by Bishop and Bahr (1973). Bahr (1974) 

 reported no evidence of orientation of 

 individual oysters with respect to cur- 

 rents in reefs in Doboy Sound (in contrast 

 to the studies by Lawrence 1971); but R. 

 Frey (University of Georgia Marine Insti- 

 tute, Sapelo Island; pers. comm. ) detected 

 such orientation among oysters in reefs 

 located in Blackbeard Creek, which is 

 characterized by strong bidirectional cur- 

 rents. 



All reefs studied at Sapelo Island, 

 Georgia, were identical in height, 150 cm 

 above mean low water (MLW), except the 

 lower immature reefs, which presumably 

 were still growing. Peak height appears 

 to represent maximum equilibrium eleva- 

 tion, given present sea level and the 

 local tidal amplitude. Generally flooding 

 tides reach the lowest portion of the 

 reefs approximately 2 hours following 

 slack ebb, completely covering the upper- 

 most oysters approximately 2 hours before 

 peak flood. On an ebbing tide, the tops 

 of the reefs become visible about 2 hours 

 following peak flood, with the result that 

 the tops of the reefs are inundated only 

 4 hours per tide, or 8 hours per day. The 

 relationship between reef elevation and 

 tidal amplitude is unknown for other 

 areas. 



Exposure to air during ebb tides 

 allows the visible portion of a reef to 

 dry. Only the upper layer (5-10 cm) of 

 oysters and dead shells actually dries 

 out, however. The underlying shell layer 

 remains moist and appears reddish-brown 

 when the dry shells are removed because a 

 thin layer of detritus covers each shell. 

 This lower layer of shells and living oys- 

 ters appears to lack the film of algae 

 characterizing the upper layer. A reef 

 can thus be considered as consisting of 

 three "horizons," one pale greenish-gray, 

 one reddish-brown, and one silver-black, 

 color characteristic of shells buried in 

 an anaerobic environment high in ferrous 

 sulfide (Wiedemann 1971). Fine scrape 

 marks appear on many shells from the green 

 and brown horizons, indicating that the 

 organic film is constantly grazed. Mud 

 crabs ( Panopeus herbstii and Eurypanopeus 

 depressus ) graze these films on partially 

 inundated reefs (Bahr 1974). 



Oysters in the upper (green) horizon 

 have sharper growing edges than those in 



the brown layer, indicating faster growth. 

 Presumably this is a function of extreme 

 crowding and sediment encroachment on the 

 lower oysters. Many dead oysters are 

 found in the black and lower brown hori- 

 zons, with the valves still together, but 

 ful 1 of silt and clay. 



Approximately 61% (by volume) of the 

 reef material collected from the upper 

 surface down to the black horizon consists 

 of living oysters, 21% consists of dead 

 shells, and the remaining 18% consists of 

 silt, clay, and nonoyster macrofauna. 



Vertical Zonation 



Although the three horizons described 

 for the oyster reef are somewhat arbitrar- 

 ily defined, there is a definite vertical 

 change in reef macrofaunal composition. 

 This is a result of interspecific toler- 

 ance to desiccation (drying) rather than a 

 feeding limitation resulting from reduced 

 inundation time. The pattern of zonation 

 in the study area (Figure 13) is typified 

 by the zonation pattern on dock pilings 

 from the lower Duplin River examined after 

 years of exposure to fouling organisms. 

 From these pilings one can extrapolate the 

 optimal elevations for oysters and other 

 epifauna of the reefs. 



At Sapelo Island, Georgia, oysters on 

 pilings are virtually limited to an eleva- 

 tion (1.5m above MLW) corresponding to 

 the maximum elevation of reefs. One could 

 assume that this pattern of vertical zona- 

 tion would be compressed in areas of lower 

 tidal amplitude. Oyster growth is maximal 

 from about 60 to 70 cm above MLW, the ele- 

 vation corresponding to the level of the 

 sediment surface on which these reefs were 

 located. Dean (1892), observing growth 

 patterns on pilings, reported that oyster 

 growth in South Carolina was maximal in 

 the mid intertidal zone. 



Populations of the barnacle Chthama- 

 lus fragilis dominate the upper 60 cm of 

 tidal range. Other barnacles ( Balanus ) 

 and two mussels ( Ischadium and Guekensia ) 

 representative of the reef community oc- 

 cupy the lower intertidal and upper sub- 

 tidal ranges on the pilings, which repre- 

 sent a zone extending beyond the lower 

 limits of the reef. In fact, optimum ele- 

 vation for these species appears to be be- 

 low the limits of the reef zone. Wiedemann 



40 



