SUBTIDAL ENVIRONMENTS AND ECOLOGY 



103 



A different situation exists on the lagoon margin on the 

 southwestern, western, and northwestern sides (leeward). 

 Because of exposure to prevailing winds across the fetch 

 of the lagoon, these areas often possess an almost barrier- 

 reef type structure with small patch reefs inside it. The 

 sediment bottom often slopes upward steeply near this 

 structure. This is discussed subsequently. 



Hiatt and Strasburg (1960), in their classic study of 

 reef fish feeding ecology, presented a brief summary of 

 Enewetak reefs. They reported that in the lagoon "in pro- 

 tected areas there is a discontinuous series of irregular 

 patch reefs which extend from nearshore to the outer reef 

 slope leading to the deeper parts of the lagoon." On the 

 western side of the lagoon, "the lagoon reefs are better 

 developed and frequently are continuous, because they 

 receive fairly strong waves engendered by the prevailing 

 winds" across the lagoon. In some respects, they come to 

 resemble reefs of the windward shore. Hiatt and Strasburg 

 (1960) provide drawings of typical reef environments (tidal 

 pools, seaward reef flat, spur and groove surf zone, patch 

 reefs and coral heads, mid-water) with the characteristic 

 fishes found there. 



The patch reefs of the windward lagoon margin have 

 particularly well-developed coral communities where the 

 water flow across the reef is unimpeded by islands. The 

 vertical relief of the reef generally increases with size, but 

 in many cases small reefs have a relief about one-half their 

 diameter, up to a maximum of about 6 m relief. Table 

 Acropora sp. corals are abundant on these patch reefs, 

 whereas other corals grow well on the sides of the patch 

 reefs and even under overhangs because of the reflection 

 of light from the white bottom. Relatively few soft corals 

 occur in such areas. 



Sand areas in between the lagoon rim patch reefs are 

 areas of high grazing pressure by surgeonfishes and parrot 

 fishes. Burrowing activity in the sediments is also high, 

 mainly through the activities of a variety of fishes. 



An important factor determining the distribution of 

 windward lagoon margin patch reefs is the effect of lagoon- 

 ward sediment and rubble movement from the reef flat. 

 Between Enewetak and Medren such patches are abun- 

 dant, but they are best developed in areas protected from 

 sediment "overwash." Leeward of Bokandretok is an area 

 of numerous patch reefs, whereas north and south of this 

 the island rips have covered the area with sediment where 

 the reefs occur. Farther north along the reef, areas of sedi- 

 ment overwash have at best reduced numbers of patch 

 reefs. In areas protected by structures diverting the cross- 

 reef flow of sediment, patch reefs are better developed, 

 coming close in behind the reef flat. Nolan (1975) used a 

 large series of patch reefs in the lee of "Isaac's Island," a 

 small rock and sand spit, for his fish community studies. 



Nolan (1975) described some patch reefs between 

 Medren and Enewetak Islands where he analyzed and 

 manipulated reef fish populations on these and artificial 

 reefs. He felt coral development was particularly luxurious 

 on the patch reefs on the lagoon side of Isaac's Island. 

 Nolan (1975) pointed out that many of the patch reefs to 



leeward of Enewetak and Medren Island were predom- 

 inantly dead coral. He provided a detailed map locating his 

 study reefs and chose reefs of about 3 X 3 X 3 m in 

 size, which were abundant, in depths of 5 to 7 m. He 

 noted that the reefs in the lee of Isaac's Island were pro- 

 tected from the full brunt of the cross-reef currents but 

 that an eddy pattern existed on the leeward side of this 

 small outcropping which provided abundant water circula- 

 tion. 



Nolan's (1975) study reefs were predominated by mas- 

 sive "table" Acropora cythera, but during his study in 

 1972, heavy surge from the leeward side of the atoll 

 dislodged many of these corals on his study reefs. Sand in 

 this area was also removed and deposited in shallow water 

 creating a 3 m high sand bar continuous from Medren to 

 Enewetak. This sand ridge was destroyed and moved into 

 the lagoon with the resumption of normal trade wind 

 weather and sea swell. 



Similar destruction of A c\^thera on patch reefs was 

 observed during southwesterly to westerly storms in March 

 1981 and July 1982. The tables of A. cythera were bro- 

 ken loose at their bases and moved. Many specimens 

 ended up on island beaches with the corallum nearly 

 intact, testament to the strength of this form. 



North from Japtan to Ananij, no significant lagoon mar- 

 gin patch reefs exist between islands. The bottom slopes 

 relatively steeply into the lagoon, and the reef from ocean 

 to lagoon is narrow. The zonation across the reef is dis- 

 tinct (Fig. 10) and is described subsequently. Chinimi, the 

 only island interrupting this 4 km stretch of open wind- 

 ward reef, has the lagoon margin protected from reef flat 

 "outwash," and patch reefs are well developed in the lee 

 of the island. The change in zonation of the lagoon margin 

 is really visible north from Japtan. Island rips occur north 

 and south of Chinimi and lagoonward depth contours veer 

 close to Chinimi 's shore in its lee. This cusping of the atoll 

 rim behind islands is seen in other areas of the windward 

 side. The area on the northern lagoon margin of Chinimi 

 has one of the best developed reefs along the shore of any 

 windward island, with lovely microatolls, although less than 

 100 m north the reef seems limited by the island rip and 

 sediment outwash. 



Ananij similarly has a large number of lagoon margin 

 patch reefs in its lee and has the most developed island rip 

 system of any island at Enewetak. Between it and Runit, 

 8 km farther north, cross reef zonation is similar to that 

 south of Ananij, but more islands are found on the reef. 

 The island cusping effect, however, is evident with many 

 patch reefs in their lee. 



A good example of a well-developed lagoon margin 

 patch reef is "Choptop Reef," located just north of "Isaac's 

 Island" between Enewetak and Medren (Fig. 8). It is large 

 for a lagoon margin patch reef, but smaller reefs adjacent 

 to it are similar and provide easy comparison. Choptop 

 has high coral cover and diversity and high fish popula- 

 tions (Fig. 8). It is located on the margin of a reef flat rip, 

 and although not in the strongest portion of the current 

 coming off the reef flat, it is in a well-flushed area. An 



