112 



COLIN 



Fig. 15 Upper leh: Larger shoal of Acanthurus triostegus in the spur and groove zone, Ananij Island, depth 4 m. Upper right 

 Unusual growth form of Acropora sp. found seaward of the area of live edgal ridge, Ananij Island, Enewetak Atoll. Lower left: 

 Grooves eroded in the side of spurs by sea urchin Echinometra mathaei, windward reefs, Enewetak Atoll. Lower right: Isolated 

 coral head located to seaward of the spur and groove zone, windward shore of EnewetiU( Island, depth 7 m. 



Harrison's (1977) study area was disrupted by a severe 

 typhoon in January 1979 (Alice) in which all the large 

 Acropora colonics at 15 to 25 m were reduced to rubble 

 (Fig. 16), confirming their suspicion that typhoon-strength 

 storms are capable of such disruption to depths near 20 to 

 25 m. 



The outer slope or "drof>-off" begins at depths of 18 to 

 23 m as a distinct change, from a gentle slope of a few 

 degrees to an angle of approximately 30° to 45°. This 

 slope rapidly increases with depth (Fig. 17). The deep 

 reefs of the windward side have been severely damaged by 

 storms so that there is relatively little live coral and 

 tremendous amounts of rubble at 15 to 30 m depth 

 (Fig. 16). Along Enewetak Island to Medren, there is gen- 

 erally a sandy zone at 30 to 40 m which appears as an 

 irregular light band from the air. Below this depth sand 

 channels alternating with reef can be seen on the outer 

 slope when viewed from the air; this sandy zone is not 

 apparent from the air on reefs of the islands farther north. 



Vosburgh (1977) experimentally determined that waves 

 of near 5 m height did not produce sufficient water motion 

 at depths of 9 to 21 m to cause breakage of the skeleton 

 of large, healthy Acropora Ci/thera. He reported that 

 although this species is found at less than 2 m depth in 

 sheltered areas of the lagoon, it occurs commonly on the 

 windward reefs only at depths below 8 to 10 m. Sheltered 

 lagoon colonies were generally larger than those on the 

 windward reef, and depth distribution and colony size are 

 related to wave exposure. Although his estimates of near 

 5 m waves are based on the highest 1% of waves 

 observed during the windiest portion of the year, he points 

 out that typhoon waves, not considered in his study, 

 "might cause catastrophic breakage over the entire species 

 range on the (windward) terrace." 



The steady seaward slope of the windward reef gen- 

 erally prevents accumulation of large amounts of sedimen- 

 tary material. At the slope break at about 18 to 20 m 

 depth, some sediment-bottomed channels occur which can 



