currents indirectly can reduce 

 oyster feeding. 



(3) Currents have been shown, how- 

 ever, to positively affect oys- 

 ter ingestion (Walne 1972). Thus, 

 an optimum low-current level 

 probably exists to stimulate 

 oyster feeding with a minimum of 

 sediment erosion. 



(4) Eroded sediments in the water 

 column can settle out on a reef 

 and bury the lower level oys- 

 ters, causing a decline in reef 

 viability. Sediment input by 

 currents, coupled with a high 

 rate of biodeposition, can suf- 

 focate all but the uppermost 

 oysters in a reef. 



(5) Oyster reef growth in a positive 

 vertical direction is limited 

 absolutely by the local tidal 

 amplitude. The highest portions 

 of the reefs examined at Sapelo 

 Island were limited to 1.5m 

 above MLW, corresponding to a 

 daily inundation tim.e of only 

 8 hours, or conversely, to an 

 exposure time of 16 hours, 



(6) Lateral extension of oyster 

 reefs apparently occurs at a 

 rate limited by suitable sub- 



strate at the proper elevation 

 in the intertidal zone, by water 

 currents, and by available food. 



(7) In addition to a minimum inunda- 

 tion time, vertical reef growth 

 is also subject to temperature 

 stress in the study area (ex- 

 tremely cold spells and hot 

 spells during reef exposure). 



(8) Reef crowding appears to buffer 

 temperature stress and to allow 

 vertical reef accretion beyond 

 the maximum level at which indi- 

 vidual oysters survive. 



(9) Downward extension of oyster 

 reefs toward the subtidal zone 

 appears limited by increased 

 predation, fouling, and shell 

 erosion by boring sponges. 



(10) Predation by filter feeding 

 organisms, nektonic, and epiben- 

 thic, reduces the available pool 

 of oyster larvae and perhaps 

 prevents overcrowding. 



(11) The gregarious behavior of oys- 

 ter larvae ensures a new crop of 

 spat to replenish mortality 

 losses and maintain the viabil- 

 ity of existing reefs. 



76 



