SUBTIDAL ENVIRONMENTS AND ECOLOGY 



121 



occur, bryozoans increase in abundance, particularly in the 

 patch reefs at depths below 9 m. Cuffey (1973) believed 

 the floor of the deep lagoon, accessible to him only by 

 dredging, lacked any diverse bryozoan fauna and only a 

 few "small detrital fragments" of bryozoa were taken. The 

 pinnacle reefs of the deep lagoon, however, contained an 

 abundant and diverse complement of bryozoans. The steep 

 leeward slope of the atoll apparently had the most diverse 

 community of bryozoa, particularly below 9 m. 



Cuffey (1973) found bryozoans more abundant in Ber- 

 muda than Enewetak, where they were infrequent in 

 depths less than 9 m. He suggested that the considerably 

 higher diversity of Enewetak corals might adversely influ- 

 ence the relative success of bryozoa when compared to 

 Bermuda. He makes the interesting observation that "the 

 leeward (southwestern) side of Eniwetok Atoll harbors 

 noticeably more bryozoans (both taxa and individuals) than 

 does its windward (northeastern) side. Bryozoan distribu- 

 tion on Eniwetok thus parallels sponge distribution within 

 Pacific atolls, as described by De Laubenfels (1954)." In 

 addition to not being principal frame builders on Enewetak 

 reefs, bryozoans do not contribute any significant amounts 

 of classic detritus to the sediments of the reef (Cuffey, 

 1973). 



Cuffey (1973), in considering the bryozoa of Enewetak, 

 found that most species inhabited the undersides of corals 

 and rocks on reefs. The most abundant bryozoans at 

 Enewetak were encrusting cheilostomes which grow as 

 thin, sheet-like crusts on the undersurfaces of corals or 

 rocks. Most bryozoa inhabited these sheltered microhabi- 

 tats and "function primarily as 'hidden encrusters,' adding 

 small quantities of calcareous skeletal material to the reef 

 framework." Cavity-filling tendency by bryozoa was noted 

 in Bermuda but not at Enewetak. 



Hydrozoans of the genus Millepora are extremely 

 important calcifying and reef-building organisms at 

 Enewetak. In many areas, such as the large coral head 

 zone of Odum and Odum (1955), Millepora spp. can form 

 heads several meters across which grow to the low tide 

 level where they form flat-topped structures (Fig. 12). In 

 deeper water — including the ocean slopes of all sides, 

 lagoon margin patch reefs, and lagoon pinnacles — 

 Millepora spp. form large delicately branched, often fan- 

 like, colonies (Fig 21). 



The stylastcrine hydrozoans, unlike Millepora, are not 

 important carbonate producers at Enewetak. The delicate 

 fan-like species of Sfy/aster are found beneath overhangs 

 and within caves of patch reefs, pinnacles, and on outside 

 reefs. Similar, but more robust, are two species of 

 Distichopora which occur in similar areas but are often 

 more exposed. These are extremely common on the 

 leeward reef slope but do not produce reef framework. 



Tubipora musica is not common at Enewetak, being 

 found only occasionally on reef fronts or on lagoon pinna- 

 cles, and therefore does not produce significant reef struc- 

 ture. 



Calcareous green algae, particularly members of 

 Halimeda, are extremely important in sedimentation and 



reef building. The distribution of Halimeda in most subtidal 

 environments at Enewetak is well documented (Hillis- 

 Colinvaux, 1977, 1980, 1986; Emery et al., 1954; Colin, 

 1986). Borings at various atolls (Funafuti, Enewetak, 

 Bikini, reviewed by HillisColinvaux, 1980) have shown 

 Halimeda segments to be the major identifiable component 

 of unconsolidated lagoon deposits. Milliman (1974) indi- 

 cates that among sand-sized components of lagoon sedi- 

 ments in Pacific and Atlantic atolls, Halimeda segments are 

 generally the first or second most common material. Hillis- 

 Colinvaux (1980) cites evidence in Couch et al. (1975) 

 that Halimeda segments make a significant contribution not 

 only to unconsolidated lagoon sediments but also to 

 material underlying the reef rim. The fate of Halimeda 

 plates in sediments varies. Some are shed intact, but a few 

 species (H. macroph^sa and H, favulosa at Enewetak) have 

 delicate segments that are easily broken (HillisColinvaux, 

 1980). 



Carbonate production rates by Halimeda at Enewetak 

 are not well known, depending on plant density, generation 

 time, and shedding rates. HillisColinvaux (1980) reports 

 that population densities in Halimeda can vary by two or- 

 ders of magnitude with concurrent effects on carbonate 

 production. Turnover rates are perhaps lower than some 

 published data (HillisColinvaux, 1980) since Halimeda is 

 predominantly a long-lived alga. One experiment at 

 Enewetak indicated that 70% of the original thalli were still 

 present after 4 months (HillisColinvaux, 1980). 



Dense populations of Halimeda at Enewetak and else- 

 where have about 100 plants m^^ of the H. incra^^sata- 

 c\;lindrica type thallus. The rock-growing H. opuntia type 

 can have higher densities of plant material, although abso- 

 lute numbers of plants may be less. HillisColinvaux (1980) 

 estimates that the H. incrassatact^Hndrica types would pro- 

 duce only about 10% of the total carbonate accumulation 

 in the lagoon (Smith and Kinsey, 1976) if they covered the 

 major portion of the lagoon bottom. She was not aware at 

 that time of the presence of the "Halimeda meadows" and 

 the estimated percent coverage of the deep lagoon bottom 

 predominantly by Halimeda. The contribution of Halimeda 

 segments from lagoon pinnacles may be smaller than 

 HillisColinvaux (1980) calculated when a comparison was 

 made to Halimeda from flat lagoon bottoms. 



Bioerosion of Reefs 



The agents of bioerosion at Enewetak act in a variety 

 of ways. Some, such as the boring sponges of the genus 

 Cliona, excavate chambers on the carbonate skeletons of 

 living corals and virtually any other carbonate substrate. 

 The shells of molluscs, coral rubble, and other small car- 

 bonate fragments can be attacked. Other organisms, in the 

 course of feeding activities, rasp away the surface layers of 

 carbonate while grazing the thin film of algae which covers 

 such surfaces. The parrot fishes, surgeonfishcs, various 

 echinoderms, and other such herbivores generally pass the 

 carbonate material through their gut, subjecting it not only 

 to mechanical effects but also to chemical effects. Other 



