SUBTIDAL ENVIRONMENTS AND ECOLOGY 



99 



the deep lagoon by observations and photographs from the 

 Makali'i. Such areas appear much lighter when viewed 

 from above because the algal mat has not had time to 

 regrow. The distribution of stations with over 25% algal 

 mat is shown in Fig. 3. 



Algal flats are dominated by macroalgae, particularly 

 species of Halimeda and Caulerpa. Algal films can also 

 occur with macroalgae. Individual plants are often 

 separated by areas of open sand, but densities can be high 

 enough that relatively little sediment bottom is visible. 

 Caulerpa sp. grows via rhizomes, spreading out over the 

 bottom in easily distinguishable patterns. 



Areas of algal flat can range from a few meters to over 

 hundreds of square meters. On benthic photographic 

 traverses of large algal flats, decreasing density of thalli on 

 their edges was often seen grading into open sand. The 

 situation also exists where dense macroalgae changed 

 abruptly into open sand. The distribution of stations with 

 more than 25% algal flat is shown in Fig. 4. 



The algal flat community is a diverse soft-bottom com- 

 munity with many epiphytic organisms with an apparently 

 higher biomass than open sand and algal films. A 

 shallow- water algal flat community, dominated by Halimeda 

 spp., occurs as shallow as 18 m and is a shallow lagoon 

 community accessible to divers, comparable to the algal 

 flat of deeper water. Termed "Halimeda meadows," these 

 algal communities extend shallower than the 30 m arbi- 

 trary cutoff in the "deep" lagoon, but because of strong 

 similarities these are considered representative of deep 

 lagoon Halimeda communities. These consist of extremely 

 dense stands of Halimeda spp., associated with other abun- 

 dant macroalgae, which often form distinct circular or 

 irregular algal communities from a few to many tens of 

 meters across. The meadows arc often a slightly elevated 

 mound, perhaps 1 to 3 m higher than surrounding sedi- 

 ment bottoms. Some stony corals, principally finely 

 branched Acropora and small head coreils, often occur on 

 the mound. The skeletons of these combined with 

 Halimeda and other calcareous algae plates produce a 

 higher percentage of coarse material in the surface sedi- 

 ment than in surrounding op)en sediment areas. The eleva- 

 tion of the Halimeda mound is probably due to accumula- 

 tion of carbonate material produced at the mound, 

 whereas adjacent sediment areas have not kept pace with 

 the relatively rapid carbonate accumulation on the mounds. 

 Information is lacking concerning the growth and longevity 

 of Halimeda mounds; such information would be of particu- 

 lar interest. The mounds/meadows are also foci for high 

 animal abundance. Small fishes, particularly herbivores and 

 bottom feeders, are abundant, as are benthic invertebrates. 

 Preliminary production/respiration data (MPRL, 1981) indi- 

 cated that in spite of high algal biomass there is little or no 

 net productivity by the Halimeda mound community, the 

 production of the high algal biomass being usurped imme- 

 diately by resident animal populations. 



A community of smeJl coretls of the genera Cvchseris 

 and Diaseris growing unattached on sediment substrates 



has been found in the deep lagoon (Colin, 1985). In this 

 community, the small (2 to 6 cm) corals, called "button 

 corals," occurred in densities of up to 100 m~ and were 

 photographed at only six benthic stations (Fig. 4). All these 

 were in the deep)er portions of the lagoon below 50 m. 

 The photographs at that station were all, or nearly all, of 

 C^icloseris- and Diaseris spp.-dominated bottom. Obviously 

 beds of this coral can cover areas many hundreds of 

 square meters. Also, macroalgae — fiarticularly species of 

 Halimeda and Caulerpa — small sponges, and other inver- 

 tebrates often occurred among the button corals. 



Some elements of the fauna of soft substrata at diving 

 depths at Enewetak are fairly well known. In the case 

 where sp>ecies occurring at Enewetak are documented, 

 other biological information is usually not known. In one of 

 the few instances where more than the base essentials are 

 known, several species of irregular sea urchins occur 

 buried in, or on, sediments. The density of given sp>ecies In 

 apparently similar areas of the lagoon margin has been 

 documented to vary by well over an order of magnitude 

 (V. S. Frey, unpublished data). Similar population varia- 

 tion has also been observed at a single station over severed 

 months. Although these variations have been documented, 

 the many factors determining papulation structure of infau- 

 nal organisms are poorly understood. 



The smaller organisms dwelling in sediment bottoms 

 are more jxxjrly known. For example, using a technique 

 where an area of bottom is covered by a plastic sheet and 

 rotenone, or another toxicant, introduced beneath the 

 sheet for a time, lancets (Branchiostomldae) have been col- 

 lected recently at a density of approximately 100 individu- 

 als m~^ on sediment bottoms below 15 m at Enewetak 

 (Suchanek and Colin, 1986). Schultz et al. (1952), in spite 

 of their collecting efforts in the Marshedl Islands, took only 

 a single specimen of lancet at Bikini Atoll. Approximately 

 50 small unidentified ghost shrimps were collected per 

 square meter using this technique, a density far greater 

 than imagined. The only visible evidence for the presence 

 of these small calllanassids is small-scale conical mounds 

 present in combination with larger mounds produced by 

 larger species. Also collected were stomatopods, sipuncu- 

 lids, molluscs, and echinoids (Suchanek and Colin, 1986). 

 Interestingly, in sediment-leveling experiments the number 

 of small-scale mounds (less than 5 cm diameter) was an 

 order of magnitude or more greater than large-scale 

 mounds, supp>orting evidence of the high populations of 

 small callianassids (Suchanek et al., 1986). 



Pinnacle Reefs of the 

 Deep Lagoon 



It is impossible to draw an absolute line where the 

 f)atch reefs on the margin of the lagoon and pinnacle reefs 

 begin. A working distinction can be made between "patch 

 reefs" which rise from a surrounding sediment or rock bot- 

 tom which is visible from the surface under normal condi- 

 tions and "pinnacle reefs" rising from depths where the 



