Chapter 8 



Intertidal Ecologv; of Enewetak Atoll 



ALAN J. KOHN 



Department of Zoology. (Jniuersify of Washington 

 Seattle. Washington 98195 



INTRODUCTION: THE INTERTIDAL 

 ENVIRONMENT 



At an atoll, land and sea meet at a precarious, 

 dynamic interface that often bears evidence of horizontal 

 and vertical movements over past centuries and millennia. 

 In the shorter-term dynamics of ecological time, the inter- 

 tidal region of an atoll exposes the plants and animals that 

 would earn their livelihood there to a particularly severe 

 physical environment. 



The vertical tide range on the shores of small oceanic 

 islands is narrow. At Enewetak, it averages 0.8 m, and the 

 spring tide range is 1.2 m. Over the 18-year period 1952 

 to 1969, the highest recorded tide was +1.8 m and the 

 lowest was —0.1 m, relative to prior lowest low water. 

 Despite this small range, many events within the intertidal 

 region differ markedly from those occurring below datum 

 and supratidally. Few marine organisms can tolerate the 

 supratidal fringe, wetted only by rain and sea spray and 

 subject to intense heat for long periods. Here physical fac- 

 tors profoundly affect the nature of the substrate. Within 

 the intertidal zone, conditions are less severe but still 

 stringent; substrate temperatures commonly reach 38°C 

 (Wiebe, Johannes, and Webb, 1975). Dissolution of lime- 

 stone reef rock substrate by rain and biological de- 

 struction by boring organisms are important processes 

 (Tracey and Ladd, 1974). All intertidal habitats are subject 

 to strong insolation and rain at low tide, and windward 

 intertidal environments bear the brunt of heavy wave 

 action at high tide. At the reef rim, coralline algae grow 

 and accumulate calcium carbonate rapidly. Their growth 

 also cements detritus and rubble into reef rock (Smith and 

 Harrison, 1977; Tracey and Ladd, 1974). 



This chapter is dedicated to the memory of the late 

 Paul J. Leviten. 



This chapter summarizes the present state of 

 knowledge of the ecology of intertidal environments at 

 Enewetak. It emphasizes the windward, seaward reef plat- 

 forms for several reasons: 



1. They have been the most thoroughly studied, both 

 geologically and at several trophic levels of the ecosystem. 



2. They represent a habitat type of widespread 

 occurrence throughout the tropical Indo-West Pacific (IWP) 

 region on both oceanic and continental islands and on con- 

 tinental shores (Kohn, 1971); data from Enewetak thus 

 provide a basis for comparisons with other IWP areas. 



3. They afford significant comparisons and contrasts 

 with adjacent subtidal coral reef habitats; in contrast to 

 such reefs they are physically harsh rather than equable, 

 and their topography is simple rather than complex (Kohn, 

 1971). 



In their pioneering study of the Enewetak coral reef 

 community, Odum and Odum (1955) characterized the 

 basic pattern of six physiographic zones comprising the 

 windward interisland reef 0.4 km north of Japtan (Muti) 

 Island, from the seaward edge toward the lagoon: 



1. Windward buttress zone. Spurand-groove or surge 

 channel-andbuttress zone, just seaward of the highest part 

 of the platform, the coral-algal ridge. This is the upper 

 portion of the inaccessible mare incognitum (Smith and 

 Harrison, 1977). 



2. Coral-algal ridge. "A low, narrow ill-defined strip of 

 limestone about 50 feet (16 m) wide" (Odum and Odum, 

 1955). Soft, fleshy algae (such as Dicti/osphaeria 

 intermedia, Lobophora uariegata, Ceramium, Dict];ota, and 

 Caulerpa elongata) cover the irregular surface, and the 

 crustose coralline alga Porolithon occurs in small patches. 

 Small clumps of corals (Acropora palmerae, Pocillopora. 

 Millepora p/atyphyZ/aj occur in protected sites. Algal ridges 

 are more extensive on island than interisland reefs. 



3. Encrusting zone. "The first 200 ft (66 m) down- 

 stream from the ridge is a high, gently sloping plateau that 

 at low spring tide is covered with only 6 in. 

 (115 cm) of water. It is relatively the smoothest area with 

 corals being either of a flat encrusting growth form or re- 

 stricted to low rounded 'heads' but little raised above the 

 general reef surface. The range between tops of heads and 



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