentation through biodeposition. 



Interpretation of reef effects on the 

 ecosystem over time from analyses of sur- 

 vey data of the last century is difficult 

 because, although 90 years is a long bio- 

 logical time, it is short geologically. 



61 



