SUBTIDAL ENVIRONMENTS AND ECOLOGY 



109 



Passes 



There are three passes from ocean to lagoon with suffi- 

 cient water depth for boats to regularly traverse them. 

 They are the "deep passage" (east) between Medren and 

 Japtan, the "wide passage" (south) between Enewetak and 

 Igurin, and the "southwest passage" between Kidrenen 

 (south) and Biken. Various details of these passes have 

 been discussed in Chapter 3, this volume. 



The biological communities of the deep channel and its 

 margins have not been well described. Hobson and Chess 

 (1978) discussed the patch reefs and plankton communities 

 in the area between Japtan and Jedrol Islands which are 

 affected by currents coursing through the deep channel, 

 but their study site was not in the deep channel proper. 



The northern side of the deep channel slop>es steeply 

 from depths of only a few meters. To the east of Jedrol 

 there is actually a "barrier" reef awash at low water which 

 is constantly exposed to oceanic swells entering the lagoon 

 through the deep channel. The northern slope of the deep 

 channel to depths of 30 to 40 m is a nearly 45° angle 

 rocky slope with abundant corals and reef-associated inver- 

 tebrates. At depths of 25 to 40 m, the bottom levels and 

 the central portions of the channel are probably relatively 

 flat. There is a downslope sediment transport along this 

 face, and below 30 m where the bottom begins to level, 

 sediments also begin to dominate the bottom compared to 

 exposed rock outcrops. 



The easternmost extension of the shallow wedge where 

 the channel splits is distinct, the "cutting edge" being only 

 a few meters wide and descending at about a 45^ angle 

 from 6 m to depths below 30 m. The coral communities of 

 the shallow reef and slope are rich. The fish communities 

 of the north side of the channel are diverse and abundant 

 with zooplanktivores more dominant than in other areas. 



The south side of the deep channel is different from 

 the north, with the bottom sloping gradually as a sediment 

 slope with little or no exposed rocky substratum. A shelf 

 between 30 and 36 m in depth extends a kilometer or 

 more northwest from Medren into the lagoon. 



Little is known about the area of the wide channel. 

 Aerial photographs show large patch reefs on a sandy 

 bottom scattered across the entire 9.3 km width. The crest 

 and outer slope of the sill was examined about 1.6 km 

 west of Enewetak and had large, rocky patch reefs, not 

 unlike large lagoon margin patch reefs at 18 to 20 m 

 depth (Fig. 13). The patch reefs had relatively little live 

 coral but had abundant Halimeda spp. and Asparagopsis 

 taxiformis. The most common corals were Pocilhpora spp. 

 The sediment was coarse, dominated by Halimeda. with 

 small ripples at 22 m depth. There were small rocks 

 between the much larger reefs but little grew on them. To 

 seaward, the sediment bottom sloped perceptively. At 

 30 m, it was nearly all sediments with only a few rock 

 patches and sloped at an angle of about 15° (Fig. 13). 

 Below that depth, the slope increased to about 20° at 

 40 m and more with increasing depth. 



The southeast passage consists of sandy channels 

 between elevated fingers of reef for 6.5 km southeast of 



Biken. Atkinson et al. (1981) estimated the cross-sectional 

 area of the southwest passage as only 26% of the deep 

 passage and 6% of the wide passage with no net inflow or 

 outflow. The reef fingers have well-developed coral com- 

 munities which do not differ greatly from the interisland 



Fig. 13 Views of the bottom, wiae ^soutn; passage, 

 Enewetak Atoll. Upper: Rubble substratum at about 20 m 

 depth looking downslope. Middle: Juncture of rubble and sand 

 substratum at 30 m depth, looking downslope. Lower Sand 

 slope substratum with isolated coral l>oulders at 40 m depth. 

 There is considerable evidence of downslope transport of sedi- 

 ment in this view. 



