104 



COLIN 



Fig. 8 A lagoon margin patch reef. "Choptop Reef," from the air. The main reef (center of photograph) is surrounded by smzdier 

 "Satellite Reefs," some of which are Pontes cylindrica colonies probably broken from the main reef by storm waves. The reef flat 

 is seen in the upper left with a sediment/rubble bar, produced by a cross reef "rip" seen in the upper center. Water depth around 

 Choptop Reef is about 6 m. Upper right: Typical view of coral development on a lagoon margin patch reef (Choptop ReeO with 

 the sediment floor surrounding the reef visible in the background. Lower left: View of upper surface of a lagoon margin patch reef 

 (Choptop Reef) with abundant coral and fishes visible. Depth on the top of the reef is approximately 2 m. Lower right: "Satellite 

 Reer' located about 15 m away from the main portion of Choptop Reef. This reef is simply a smaller version of Choptop with a 

 vertical relief of about 4 m. 



aerial photograph of the reef is shown in Fig. 8, with the 

 rubble bar and outwash area of the reef flat rip clearly visi- 

 ble. 



There are several smaller "satellite" reefs close to 

 Choptop which may have resulted from storm fragmenta- 

 tion of the larger reef (Fig. 9). The sediment around 

 lagoon margin patch reefs, like Choptop, is coarse. Cal- 

 careous macroalgae, such as Halimeda spp., occur 

 sporadically on the lagoon margin (Fig. 9), not in large 

 beds as is found in the deeper lagoon. 



Coral heads on the upper surface of lagoon margin 

 patch reefs often rise to near the surface, but at Enewetak, 

 patch reefs are not planar at about mean to low water 

 levels. At Ujilang Atoll, 200 km southwest, lagoon patch 

 reefs were planar on top, reaching low water level, 

 because of growth of coralline algae. Enewetak patch reefs 

 lack abundant coralline algae on the upper surfaces which 



may account for these differences. Encrusting corallines are 

 abundant within interstices of Enewetak patch reefs, but 

 the difference, compared to Ujilang, in the amount of 

 exposed corallines is striking. 



Where the internal structure of patch reefs is exposed, 

 such as in caves or recent fractures, it appears to be com- 

 posed of accumulations of coral skeletons that are poorly 

 cemented internally. Dead branches of coral plates have 

 interstices where small sclerosponges are common. Smith 

 (MPRL, 1972) reported that an explosive blast on a lagoon 

 pinnacle west of Jedrol "exposed unconsolidated to poorly 

 consolidated coral material more or less in growth posi- 

 tion." Sclerosponges, one of the prominent inhabitants of 

 the unlighted holes in the reef, were abundant. 



The sediment in these lagoon rim areas is not neces- 

 sarily stable. At some coral patches, the sediment is 

 scoured away at the base of the patch reef. Likewise, in 



