SUBTIDAL ENVIRONMENTS AND ECOLOGY 



105 



Fig. 9 Environments of lagoon margin patch reefs. Upper left: Coarse carbonate sand bottom with Hallmeda spp. and other 

 macroalgae. depth 6 m, near Choptop Reef. Upper right: Sediment/rubble bar near Choptop Reef produced by cross reef "rip." 

 Lower left: "Satellite Reef near Choptop Reef, comprised of a single colony of Porites cylindrica, probably torn from the main reef 

 by storms. Several other satellite reefs are visible in the background. Depth on the bottom is 6 m. Lower right: Small patch reefs 

 on the lagoon margin. A large cable from the atomic testing period is draped over a small patch reef (indicating an age of at least 

 20 to 30 years) with a colony of Porites edouxi;i which has grown on the cable, depth 5 m. 



some areas sand can be piled against the reef-killing corals 

 or other sessile invertebrates. Coral colonies with half their 

 surface buried and dead and the upper half healthy can be 

 found at the point of reef-sediment contact on some patch 

 reefs. Alteration of normal tradewind sea conditions can 

 radically alter shallow water sediment distributions. 

 Beaches grow or recede, islands change, and shallow sand 

 bars on the lagoon margin appear or vanish with changes 

 produced by passage of cyclonic storms (Nolan, 1975). It 

 is not necessary for storms to pass close to Enewetak 

 because the swell produced by a distant storm can accom- 

 plish the listed changes without high winds. 



Lagoon Margin Zonation 



The area immediately lagoonward of the reef flat is 

 quite variable and of considerable biological interest. Vari- 



ous authors have described this zone, usually in combina- 

 tion with a description of a cross-reef flat transect. 



Odum and Odum (1955) described the zonation of the 

 interisland reef about 400 m north of Japtan Island. In 

 many respects this is typical of windward interisland reefs 

 of the central and southeastern portions of Enewetak. 

 They described six zones from ocean reef to lagoon (a dis- 

 tance of about 450 m). These were (1) a buttress zone, 

 (2) the algal-ridge, (3) an encrusting zone, (4) a zone of 

 small coral heads, (5) a zone of small patch reefs "larger 

 heads," and (6) a sand and shingle zone. Typical views of 

 the bottom on the Odum and Odum (1955) transect are 

 shown in Figs. 10 and 11. They make the point that the 

 interisland reefs generally had more "vigorous" communi- 

 ties as opposed to reefs seaward of islands ("island reefs") 

 where living corals were limited to the outmost portions of 

 the reef. They believed this was due largely to different 



