SUBTIDAL ENVIRONMENTS AND ECOLOGY 



115 



Fig. 18 Typical views of seaward reefs off the southwestern islands, leeward side of Enewetak. Upper left: Area of heavy graz- 

 ing, probably by parrot fishes, on rock substrata. Upper right: Seaward end of the reef flat with short "grooves" going Into the reef 

 flat. Lower left: Heavily greized substrata, with parrot fish tooth marks, depth 2 m. Lower right: Seaward end of the reef flat with 

 some large coral colonies. 



face, but they are small and do not produce significant 

 amounts of calcium carbonate. 



There are many large fan-like gorgonians along the 

 vertical face, in addition to widely scattered colonies of 

 antipatharians (black coral). Large black coral "trees" are 

 rare in these (and all other) areas. 



On the leeward side of Enewetak Atoll there is an algal 

 ridge-type structure which is not well known. Marsh (1970) 

 reported one area at Igurin to have "a relatively good 

 growth" of coralline algae. The leeward ridge is in many 

 places slightly submerged at low tides, but never as emer- 

 gent as the windward reef flat. 



The outer slope of Enewetak below scuba diving 

 depths was examined to a depth of 365 m with the 

 research submersible Makali'i during the summer of 1981. 

 Twenty-two dives were made on the seaward face from 

 Biken around the southern end of the atoll to south of 

 Runit (Colin et al., 1986). The seaward reefs of the north- 

 ern half of the atoll were not examined. 



The depth profiles of five areas on the seaward margin 

 are shown in Fig. 21. The profile of the outer slojie of 



Enewetak is very steep, an angle of about 60° between 90 

 and 360 m with the leeward slope being slightly steeper. 

 Emery et al. (1954) and subsequent writers have com- 

 mented on the steep slopes of atolls in the northern 

 Marshalls. Their opinions were based on echo soundings 

 and were confirmed by observations from the submersible 

 Makali'i. 



To depths of about 300 m the slojje is generally rock 

 with small accumulations of sediment. Every near- 

 horizontal surface has a dusting of sediment, and small 

 ledges have accumulations varying with the area where 

 sediment can rest. There is little significant accumulation 

 of talus to depths of 200 to 300 m from upper areas as 

 the slope remains steep enough to prevent talus accumula- 

 tion. Incised, highly polished vertical grooves occur in the 

 rock face serving for downslope transport of sediment. At 

 depths between 200 and 300 m, large talus begins to 

 occur in the form of broken colonies of coral and reef plate 

 brought down the slope. In some locations a Halimeda 

 sediment-dominated slope began at about 270 to 300 m 

 depth with a slight decrease in slope. Along with this were 



