(1971) remarked on the paradoxical re- 

 striction of the barnacle, £. fragilis , to 

 the uppermost oysters in a reef or to 

 blades of marsh grass above the maximum 

 height of oyster reefs. Of the three spe- 

 cies of barnacles in the reef community, 

 C^. fragilis is restricted to the upper, or 

 green, horizon. Another related barna- 

 cle, C^. stel latus , has been described as 

 an obligate intertidal form for reasons of 

 competition rather than physiology (Con- 

 nell 1961; Barnes and Barnes 1969). The 

 restriction of Chthamalus to the mid to 

 upper intertidal zone was demonstrated by 

 Connell to result, not from intolerance to 

 constant inundation, but rather from com- 

 petitive exclusion by Balanus spp. In the 

 oyster reef community, where barnacle den- 

 sity is not as great as in Connell 's 

 study, oysters seem to assume the role of 

 "squeezing out" all but the uppermost 

 individuals of C. fragilis . Many well- 

 preserved individuals of the latter spe- 

 cies are found trapped and overgrown 

 between adjoining oysters. Chthamalus 

 fragilis represents the most obvious exam- 

 ple of vertical zonation in the reefs, but 

 other evidence of similar restrictions can 

 be observed; e.g., anemones occur almost 

 exclusively in the brown horizon. 



Green and Hobson (1970) stated that a 

 difference in elevation of 6 cm in the 

 intertidal zone results in a significant 

 effect on rates of mortality; however, 

 they were describing an infaunal assem- 

 blage dominated by the little gem clam 

 ( Gemma gemma ). The oyster reef displays a 

 similar sensitivity at the upper limit of 

 its intertidal range. At slightly lower 

 elevations, however, these effects are 

 buffered by the physical complexity and 

 density of the reef, which trap and hold 

 moisture above the level of the surround- 

 ing sediment. 



Temperature Effects on Oyster Reefs 



Oysters adjacent to a hole in a reef 

 made by sampling often die after being 

 dislodged from their normal position in 

 the reef. Undisturbed oysters are normally 

 oriented vertically, (with the ventral 

 side upward) and those which collapse into 

 a sampling site are usually horizontally 

 oriented. The latter position results in 

 exposure of a greater proportion of sur- 

 face area to direct solar radiation, with 



little chance for mutual shading. The tem- 

 perature of sediment within a reef varies 

 widely with depth; e.g., temperatures were 

 35°C at the surface and 28°C at 6 cm depth 

 during one measurement in October (Bahr 

 1974). 



More critical than sediment tempera- 

 ture is the fact that the internal temper- 

 ature of an oyster is a function of the 

 orientation of the oyster with respect to 

 direct solar radiation. For example, the 

 internal temperature of a reef oyster in 

 Georgia varied (in the same October obser- 

 vation) from 34°C to over 38°C, according 

 to whether it was oriented vertically or 

 horizontally (Bahr 1974). In full shade 

 the temperature dropped to 31.5°C. This 

 implies that mutual shading of crowded 

 reef oysters is beneficial and important 

 to the maintenance of temperatures within 

 the tolerance limits of the oyster. In the 

 summer when the angle (azimuth) of the sun 

 is highest, significantly higher tempera- 

 tures result on incident surfaces; there- 

 fore, high mortalities could easily result 

 from the disruption of the angular orien- 

 tation of reef oysters which provides the 

 shading to protect the oysters. Copeland 

 and Hoese (1966) reported mass mortalities 

 of intertidal oysters in Texas during the 

 summer. Hodgkin (1959) concluded that an- 

 nual high mortalities of littoral fauna 

 and flora near Fremantle, Australia, re- 

 sulted from high temperature, which was a 

 major factor in the maintenance of charac- 

 teristic shore zonation. Thus, it appears 

 that oyster reefs grow to elevations above 

 that at which individual oysters could 

 survive the rigors of temperature stress 

 and minimal inundation time. 



Lehman (1974) examined the effects of 

 thermal loading from the discharge water 

 of a local power plant on the oyster reef 

 community at Crystal River, Florida. He 

 concluded that an average annual increase 

 of 4° C in the water surrounding experi- 

 mental reefs (relative to unaffected 

 reefs) caused an increase in oyster bio- 

 mass, metabolic rate, and turnover rate, 

 but a decrease in the diversity of the 

 reef community. 



Salinity Effects on Oyster Reef s 



Although oysters are euryhaline and 

 can tolerate low salinities, reefs are 



41 



