SUBTIDAL ENVIRONMENTS AND ECOLOGY 



113 



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Fig. 16 Upper: Carbonate rock substratum heavily bored by 

 the sponge Cliona sp. on the windward reef, Enewet€U< Island, 

 depth 8 m. The dark oscula of the sponges are visible over 

 much of the substratum, although the tissue of the sponge is 

 located internally beneath the surface of the rock. Middle: 

 The area of the shelf edge break (20 m depth) off Enewetak 

 Island. There is very little live coral in this area with only a 

 single table Acropora visible among large amounts of coral 

 rubble. Lower: Outer slope at 25 m depth. Enewetak Island, 

 coral rubble going down the slope into deeper water. 



serve to transport sediment into deep)er water. Below the 

 shelf break, larger amounts of sediment are visible on 

 relatively horizontal areas, but the slope limits the amount 

 of build up. 



The reefs of the leeward side have extremely steep 

 slopes. The distance between the reef crest and the steep 

 slope into deep water changes with location. Along the 

 southwestern islands (Ikurin through Kidrenen) there is a 

 narrow shelf sloping gently from about 3 to 15 to 18 m. 

 This shelf is generally about 100 to 150 m wide and has a 

 well-developed coral community on the rocky shelf. Most 

 of the corals are small, less than 10 years old, implying 

 recent devastation, probably by storm waves. Sand chan- 

 nels occur perpendicular to the reef front which is at the 

 head of reentrants on the reef face. The change to a steep 

 slope occurs at about 15 m where it becomes a 45° to 

 60° slope to the limit of scuba diving. A typical profile of 

 a southwest island reef is shown in Fig. 17. 



To the west of Kidrenen, the reef remains unbroken 

 until the southwest passage. The bottom slopes gently, 

 then progressively becomes steeper with virtually no shelf 

 to a near-vertical face at about 10 m depth. The horizontal 

 distance from water a few meters deep to the vertical face 

 is less than 50 m. This extremely steep profile is even 

 more pronounced on the reef north of Biken to the West 

 Spit. Reentrants penetrate the reef face with Halimeda 

 dominated sediments on shelves on a steep slope into the 

 deep water (Fig. 17). 



The leeward reef crest near the island of Ikuren has a 

 healthy cover of coralline algae on its upper surface, even 

 though on the leeward side of the atoll, small to moderate 

 surf usually occurs, which is produced by the long, low 

 swell from the west. Large numbers of herbivorous fishes 

 occur here, essentially the same species as are found on 

 the windward spur and groove areas. The two areas are 

 similar; but near the southwest islands the grooves, 

 strength of surf, and various invertebrates are lesser 

 developed. Seaward of the reef flat are often small high 

 relief rocky structures with flattened tops and abundant 

 coral (Fig. 18). Species of Acropora, Pocillopora, and 

 Heliopora axe common on the edges of the coralline flat. 

 The cidaroid sea urchin, Heterocentrotus trigor)arius, is 

 found deep in small caves and crevices of the outlying rock 

 structures among coralline-covered fossil coral branches. 

 Around and to seaward of these structures is often a bot- 

 tom at 5 m depth composed of large coral boulders and 

 shingle. Much of the hard substrate in this area not 

 covered by hard or soft corals has coralline algae growing 

 on it. These algae often have large numbers of grazing 

 marks almost cerlainly from parrot fish (Fig. 18). The alga, 

 Asparagopsts taxiformis, is extremely abundant; its upright 

 tufts in evidence on nearly all rocky surfaces (Figs. 19 and 

 20). 



A rock substrate begins within 20 to 40 m of the reef 

 flat with occasional large vertical knobs of rock covered 

 with hard and soft corals. Urchin grooves are evident in 

 the rock, but diademnid urchins were seen much more 

 often in them than Echinometra mathaei. 



