114 



COLIN 



30 60 



120 



DISTANCE, m 



180 240 300 



360 



420 



5X Vertical Exaggeration 



Fig. 17 Typical slope profiles of Enewetak Atoll seaward reefs. The profiles, which are vertically exaggerated, are from a wind- 

 ward reef off Enewetak Island (upper), a leeward reef off Ikuren (southwest islands) (middle), and a leeward reef north of Biken 

 (lower). Dotted lines represent the bottom in areas of sand channel reentrants of the reef face. Waves shown on the surface reflect 

 the normal wave conditions on these different areas. 



The rocky shelf slopes gradually seaward, and at about 

 8 to 10 m depth sand channels begin to appear on its sur- 

 face. There is considerable relief between the reef fingers 

 at about 9 to 10 m and the channels at 12 to 14 m (Figs. 

 19 and 20). The sides of many of the fingers are nearly 

 vertical and often undercut. These overhanging walls have 

 dense coverage of coralline algae and abundant Haliweda. 

 The sediment in the channels is coarse, derived largely 

 from Halimeda flakes and often has wave ripples on its sur- 

 face from the long period swells. The upper portions of 

 the reef fingers have dense coral on their tops and sides. 

 Coral coverage at 12 m depth on the top of the fingers at 

 the shelf break is 80 to 90% in some areas. A few large 

 head corals occur but most are small to medium acro- 

 porids. They are at most 25 to 40 cm across and probably 

 reflect recruits after storm destruction of most of the previ- 

 ous acroporids (Fig. 19). 



The bottom slopes away at the shelf break (12 to 

 15 m) at a 45 to 60° angle. Most of the sand channels 

 continue down the slope as sediment chutes into deep 

 water. These chutes are cut back into the reef face and 



have sediment down them to the limit of visibility 

 (Fig. 20). Adjacent rock surfaces have abundant corals, the 

 same types of species that occur in shallower water. Live 

 Halimeda is abundant all down the slope to over 60 m. 



Hillis-Colinvaux (1980) found four species of Halimeda 

 on the seaward reef off Mut at 10 to 15 m depth. She 

 estimated cover of Halimeda on this bottom as about 15% 

 and commented that Halimeda was much more conspicu- 

 ous on the spur reef structure than she would have 

 expected on a reef buttress in Jamaica. 



Halimeda flakes dominate the sediments of all leeward 

 side oceanic reefs. Below about 20 m depth, sediment 

 builds up on any nearly horizontal surface, particularly near 

 the reentrants which are the primary "down chutes" for 

 sediment. 



There are many overhangs and small caves formed by 

 coral plates on the leeward reefs. Incredibly delicate large 

 colonies of stylasterine corals grow in their dim recesses. 

 Three species of sclerosponges — Astrosclera willei/ana, 

 Acanthochaetetes welisi, and one unidentified species 

 (Basile et al., 1984) — are found in caves along the reef 



