SUBTIDAL ENVIRONMENTS AND ECOLOGY 



107 



Fig. 11 Views of tiic sand-shingle zone of tfie Odum and Odum (1955) transect at Enewetak Atoll. Upper left: Coarse carbonate 

 rubble and sand immediately behind the large coral head zone (depth 1.5 m). Upper right: General view of rubble area behind the 

 large coral zone. Lower left: Carbonate sand farther lagoonward from the large coral head zone, depth 3 m. Lower right: Break in 

 slope of sand-shingle zone where the slope increases considerably (to the right) toward the deep lagoon. 



of them. From aerial photographs it appears many of the 

 coral heads are arranged in a serial fashion across the reef 

 with large numbers of them resembling striations across 

 the bottom. 



The clustered coral head zone is about 600 m across 

 and has a lower density of coral heads than the previous 

 zone. Those present are grouped together somewhat. 

 Finally there is a zone of large patch reefs about 400 m 

 wide. These patch reefs appear comparable in size to the 

 larger patch reefs of the windward lagoon margin. 



Channels Between Northern Islands 



The channels between the closely spaced northern 

 islands are of special beauty and biological interest. They 

 are not true passes from ocean to lagoon because they 

 draw their flow from the shallow reef flats to seaward but 

 channelize the flow of water off the reef flat between 



islands. Viewed from the air, their bottom features show 

 strong orientation to the current which funnels between the 

 islands from ocean to lagoon, with reefs often elongated 

 with the current and sediment washed out between patch 

 reefs. These "passes" have a reef flat on their seaward 

 end, but the cross-reef flat flow from an area of reef front 

 several times broader than the channel is funneled into 

 each one. The channels are often deep, but where current 

 flow slows on their lagoonward end, they usually have a 

 shallow, delta-like bottom. 



A good example of a northern island channel is that 

 between Lojwa and Aomen. At very low tides water flow 

 across the reef flat is completely eliminated, with no 

 current in the channel. At high tides with strong waves 

 pumping, the current is swift, sufficient to deeply churn 

 sediment from around patch reefs in the channel. The gaps 

 between reefs have the sediment scoured away, appearing 

 darker blue when viewed from above, whereas areas on 

 the sheltered, downcurrent side of the patch reef have 



