(S. Stevens, University of Georgia Marine 

 Institue, Sapelo Island, Georgia; pers. 

 comm. ) . 



Present Distribution 



Current speed and bottom roughness 

 have been theorized as controlling the 

 distribution of estuarine suspension-feed- 

 ing macrobenthos (Wildish and Kristmanson 

 1979). The distribution of intertidal oys- 

 ter reefs in the study area is described 

 in terms of the three hydrographic zones 

 of the estuary (see Section 1.2). The 

 zones are (1) the lower sound and inlet 

 areas between barrier islands; (2) the 

 middle region of the estuary, including 

 the major rivers feeding the sounds; and 

 (3) smaller tidal creeks draining the 

 marshes (Figure 2). 



The typically high energy regime and 

 sedimentary instability of the lower sound 

 region render this zone the least favor- 

 able for reef development. Where reefs 

 are found in the lower sound areas, they 

 presumably indicate local pockets of shel- 

 ter from storm surges. 



From the lower to middle estuarine 

 zone, wave energy is probably the control- 

 ling factor. The middle zone is charac- 

 terized by an optimum current regime for 

 reefs; the regime of the lower zone is too 

 turbulent, and the upper zone is too slug- 

 gish. Oyster reefs, sometimes exceeding a 

 kilometer in length, in the middle estua- 

 rine zone are predominantly (but not ex- 

 clusively) oriented along the banks of 

 rivers. Circular reefs and oyster reef 

 islands also occur infrequently in this 

 zone. Many reefs in the middle estuarine 

 zone are near the entrances to small tidal 

 creeks that feed the larger rivers. This 

 orientation is not accidental and may 

 indicate the importance of slight differ- 

 ences in current regimes, which are en- 

 hanced at the confluence of water bodies. 



The complex network of tidal creeks 

 and small rivers that drain the marshes is 

 also an area of significant oyster reef 

 development. The distribution of inter- 

 tidal oyster reefs within this zone is 

 perhaps the most consistent and predict- 

 able of the three estuarine subdivisions. 

 The pattern of oyster reef development and 

 tidal creek meander systems are strongly 



correlated. Oyster reefs are likely to 

 occur in three zones within a tidal creek 

 system (Figure 17): (1) on the concave 

 outer banks of meander loops, (2) in areas 

 immediately adjacent to smaller tidal 

 tributaries, and (3) at points of tidal 

 stream confluence. 



The oyster reef tendency to develop 

 on the concave outer banks of tidal creeks 

 is predictable from the hydrography of 

 stream meanders. The outer or cut-bank of 

 the meander loop is the zone of highest 

 current velocity within the channel. The 

 sediment substrate, therefore, tends to 

 consist of firm, consolidated mud, swept 

 clean of soft mud and slime unsuitable as 

 a spat settlement surface. Once the reef 

 colony is established, these higher veloc- 

 ity currents provide nutrients and remove 

 fecal matter. Keck et al. (1973) discussed 

 this same relationship between meander 

 morphology and oyster distribution in the 

 Murderkill River, Delaware. Reefs in that 

 region tend to form in areas adjacent to 

 smaller tidal tributaries where important 

 marsh-derived nutrients are. Oyster reefs 

 at points of tidal stream confluence are 

 also influenced by hydrographic factors. 

 During flood tide, the confluence of flow 

 between the two tidal creeks results in a 

 zone of circular back-eddy formation lo- 

 cated at the point bar (Figure 17). The 

 turbulence associated with this process 

 provides nutrients to the reef. During 

 flood tide, the point bar is an area of 

 relatively higher current velocity and 

 little deposition. 



Historical Changes in Reef Distribution 



Four surveys of intertidal oyster 

 reefs along the Georgia coast demonstrate 

 changes in oyster distribution from 1889 

 to 1977. These are Drake (1891), Galtsoff 

 and Luce (1930), Linton (1968), and Harris 

 (1980). The survey results reveal two 

 aspects of the change in oyster reef dis- 

 tribution over time: a change in total 

 reef area, and local changes (increases or 

 decreases) in specific areas. 



Galtsoff and Luce (1930) reported few 

 significant changes occurring in the dis- 

 tribution and extent of natural oyster 

 beds between the years 1889 (Drake 1891) 

 and 1925. They reported, however, a de- 

 cline in the health of many intertidal 



59 



