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COLIN 



surrounding bottom is not visible. Emery et al. (1954) used 

 the term "coral knoll" for such structures, but this author 

 thinks it is not truly a descriptive term in this case. 



The pinnacle reefs of Enewetak Lagoon cover only a 

 small percent of the bottom area but are areas of great 

 biological diversity and interest. Their presence in the deep 

 lagoon, appearing as light areas among the dark waters, 

 parallels on a smaller scale the presence of atolls in the 

 deep ocean. Pinnacle reefs vary greatly in size, from a few 

 tens of meters to over 1 km in diameter at their base. 

 Emery et al. (1954) pointed out that among the 20 largest 

 pinnacle reefs, they are quite evenly spaced throughout 

 the lagoon. For several reasons the distribution of smaller 

 pinnacles, though, is not as well known. The tops of most 

 are not visible from the surface and because of their small 

 size, they are easily missed by echo sounding surveying. 

 Emery et al. (1954) estimated there were about 3000 

 coral pinnacles in the Enewetak Lagoon but ignored any 

 which did not have a relief of more than 4 m. There are 

 about 150 to 180 pinnacles which should be visible from 

 the surface (<18 m depth), rising from depths of about 

 35 m or more. 



The surface-visible pinnacles are the best known 

 because they can be located relatively easily for diving and 

 are shallow enough for prolonged scuba diving. They have 

 been used as sites for a variety of studies, but their origins 

 and underlying structure are not well known. 



The slope of the sides of pinnacle reefs can vary 

 greatly. In general, the smaller a pinnacle reef diameter, 

 the steeper its slope. On small pinnacles much of the sloF>e 

 is nearly vertical. The largest pinnacles are somewhat flat 

 on top for much of their diameter but still slope to the 

 lagoon floor at an angle of at least 10 to 20°. 



Those pinnacles closest to Enewetak Island are best 

 known because of their closeness to MPRL. Figure 6 indi- 

 cates the location of many of these and the names applied 

 to them. There is, however, considerable variation in the 

 biological communities between pinnacles, even among 

 those of similar size and shape. A few pinnacle reefs are 

 described subsequently in greater detail. 



An example of a well-developed small, but not typical, 

 pinnacle reef is "Pole Pinnacle," so named because of a 

 toppled marker pole and anchor block on its upper sur- 

 face. It is located 1.6 km from Jedrol Island (Fig. 6). Pole 

 Pinnacle actually rests on the edge of the deep channel on 

 an extension of the wedge of shallow reef produced by the 

 split of the deep channel west of Jedrol. The entire upper 

 surface of the pinnacle is dominated by the coral Pontes 

 rus, the P. iuxjyamaensis of Wells (1954) (Veron and 

 Pichon, 1982). On the upper surface at 3 to 5 m depth, 

 the columnar form of P. rus occurs, but on the sides of 

 the pinnacle where P. rus also dominates, the plate- 

 columnar form occurs. The vertical distribution of P. rus 

 varies on different sides of the pinnacle. On the northern 

 face, little occurs below 8 m, whereas on the south side a 

 solid cover is found above 12 m. The eastern face has its 

 first colonies of P. rus at about 26 m, with large patches 

 starting at 18 m. The western face has some large clumps 

 as deep as 15 m. Below the steeply sloping upper portion, 



the bottom becomes less steep, having an angle of about 

 45° to depths of 30 m. The bottom around the base of 

 the pinnacle becomes relatively flat with coarse 

 Ha/imeda-dominated sediment and occasional small reefs. 

 On the eastern side, which abuts the side of the deep 

 channel, the bottom slopes away farther to about 40 m. 



Below the depth of P. rus dominance, the coral cover 

 is low. The bottom is largely rocky substrate with shelves 

 on which considerable quantities of sediment are retained. 

 Hillis-Colinvaux (1980) reported that Pole Pinnacle "pos- 

 sessed the same high Halimeda species richness" encoun- 

 tered in some shallow water interisland channels. She felt 

 the Halimeda species populations of the sides of all pinna- 

 cles "may well be principal suppliers of carbonate to the 

 reef floor." In light of recent information on "Halimeda 

 meadows" and the occurrence of Halimeda in the deep 

 lagoon, pinnacle reefs may be less important as carbonate 

 producers than previously suspected, but they are still sig- 

 nificant. Both small and large pinnacles are definitely 

 Halimeda spp. sediment source points; their sloping sides 

 and shallow depths producing a potentially radial dispersal 

 of Halimeda plates from shallower depths to the deep 

 lagoon. 



A well-known example of a "larger" pinnacle reef is 

 South Medren Pinnacle, located 1.7 km west of the south 

 end of Medren (Fig. 4). It is about 100 m in diameter, 

 roughly circular, and slopes off at about a 30° angle to the 

 lagoon floor at 35 to 40 m. Its upper surface is rugged, 

 with coral ridges and heads interspersed with deeper rub- 

 ble areas. Coral coverage is not as high as Pole Pinnacle 

 but seems average (10 to 30%) for most pinnacle reefs. 



Coral distribution on the tops and flanks of pinnacles, 

 particularly larger ones, seems somewhat patchy (Fig. 7). 

 Definite sediment downfall areas exist on large pinnacles 

 which restrict corals. Medren Pinnacle has several on its 

 southern face, and near the base of the pinnacle at 35 to 

 40 m only isolated areas of reef exist. These small patch 

 reefs are generally of low relief, somewhat rounded with 

 abundant macroalgae populations. Here the large blue 

 tubular to vasiform sponge, Cribochalina olemda, is often 

 common. 



Gilmartin (1966) found the green alga, Tydemania 

 expeditionis, along with species of Caulerpa, Halimeda, 

 and Dictiiota to form the bulk of algal biomass on the deep 

 lagoon coral patches at depths greater than 40 m. Previ- 

 ous dredging work on 7. expeditionis had indicated it to be 

 uncommon, but Gilmartin (1966) found it to be first or 

 second in abundance among algae on deep lagoon coral 

 patches, equal to or exceeded only by Halimeda at 51 to 

 62 m depth. 



Ship Channel #1 Pinnacle (not shown in Fig. 6), 

 located some 6 km west of Ananij Island, is unusual. It is a 

 fairly small pinnacle, about 100 m in diameter, rising 

 within about 3 m of the surface with the lagoon about 

 40 m deep around it. The eastern end of its top is dom- 

 inated by Porites rus, similar to that found at Pole and 

 Tunnel Pinnacles, while its western end has almost 

 exclusively table Acropora corals and appears to have 

 been devastated by a storm several years ago. 



