ECOLOGY AND TAXONOMY OF Holimeda 293 



Marine botany began with Taylor's (1950) survey as part of the 

 preparations for Operation Crossroads. Taylor's monograph is essen- 

 tially the results of many dredge hauls made as the requirements of 

 military logistics allowed. Dawson (1957) added to the algal collections 

 by diving, but the main effort at ecological marine botany by diving 

 was the work of Gilmartin (1960, 1966). In the first paper Gilmartin 

 described the lagoon bottom, and collections of algae at 21 stations, 

 comprising a complete east-west transect of the lagoon and reaching 

 to the maximum depth at about 65 m. It should be noted that the 

 sand-dwelling Halimeda found most commonly by Gilmartin and 

 described as monile is now properly known as cylindracea. The only 

 report on algae at Enewetak since Gilmartin appears to be that of 

 Hillis-Colinvaux (1977). 



1. Enewetak Atoll and the reef profile 



Enewetak Atoll consists of approximately 40 low islets on a broadly 

 elliptical reef surrounding a large and deep lagoon (Fig. 97). Prevailing 

 winds are important in shaping a reef (Yonge, 1951), and Ladd (1973) 

 describes Enewetak as a typical "rough-water reef" since it lies in the 

 belt of the north-east trade winds. Its windward side is consequently 

 subjected to steady wave attack for about 9 months of the year. Its 

 southern reefs are protected from the trade winds, but are periodically 

 damaged by powerful long-period swells from the southern hemisphere, 

 whereas the reefs of the west and north-west are exposed to relatively 

 calm seas. These differences in the physical environment of the portions 

 of reef bordering the small islets create an initial series of varied 

 macrohabitats, to which are added the different conditions presented 

 by the channels between the islets, and a lagoon of approximately 39 km 

 in diameter, about 65 m at its deepest, with more than 2000 coral knolls 

 or pinnacles (Fig. 83, bottom) and its own circulation system (von Arx, 

 1948). 



Basic regions and features of the reef of this atoll are shown in 

 profile in Fig. 98. Weather, accessibility of sites and work facilities 

 severely limit the number of studies that can be made on the seaward 

 portions of such reefs, so that we have few data of structure or com- 

 munities to windward of the reef ridge. 



The reef profile of Fig. 98 begins, therefore, just below the approxi- 

 mate depth (range c. — 14-5 m to —25 m) at which a pronounced 

 change of slope is known to occur. The region preceding it has been 

 frequently called the "ten-fathom terrace". But since the slope break 

 occurs over a range of many metres (Stoddart, 1969 ; Orme, 1977 ; Smith 



