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COLIN 



MPRL files, and unpublished data from numerous scien- 

 tists. 



No comprehensive descriptive account of the subtidal 

 environments of Enewetak has been attempted. Only a few 

 specialists have endeavored to discuss atollwide distribu- 

 tion and contributions of their restricted groups of organ- 

 isms. Cuffey (1973 to 1978) examined the role of bryozoa 

 at Enewetak with comparison to other reef areas. He listed 

 three major marine benthic "macrohabitats" at Enewetak: 

 the coral-dominated, the bedrock-dominated, and the 

 sediment-dominated. He distinguished between "larger 

 patch reefs (from 25 to more than 150 ft height)" and 

 "smaller coral knolls (from 1 to 25 ft high)" in examining 

 bryozoan distribution at Enewetak. He also distinguished 

 between biohcrms ("coral-dominated macrohabitats with 

 significant height") and biostromes ("coral-dominated 

 macrohabitats lacking significant height"), such as his 

 "coral pavement." 



Allen (1972), in his work on anemonefishes, provides 

 brief descriptions of some Enewetak habitats. The major 

 physiographic features of environments from outer reef 

 slope, reef flat, shallow lagoon margin, and deep lagoon 

 arc mentioned. He thought that the deep lagoon floor 

 "appears to be of a fairly uniform nature" and had "large 

 stretches of sand with orcasional small patch reefs." Deep 

 lagoon pinnacles were described as "an oasis, rising from 

 the barren lagoon floor" and harboring "an extraordinary 

 wealth of marine organisms." 



The deep lagoon can be characterized as dominated by 

 sediment substrates but with reefs of widely varying size 

 and vertical relief, distributed fairly evenly throughout the 

 lagoon. The soft substrate supports several different biolog- 

 ical communities, often occurring within short distances of 

 other soft substrata and reef substrata. Extensive distur- 

 bance of the sediments is evident in many of the benthic 

 photographs and from submersible dives. 



Based on point counts of the benthic survey photo- 

 graphs, approximately 859b of the deep lagoon has soft 

 substrata, with the remaining 15% hard substratum. Nearly 

 half of the stations had 100% coverage of soft substrata, 

 more than 60% were 90% or more soft substrata, and 

 more than 75% were 75% or more soft substrata cover- 

 age. If the individual photographs are considered, rather 

 than entire stations, slightly higher percentages of 100% 

 and 90% soft substrate coverage are found. 



The soft substrate biological communities comprise 

 four identifiable types. These include (1) open sand sub- 

 strate without a visible algal mat, (2) sand substrate with 

 visible algal mat on its surface ("algal film"), (3) sand sub- 

 strate with macioalgae, particularly species of Halimeda, 

 on its surface ("algal flat"), and (4) sand with large popula- 

 tions of an unattached Fungiid coral, Cxjclosersis and 

 Diaseris spp. ("button corals"). Typical views of these com- 

 munities from benthic survey photographs are shown in 

 Figs. 2 and 3. Interpretation of the benthic survey photo- 

 graphs has been facilitated by observations and photo- 

 graphs from the Enewetak submersible project and scuba 

 dives in shallower areas where similar communities occur. 



The soft substrate communities often intergrade, for 

 example, the macroalgae of the "algal flat" community 

 decreasing in density until only open sand remains. Arbi- 

 trary points at which one community "becomes" another 

 have been used in interpreting the photographs, but abso- 

 lute distinctions among types of soft substrate communities 

 are often impossible. The distributions of community types 

 in the deep lagoon as based on benthic survey 

 photographs are shown in Figs. 4 and 5. 



Deep lagoon sediment substrata with no visible algal 

 cover are qualitatively similar to areas of the lagoon mar- 

 gin as shallow as 15 m. They are usually heavily 

 bioturbated, dominated by the conical mounds produced 

 by callianassid shrimp. The occurrence of open sand sul>- 

 strates, based on benthic survey photographs, is shown in 

 Fig. 4. Although this covers only a limited number of sta- 

 tions, it does indicate "barren" soft substratum can occur 

 throughout the lagoon. Open sand substratum, however, 

 can change within a few meters horizontally to soft sub- 

 stratum covered with macroalgae. Such short-scale horizon- 

 tal changes among soft-substratum communities and hard 

 substrata are common throughout the lagoon. 



It is possible that the rapid sediment turnover in open 

 sand areas is responsible for the lack of dark algal films of 

 macroalgae. However, algal mats over 1 m in diameter do 

 occur in heavily bioturbated areas but are capable of form- 

 ing in only a few days time. Biological sediment overturn is 

 concentrated at specific points in the short term (days) pro- 

 ducing "splotching" of algal mats when viewed from above. 

 Other factors affecting density of algal films (nutrients in 

 water or sediment, water clarity, standing crop already 

 present, etc.), may be critical in controlling the presence of 

 dense algal mats. 



The presence of algal films, particularly diatoms and 

 blue-green algae, on sand bottom without visible algal mat 

 is well known (J. T. Harrison, personal communication). 

 The population level at which an algal mat becomes visible 

 in a photograph or to a human observer is dependent on 

 the standing crop per unit area and the plants involved. 



Water visibility over open sand bottoms is often limited 

 to only 5 to 10 m, even in the deep lagoon. Considerable 

 amounts of suspended particulates were observed from the 

 submersible Makali'i, using its lights, in the deep lagoon; 

 but relative densities varied considerably from day to day 

 at one location. It was noted, however, that suspended 

 particulates were often elevated closer to the bottom than 

 near the lagoon surface. Similar observations commonly 

 have been made by scuba divers over open sand substrates 

 at depths around 20 m. 



Sediment will often have a clearly visible thin layer of 

 microalgae on its surface. Algal films are seen in the 

 lagoon margin areas as shallow as 15 m. At depths of 15 

 to 30 m, small dense algal mats (only a few centimeters in 

 diameter) are often seen on otherwise clear bottoms. 

 Where a film of algae exists, any recent disturbance of the 

 sediment is clearly indicated by lack of, or disturbance of, 

 the algal mat. This relationship has been closely examined 

 at diving depths from 15 to 30 m and has been verified in 



