30 



COLIN 



Fig. 3 Aerial view of the wide, south channel passage to the lagoon looking from Enewetak 

 Island (lower right) to Ikuren Island (upper left). The shallow bottom of the sill at the passage 

 is visible. [Photo by P. L. CoHn.] 



shoaling nature of the bottom at the wide passage, 

 produces rough conditions with standing waves and steep 

 waves in the western half of the wide passage. 



A series of shallow open passes with fingers of emer- 

 gent to near emergent reef intersjjersed between them is 

 called the "southwest passage," an additional passage 

 between the lagoon and ocean. These openings cover 

 about 6.7 km of the atoll margin from the island of Biken 

 to the beginning of unbroken shallow reef to the southeast. 

 The sand-bottomed passes appear deeper to the south — as 

 much as 8 m deep in places. While significant, the 

 southwest passage is p>erhaps an order of magnitude less 

 important in lagoon-ocean water transport than the deep 

 and wide channels (Chapter 5 of this volume). 



The reef flat is also a major source of water movement 

 into or out of the lagoon. The amount of such transport is 

 dependent on the height of the tide and the wind and 

 waves which influence the wave pumping of water from 

 ocean to lagoon. Where islands disrupt the free flow of 

 water across the reef flat into the lagoon, water flow is 

 channeled into narrow, deeper areas where current speed 

 can be relatively high. These channels are variously termed 

 "rips" or "gutters" and can also occur on intraisland reef 

 flats where there are areas of higher current flow. 



The biological communities and environments of the 

 lagoon are discussed in Chapters 7 and 8 of this volume. 

 They are quite variable from place to place, varying from 

 sediment-bottomed areas devoid of hard substratum to 

 well-developed coral reefs. The diversity of plants and 

 animals is as high in the lagoon as it is in other areas of 

 the marine environment. 



Emery et al. (1954) reported over 2000 "coral knolls" 

 in the lagoon with some suggestion that they "belong to 2 

 distinct size categories; nearly all the large coral knolls 

 have a diameter in excess of 1 mi whereas nearly all the 

 rest are smaller than Vi mi, and intermediate sizes are not 

 common." Most of these do not reach sufficiently close to 

 the surface to be visible and can be detected only by echo 

 sounding. Emery et al. (1954) distinguished between the 

 term "coral pinnacle" and "coral knoll," preferring the 

 latter term, but did not clarify how the reef structures of 

 the lagoon margin were considered. In essence an inter- 

 grading series of reef structures exists within the lagoon. 

 Although distinct types — such as coral knolls (broad, rela- 

 tively low structures), coral pinnacles (high relief relative to 

 diameter), and patch reefs (small structures, often in sheil- 

 low water) — can be identified, intermediates are common. 

 Those reef structures that are present on the bottom and 

 visible from the air are generally, in this treatment, con- 

 sidered to be "patch reefs." 



The Reef Flat 



The shallow reef flat, much of which is emergent at 

 low tides, around the rim of the atoll has been the most 

 intensively examined marine environment. It consists of 

 areas of rock pavement with seaward algal ridge structures 

 and lagoonward rubbly bottom. The reef flat varies consid- 

 erably in different areas of the atoll, particularly between 

 the windward and leeward sides but also over relatively 

 short distances on the windward shore. Very little of the 

 algal ridge, normally produced by coralline algae, is "live" 

 at Enewetak. Instead of the healthy pink corraline areas 



