SUBTIDAL ENVIRONMENTS AND ECOLOGY 



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Fig. 14 Typical views of the spur and groove zone off Enewetal( Island. Upper left: Shallow groove with large coral boulders In 

 its center which are set in motion during periods of high waves. These effectively keep the grooves free of sessile benthic macroor- 

 ganisms. Upper right: A variety of herbivorous fishes at seaward end of a spur. Lower left: Spur seaward of the Enewetak Island 

 reef flat with breakers rolling over it. Lower right: Seaward end of a spur with a breaker forming at its prow. The shallower water 

 depth on the spurs causes waves to break there sooner than over the grooves. 



appears to be a uniform light color. The rock shelf width 

 varies around the atoll. Off Enewetak Island it is relatively 

 wide, about 200 to 300 m, but farther north it becomes 

 narrower, probably less than 100 m wide. 



The rock surface of the shelf often has evidence of 

 extensive boring by clionid sponges. Large areas of 

 substrate may have the tiny, dark oscula visible, but these 

 are not apparent on superficial examination (Fig. 16). 



Smith and Harrison (1977) have described a windward 

 reef slope from off Jinimi Island in connection with 

 estimates of calcium carbonate production there. The reef 

 crest had essentially no corals. Moving seaward from the 

 reef crest and spur and groove zone, the bottom slopes 

 gradually from 4 to 5 m depth to about 8 m and is essen- 

 tially a rocky pavement with minor surface undulations. 

 Smith and Harrison (1977) estimated only 10% coral cov- 

 erage in their study area at 7 m depth. Seaward, the 

 amount of coral cover increased with depth, although the 

 slope may increase only slightly with 15, 20, and 25% at 



11, 15, and 21 m depth, respectively. At 50 m, coral cov- 

 erage was virtually zero. Smith and Harrison (1977) found 

 that the vasiform Acropora cvthera was the most conspicu- 

 ous coral in their study area, with its nearly flat upper sur- 

 face well adapted for capturing sunlight. They performed 

 coral and coralline algae incubations using clear acrylic 

 domes, where possible, at depths to 21 m. Steadily 

 decreasing rates of calcification with increasing depth were 

 found. Overall they believed the seaward slope of wind- 

 ward reefs at Enewetak (the "mare incognition" of Ladd, 

 1961) has only a small role in the CaCOa mass balance of 

 the atoll. 



Large numbers of vasiform Acropora cilthera colonies, 

 up to 2 m in diameter, were found by Smith and Harrison 

 (1977) at 15 to 25 m at their study area (Fig. lA of that 

 paper). Colonies had a maximum of 13 growth bands 

 (annual), and they considered that the major typhoon in 

 late 1962 (their observations were in late 1976) may have 

 devastated Acropora corals in that area. Smith and 



