124 



COLIN 



the isolated bacteria were able to digest chitin in vitro, and 

 DiSalvo suggested these might weaken the skeleton by 

 breaking down the organic matrix. DiSalvo (1969) also 

 found that sediments in proximity to the bases of corals 

 had 10^ to 10* bacteria g dry wt^' of which 10 to 20% 

 were chitin-digesting varieties. Thus there is a ready 

 source of suitable bacteria close to the coral's skeleton. 



Invertebrate Coral Predators 



Some invertebrates are also coral predators at 

 Encwetak. The crownofthorns starfish, Acanthaster plana, 

 is found in many areas. Most lagoon pinnacles have one or 

 more A planci, and evidence of their feeding activities on 

 corals is apparent. The status of A. planci populations at 

 Enewetak, population changes, and impact on the reefs in 

 recent times arc not well known. Population levels are cer- 

 tainly below "plague" levels. Allen (1972b) noted that 

 "during the summer of 1970 the author observed an 

 increasing number of Acanthaster at Eniwetok. Prior to this 

 date relatively few were observed." Starck (MPRL, 1971, 

 1972) found "no unusual populations of Acanthaster." 

 About a Ihour (one dive) search during the day would 

 result in one to two individuals, whereas a nighttime 

 search of about only one-quarter the area revealed 10 to 

 12 specimens. Starck found a wrasse, Cheilinus undulatus, 

 of about 50 kg weight (of four examined) with a large, 

 nearly intact A. planci in its gut contents. Starck, in Ander- 

 son (1979), reported a sizeable population, perhaps as 

 many as 50 to 100 A planci on Pole Pinnacle, but stated 

 that there was no extensive damage to the coral there. 



The juveniles of A planci apparently occur beneath 

 rubble on reefs. Lisa Boucher and Scott Johnson (personal 

 communication) report finding numerous examples from a 

 few centimeters to less than 1 cm disk diameter on pinna- 

 cles near Enewetak Island. Although they never found 

 these A. planci to be very common, a distinct increase in 

 the numbers of juveniles encountered was noted in April 

 1982. 



Storm Destruction of Reefs 



The effect of subtyphoon storms (tropical storms, tropi- 

 cal depressions) on subtidal environments can be devastat- 

 ing. Many such storms occur compared to full strength 

 typhoons and are often not noted in historical records. 

 Damage from wind and rain to terrestrial areas may be 

 minor, but the swell produced by such storms can wreak 

 havoc in shallow-water communities. The production of 

 boulder ramparts by storms is well known, and such struc- 

 tures occur on the southwest islands of Enewetak. Since 

 the ocean shores of these islands are normally in the lee 

 and the reef slope is steep and close to shore, the infre- 

 quent reversal of wind and waves can cause catastrophic 

 destruction of corals in shallow water, moving vast quanti- 

 ties of material onto the shore or into deeper water. 



The movement and effect of ocean swells in the lagoon 

 are important. The wide pass at Enewetak is sufficiently 

 deep to allow ocean swell from the southeast to southwest 



to enter the lagoon. Ocean swell is also refracted at the 

 pass so that wave trains moving from the west and 

 southwest can come through the pass and proceed north 

 to northeast to reach lagoon shores. These long period 

 swells have no direct effect on the deep lagoon communi- 

 ties. However, when they reach the lagoon shore of wind- 

 ward islands or shallow pinnacle or patch reefs, they can 

 turn these shallow-water communities into churning mael- 

 stroms of breaking waves. One such period of swells from 

 the southwest to west for 3 days in July 1982 turned the 

 lagoon shore of Enewetak Island and other southern 

 islands into a mass of dark brown water (with essentially 

 zero visibility) above 6 m depth with breakers to 2 m high 

 where depths were less than about 4 m. Significant swells 

 and breakers persisted for nearly 1 week. Many fishes, 

 molluscs, and other invertebrates were killed and cast up 

 on the beaches. In places sediment and rubble were eroded 

 away as much as 1 m or more. Many of the delicate corals 

 on shallow reefs {Pocillopora edouiixi. Millepora spp.) were 

 broken to stubs. 



Carbonate material from shallow water, particularly 

 large pieces such as coral boulders, can be deposited on 

 the islands by storm waves, transported into the lagoon or 

 transported downslope on the seaward reefs. Various 

 islands of Enewetak are densely covered with recently 

 deposited coral boulders, and boulder ramparts are evident 

 on the seaward beaches of some islands. Less visible, but 

 perhaps more significant, is downslope transport of rubble 

 on seaward reefs. Talus was evident at many locations 

 examined by the submersible Makali'i. and shallow-water 

 coral rubble was extremely evident in the material photo- 

 graphed. At 300 to 360 m, the maximum depths visited, 

 the slope of the bottom was generally too steep for 

 extremely large talus accumulations. Larger accumulations 

 of talus should lie below those depths where the bottom 

 slope is less steep. 



Storm swell within the lagoon may be a major factor 

 controlling the morphology of lagoon margin patch reefs 

 and shallow pinnacle reefs. Choptop Reef had moderate 

 damage from swells entering the lagoon in July 1982. 

 Some large coral heads, their bases weakened, were tum- 

 bled over. Pieces of Porites c^ilindrica colonies as much as 

 a meter across were torn loose from larger colonies and 

 rolled a few meters over the bottom. Although individual 

 branches were often broken, such pieces formed satellite 

 patches of P. cfjiindrica which survived and grew. In 

 another instance, a tunnel torn through a huge mass of 

 P. ci;lindhca at Choptop Reef during a tropical storm in 

 March 1981 was collapsed by the July 1982 storm. In 

 both instances the total structure was fractured. Swell 

 within Enewetak Lagoon seems capable of breaking apart 

 patch reef features which reach too near the surface. 

 Where the internal structure of lagoon margin patch reefs 

 is visible, they seem little more than accumulations of 

 poorly cemented coral rubble. One well-known lagoon pin- 

 nacle, "Tunnel Pinnacle" (Fig. 6), has had the "tunnel" col- 

 lapsed, almost certainly by storm swell, during the past 

 few years. Reese (1981) provides a description of the 



