For example, the average sediment deposi- 

 tion rate in the study area is less than 4 

 mm/yr (Letsch and Frey 1980). This means 

 that from the years since the first reef 

 survey in 1889, theoretically only about 

 one-third of a meter of sediment has 

 accumulated. 



Oyster reefs undoubtedly dampen tidal 

 current velocities over the entire ecosys- 

 tem because of friction, but the magnitude 

 of the drag coefficient of a unit area of 

 reef is unknown, as is the overall effect. 

 Reefs also augment current velocity in lo- 

 cal areas by constricting tidal streams, 

 but no quantitative data are available to 

 detail the specific effects. 



Grave (1905) noted that oyster reefs 

 are wave- and current-resistant structures 

 that exert a physical influence over the 

 marsh system. He observed that small reefs 

 originating at points along a tidal stream 

 accrete laterally across the stream (into 

 the current), and by displacing and con- 

 stricting the current cause erosion of the 

 opposite marsh bank. This process may re- 

 sult in the formation of marsh islands. 



Passive sedimentation due to the 

 presence of reefs is qualitatively obvious 

 but has not been quantified. The magni- 

 tude of this effect would be related to 

 the overall reduction in tidal current 

 velocities and turbidity levels. Active 

 sedimentation through biodeposition can be 

 estimated (see Appendix). The biological 

 process of aggradation increases the size 

 of suspended particles and increases their 

 effective settling rates. The dominant 

 oyster reef zone's coinciding with the 

 maximum turbidity zone in estuaries in 

 the study area indicates that this effect 

 may be significant. Lund (1957a) reported 

 that oysters biodeposited or "self-silted" 

 eight times the volume of sediment in test 

 containers than would have deposited in 

 the same time due to gravity alone. He 

 calculated that a uniform single layer of 

 oysters in a natural setting with rela- 

 tively low turbidity water could biode- 

 posit sediment at a rate of about 280 

 tons/acre/yr (6 x lO** g/m^/ yr). 



4.4 AREAL EXTENT OF OYSTER REEFS IN THE 

 COASTAL ECOSYSTEM 



The most obvious criterion by which 

 to assess the importance of oyster reefs 



on the marsh-estuarine ecosystem is the 

 relative proportion of reef surface area 

 to the total surface area of the system. 

 Planimetry on maps of the Georgia coastal 

 zone (Galtsoff and Luce 1930) indicated 

 that the total intertidal and subtidal 

 zones of the entire area occupied approxi- 

 mately 1.8 X lO^m^. Of this area, approxi- 

 mately 75% was marsh and tidal creeks, and 

 25% was open water (wider than about 350 

 m). The linear extent of the oyster reefs 

 measured about 403,000 m. If the average 

 reef were estimated as 2 m in width, the 

 total reef area in 1925 would have com- 

 prised about 8 X 10^ m, or 0.04% of the 

 marsh-estuarine area. If the mean reef 

 width were 3 m, reef area would increase 

 to 1.2 X 106 m2^ or 0.06%, Harris (1980) 

 estimated that the total viable reef area 

 in the Georgia coastal zone in 1977 was 

 equal to 102 ha, or about 0.05% of the 

 marsh-estuarine area. This presumably rep- 

 resents a decline from 1889, when Drake 

 (1890) estimated that 6.8 x 10& square 

 meters of reefs existed, or 0.3% of the 

 total marsh estuarine zone was occupied by 

 oyster reefs. In a detailed survey of the 

 Duplin River drainage basin, Bahr (1974) 

 estimated that about 0.06% of the marsh 

 estuarine zone was occupied by viable 

 reefs. 



The absence of quantitative informa- 

 tion about the areal extent of intertidal 

 oyster reefs in South Carolina and north- 

 eastern Florida does not allow a compari- 

 son with Georgia. Apparently oyster reefs 

 comprise a larger percentage of the marsh 

 estuary in the South Carolina area than in 

 Georgia, but the relative difference is 

 unknown. A detailed analysis of the rela- 

 tionship between reef area and tidal 

 amplitude in the study area would be 

 interesting. A small area of the Savannah 

 River basin in South Carolina surveyed by 

 McKenzie and Badger (1969) indicated an 

 extremely high oyster reef density (9%), 

 Lunz (1943) reported an extremely high 

 density of reefs along a 1-mi wide and 

 40-mi long strip surrounding the intra- 

 coastal waterway in South Carolina from 

 Charleston to the Santee River, He report- 

 ed that 33,6% of the total creek banks was 

 lined with reefs, Lunz (1943) also report- 

 ed that these reefs were populated by 

 about 136 oysters/yd^, (or about 114/m ) 

 of 2-inch (50-mm) or larger sized oysters. 

 This represents a biomass of approximately 

 50 g/m^ afdw, much lower than that for the 



62 



