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COLIN 



patch reefs fin the windward side. The sand channels shoal 

 gradually from the lagoon to their shallowest fxsint, then 

 again gradually deepen to seaward. Near the precipitous 

 reef edge to seaward, the channels quickly steepen, then 

 plunge down the near vertical slope. Sediment is trans- 

 ported over the drop-off here with heavy scouring of the 

 reef face below the sand chutes. 



Algal Ridge 



Before considering the true seaward reefs, it is 

 worthwhile to mention the zone marginal to the reef flat. 

 This is the "algal ridge" which is truly intertidal but has 

 extreme relevance to subtidal seaward areas. 



The seaward reef on the windward side of Enewetak is 

 mostly devoid of live coralline algal ridges. Live algal ridge 

 (often termed "Lithothamnion ridge" by earlier authors) 

 occurs only along one section of windward reef about 

 200 m in length at Ananij Island. This section is readily 

 distinguished by its pink coloration, produced by the abun- 

 dance of Porolithon species, as compared to the dull sur- 

 face of the algal ridge dominated by macroalgae. 



Three species of Porolithon, as identified by Lee 

 (1967), have been found on the Ananij algal ridge. Large 

 portions of the surfaces of the spurs are covered with crus- 

 tose corallines, probably Porolithon onkodes. Distinct colo- 

 nies of Porolithon craspedium, often with blunt fingers 

 forming a lobate mass, occur scattered on the upper sur- 

 face of the spur. Porolithon gardineri seems the least com- 

 mon species, although its colonies are often irregular 

 masses 20 cm or more across. It appears limited to the 

 sides of the spurs, not being found on the upper surface 

 among P craspedium^ Within the sponge-like structure of 

 the spurs at Ananij, virtually all visible internal surfaces are 

 covered by coralline algae, but the species involved are not 

 known. 



Inshore from the live algal ridge at Ananij is a slight 

 depression of the reef flat where colonies of Acropora sp. 

 flourish. Small patch reefs occur on the hard pavement 

 here which has water on it even at low tides. The 

 Acropora sp. colonies are emergent at low tides. The small 

 coralline algae Neogoniolithon rutescens is occasionally 

 found among these patch reefs but not on the more 

 exposed spur and groove areas. 



Seaward Reefs 



Smith and Harrison (1977) described the windward 

 reef slope off Chinimi Island, and since their study other 

 areas have been examined. The spurs are relatively flat on 

 top and occasionally have undercut, overhanging edges 

 (Fig. 14). Algae and invertebrates are abundant on the 

 sides of these spurs. Sea urchins have eroded elongate 

 grooves in the rocks on the sides of the spurs which afford 

 protection from wave action and predatory fishes (Fig. 15). 



The bases of the grooves are floored with boulders and 

 cobbles, precluding any significant benthic invertebrate 

 populations (Fig. 14). The walls of the grooves, however. 



have on them small corals and invertebrates adapted to 

 withstand the wave surge. On the upper surfaces of the 

 spurs, small corals grow with an abundant film of algae on 

 rock surfaces (Fig. 14). 



Smith (MPRL, 1972) dissected a spur and groove sys- 

 tem north of Japtan using explosives. "The spur proved to 

 be dense, well-cemented coral rubbles covered by a veneer 

 of live encrusting coralline algae." He felt that, except for 

 relatively minor growth by the coralline algae, the spur and 

 groove systems are erosional features. 



On windward reefs the spur and groove zone and the 

 area immediately seaward of it are areas of very high fish 

 abundance (Fig. 15). Herbivorous parrot fishes and sur- 

 geonfishes feed in this productive area and range on to the 

 algal ridge and reef flat from there. At low tide these shal- 

 lower areas are dry, requiring their exploiting fish popula- 

 tions to move elsewhere. 



The spur and groove zone seaward of the areas of 

 Porolithon algal ridge at Ananij is different from other 

 areas examined where the ridge is "dead." The cover of 

 benthic invertebrates appears higher there. This is the only 

 area on the windward shore where the club-spined urchin 

 Heterocentrodus trigonarius is known to be abundant, both 

 in holes on the sides of the spurs and on the algal ridge. A 

 form of branched Acropora sp. coral with other corals and 

 Halimeda sp. algae with distinct laminations occurs there. 

 This form of Acropora has not been seen elsewhere 

 (Fig. 15). 



Off the north end of Enewetak Island, the sides of the i 

 spurs are lined with grazed macroalgae and occasional 

 patches of coralline algae. The rock-boring urchin, 

 Echinometra methaei. is abundant in grooves in the sides 

 of the spurs. In small caves and on overhangs a wide 

 variety of benthic invertebrates occurs. 



On the sides of the spurs' upper surface are small head 

 corals and soft corals. There is less coral on the tops of 

 the spurs, and the area is more dominated by macroalgae. | 

 At the seaward end of the spur, colonies of stony corals, 

 Heliopora caerulea, and soft corals are common. These are 

 larger than those of the top or sides of the spur. Some 

 sizeable encrusting sponges may also occur in this area. 



Several herbivorous fishes are characteristic of this j 

 spur and groove zone. The surgeonfishes, Acanthurus 

 achilles. A. guttatus, A. thostegus, and especially A. lin- 

 eatus are generally found in any abundance only in this 

 area on the windward shore. One small damselfish, 

 Plectrogliiphidodon phoenixensis, is common on the wind- 

 ward shore and occurs only in the spur and groove area. 



Seaward of the spur and groove, the rocky bottom lev- 

 els somewhat with only a slight seaward slope (Fig. 14). 

 The bottom often has minor undulations of its surface, 

 occasionally with small shallow grooves oriented perpendic- 

 ular to the reef front, but generally it has few distinguish- 

 ing features. The irrejular pits and grooves of rock-boring 

 sea urchins, Echinometra mathaei. and lesser numbers of 

 some diademnid urchins (Fig. 16) are often abundant. A 

 few small- to medium-sized corals occasionally occur on i 

 this "barren" zone (Fig. 15). Viewed from the air, this zone - 



