SUBTIDAL ENVIRONMENTS AND ECOLOGY 



93 



side of the wide passage is an area of merging waves from 

 the lagoon and refracted oceanic waves and swell with 

 strong southerly currents producing standing waves and 

 short, steep seas. 



During summer calms and at odd times during the rest 

 of the year, the lagoon and surrounding ocean become 

 smooth. At such times it is possible to swim off the reef 

 flat to the windward ocean reefs because the surf is small 

 and gentle Surface slicks are found in lagoon waters. The 

 windrows of phytoplankton from blooms have been found 

 during such calm periods. 



Swells from distant storms create a different situation 

 in which shores exposed to the swell (which can be either 

 windward or leeward) are heavily pummeled, whereas the 

 waves produced by wind in the immediate area may be 

 small. Most impressive are those rare times when large 

 swells thunder against the reef while calm trade winds pro- 

 duce an almost mirror like surface elsewhere. On shores 

 normally lacking high surf, these waves can cause consider- 

 able damage. Such swells can also enter the lagoon 

 through the southern pass to break on the lagoon shore of 

 Enewetak, Medren, and other islands. 



Several typhoons and near-typhoon strength storms 

 have passed by or over Enewetak during the last decade. 

 Although the atoll is generally not considered in the 

 typhoon belt, these storms have had a significant and 

 readily visible effect on Enewetak reefs. Often a storm 

 whose center does not pass especially close to Enewetak 

 can produce storm waves which severely damage reefs, 

 although above-surface damage from winds may be very 

 light. 



Deep Lagoon Biological Communities 



The lagoon bottom below 30 m depth consists largely 

 of soft substratum with small to large reef structures (pin- 

 nacle reefs) spread randomly throughout the area. Except 

 for the pinnacle reefs, relatively little published information 

 exists regarding in-situ observations of either the lagoon 

 slope or deep lagoon benthic biological communities or 

 geology. Nearly all published information is based on 

 surface-lowered grab samplers or dredges. Emery et al. 

 (1954) reported on results of samples taken by an "under- 

 way" bottom sampler and about 50 photographs taken by 

 a remote camera at unspecified locations in the deeper 

 lagoon. More recent researchers (Nelson and Noshkin, 

 1973; Noshkin, 1980) have relied on grab samplers or 

 short-core samplers to obtain bottom samples for analysis. 



The only published in-situ observations of the deep 

 lagoon are those of Gilmartin (1960), made during deep 

 scuba dives on a transect across the southern lagoon. 

 Twelve of these stations were below 30 m in depth. All 

 stations below 30 m had coral patches present within the 

 range of visibility; these varied from only a few small 

 heads in one instance to massive patch reefs rising 15 m 

 or more above the surrounding bottom. Some stations had 

 the substratum covered with "mounds of sand and cast- 

 ings," but for most of the deeper stations, the presence or 



absence of mounds was not noted. This study confirmed 

 that abundant algal communities exist in the deep areas of 

 Enewetak Lagoon, many occurring at the deepest depths 

 reached (62 m). Coral patches at these depths seemed 

 more densely populated with algae than adjacent sand. 

 Eight species of Halimeda were found primarily between 

 42 and 62 m, supporting previous reports that the genus 

 was "most common and luxuriantly developed at the 

 deeper levels." Gilmartin (1960) was the first person to 

 realize the intense bioturbation of the soft substrate bot- 

 toms of the lagoon, commenting that "the continual 'churn- 

 ing' of the substratum by these benthic organisms has 

 prevented algae, which might occur elsewhere on the same 

 stations, from starting and growing to the fX)int where they 

 would not be 'uprooted' or buried by the sand displace- 

 ments." 



During 1980 and 1981, a distributional survey of deep 

 lagoon benthic communities was conducted using a 

 lowered camera system. During this "Enewetak Benthic 

 Survey" over 2000 photographs covering 24 m each 

 were taken at 190 stations throughout the deep lagoon 

 (Colin, 1986). Additionally, in the summer of 1981 the 

 submersible Makali'i was utilized for a series of dives in 

 several areas of the deep lagoon to augment the photo- 

 graphic survey. 



STATE OF KNOWLEDGE OF SUBTIDAL 

 MARINE ENVIRONMENTS 



With certain exceptions, the subtidal marine environ- 

 ments of Enewetak cannot be characterized as well known. 

 Often our knowledge is based on studies in the southern 

 lagoon close to the lee of the southern islands. The dis- 

 tances from support bases, the generally rough conditions 

 of the lagoon outside protected lee areas, and the rapidly 

 increasing water depth have severely limited work in both 

 the northern and central lagoon. Much of the work accom- 

 plished in the southern lagoon is of equal applicability to 

 the entire lagoon, but differences do exist between these 

 areas. 



Even the southern areas of the lagoon below 20 to 30 

 m depth are poorly known. This is due to the limited 

 access of scuba-diving scientists to the deep lagoon bot- 

 tom, particularly at its most common depths of 40 to 60 

 m. Although a few hardy souls have ventured to dive in 

 these areas, working time is limited, nearly precluding 

 studies providing an understanding of overall conditions in 

 the deep lagoon. Significant work on the deep lagoon floor 

 requires either specialized instrumentation and recording 

 methods or suitable vehicles for in-situ work. 



The present work is intended to provide descriptive 

 information about subtidal habitats in the following order: 

 (1) deep lagoon; (2) shallow lagoon; (3) lagoon-ocean 

 passes; and (4) the seaward reefs, from the center of the 

 lagoon outward. The information has been drawn from 

 publications, annual reports, unpublished information in 



