stable, the newly formed reef accretes 

 laterally and vertically within the inter- 

 tidal zone. Dead shell material scattered 

 around the reef aids in building up the 

 channel floor or reef platform, paving the 

 substrate for the reef to spread laterally 

 (Wiedemann 1972). Lateral reef accretion 

 generally occurs in a direction perpendic- 

 ular to tidal currents so that the effec- 

 tiveness of currents in transporting 

 nutrients and removing fecal material is 

 exploited (Grave 1905; Grinnell 1971). 



On a still smaller scale, individual 

 oysters on the reef surface tend to orient 

 themselves so that their planes of commis- 

 sure (i.e., opening between the valves) 

 are alined roughly parallel to the current 

 direction (Lawrence 1971). Lawrence (1971) 

 found that either the anterior or poste- 

 rior shell margin may face the oncoming 

 current direction, a fact suggesting that 

 this alinement is necessary for the hydro- 

 dynamic stability of the individual oys- 

 ters. The macro-orientation of a reef with 

 respect to the local current regime and 

 the micro-orientation of its constituent 

 oysters are only demonstrable where the 

 currents are uni- or bi-directional. For 

 example, most of the reefs examined by 

 Bahr (1974) were located at the southern 

 edge of Sapelo Island in Doboy Sound, an 

 area with multidirectional currents, and 

 no definite macro- or micro-orientation 

 was observed. 



Vertical accretion continues as long 

 as the upper (living) layer of the oyster 

 reef remains within the portion of the in- 

 tertidal zone in which oysters are viable. 

 Bahr (1976) found the maximal reef height 

 for oysters to be a constant feature of 

 the intertidal oyster reefs in Doboy 

 Sound. No reefs in this area exceeded 

 72 cm above the surrounding mud surface or 

 1.5m above mean low water. 



At this stage of development, the 

 reef consists of an approximately 1-m 

 thick accumulation of live oysters, dead 

 shell, and mixed shell and mud (Figure 

 13). The uppermost portion of the reef is 

 level, sloping off steeply at the edges. 

 The living portion of the reef is thicker 

 at the edge than in the center because of 

 mud trapped by the reef. The central core 

 of the reef is corposed of mixed dead 

 shell and mud. If, for example, the reef 



is formed on a soft mud substrate, its 

 weight will cause the entire structure to 

 slowly subside or sink. Vertical upbuild- 

 ing in a viable reef keeps pace with grad- 

 ual subsidence, and the upper reef surface 

 remains at a steady state with respect to 

 mean water level. The reefs examined by 

 Bahr (1974) were typical of this stage of 

 development. 



Senescent Stage 



A senescent stage of intertidal oys- 

 ter reef development is reached when the 

 upper surface of the reef can no longer 

 accrete vertically and the majority of 

 live oysters populate only the flanks of 

 the reef. The mature reef will have a bar- 

 ren central zone, or ridge in the case of 

 long linear reefs, comprising dead shell 

 and various sized fragments of shell. The 

 barren central region has been referred to 

 as a "hogback" (Gunter 1979) or flatland 

 surface (Grinnell 1971). Gunter (1979) 

 suggests that for gulf coast reefs the 

 constant motion or saltation of fine shell 

 "grit" in the central zone prevents the 

 survival of new oyster spat, so that this 

 area remains void of organisms. This 

 "grit theory" would not hold, however, for 

 the smaller, relatively sheltered reefs in 

 the environment of the salt marsh estuary. 



An extension to the senescent stage 

 of reef development was proposed by Grave 

 (1905). He suggested that with time, the 

 barren central "flatland" surface would be 

 built up with thicker accumulations of 

 sand, mud, and shell debris, and would be 

 colonized by Spartina . The reef would then 

 become an oyster marsh island, with a 

 length and width greater than that of the 

 original oyster reef, and surrounded by a 

 thin band of intertidal oysters. Little 

 Egg Island in the mouth of the Altamaha 

 River in Georgia may be an example of such 

 an oyster-formed island. 



4.2 DISTRIBUTION OF OYSTER REEFS IN THE 

 MARSH-ESTUARINE ECOSYSTEM 



This section includes some specula- 

 tive material that remains to be confirmed 

 by scientific study. There is, however, 

 ongoing research at Sapelo Island, Geor- 

 gia, that should help explain the observed 

 distribution of reefs in the ecosystem 



58 



