INTERTIDAL ECOLOGY 



141 



TABLE 1 



Classification of Windward 

 Reef Platform Substrates 



Sand 



Extensive sand patches 



Sand in depressions in reef limestone 

 Sand under coral rock 

 Coral rubble with or without sand 



On flat bench surface 



In depression on bench 

 Very thin layer of sand on bench 



On flat bench surface 



In depression on bench 

 Limestone bench bare of sand or algal turf 



Smooth 



Rough 



In depression 

 Bare beachrock 

 Algal turf on reef limestone bench, typically binding sand 



On smooth bench surface 



On rough bench surface 



In depression on bench surface 

 Dead coral boulder 

 Crustose coralline algae 



The Quaternary record shows "at least four strati- 

 graphic intervals representing reef growth and associated 

 lagoonal sedimentation during relatively brief periods of 

 Quaternary interglacial high sea levels, overlying unconfor- 

 mities representing periods of emergence and weathering 

 during glacial lowering of sea level" (Tracey and Ladd, 

 1974). The uppermost unconformity, about 10 m below 

 present sea level, separates sedimentary rocks less than 

 6000 years old from sediments about 120,000 years old. 

 The limestone rock pavement constituting the present 

 extensive windward reef platform adjacent to Enewetak 

 Island (Fig. 2) is about 4000 years old and is a currently 

 developing unconformity (Tracey and Ladd, 1974; Bud- 

 demcier. Smith, and Kinzie, 1975). The latter authors 

 characterized the rock at and near the seaward algal ridge 

 crest as well lithified "poorly sorted coral rubble, coarse 

 sand, and obvious (?) coralline algae in a fine-grained cal- 

 careous matrix." The composition of this material suggests 

 that it was formed in a subtidal, sedimenting environment 

 very different from that at the site today. Geological and 

 radiocarbon studies of this material suggest the following 

 temporal scenario of the interplay of sea level changes and 

 biotic activities during the Holocene, leading to the inter- 

 tidal environments of Enewetak at the present cross sec- 

 tion of time: The sea was less than 3 m below its present 

 level 6000 years ago (ybp), rising to a maximum 3500 to 

 2000 ybp, a period of rapid reef growth. At 4000 ybp, 

 the age of the present surface rock when the corals shown 

 in Fig. 2 were living, sea level was probably about 0.3 m 

 higher than at present. From 2000 to 1000 ybp, emer- 

 gence and extensive surficial erosion of the reef accom- 

 panied a drop in sea level to its present level. 



Fig. 2 Reef limestone surface in the inshore 20 m of the 

 windward platform at Enewetak Island, showing corals in 

 growth position and the effects of subaerial, marine, and 

 biogenic erosion. The present level of the rock is too high in 

 the intertidal zone for coral growth. Dark spots in small 

 higher pits and in the pothole at lower right are limpets. 

 Siphonaria normalis. The lens cap is 52 mm in diameter. 



Shorter-term geochemical processes have been eluci- 

 dated in studies of calcification rates to be discussed 

 below. They show that the interisland windward reef plat- 

 form is accreting vertically at a rate of about 3 mm yr 

 (4 kg m~^ yr~^) in the absence of sea level change (Smith, 

 1973). If most of the products of calcification were 

 retained on the platform during the 1952 to 1970 period 

 in which there was no net sea level change, an estimated 

 shoaling of 5 cm would have occurred; if most net prod- 

 ucts of calcification were lost by transport to the lagoon, 

 no net change in platform level would have occurred. 



Calcification rates subtidally on the windward reef 

 slope are somewhat lower, about 1 to 2 kg m yr in 

 the buttress or spur-and-groove zone, but 3 to 6 kg m 

 yr^^ where coral cover is virtually total at depths of 15 to 

 25 m (Smith and Harrison, 1977). 



BIOLOGICAL PATTERNS AND 

 PROCESSES 



Primary Productivity 



The intertidal and shallow subtidal windward reef plat- 

 forms at Enewetak are highly productive, particularly 

 where covered by a dense algal turf. The mainly subtidal 

 interisland reef north of Muti Island, close to the site of the 

 original study by. Odum and Odum (1955) has been the 

 most intensively studied. Smith and Marsh (1973) mea- 

 sured primary productivity along two transects normal to 

 the seaward reef edge by two independent methods — rate 

 of oxygen production and rate of carbon dioxide 

 fixation — which gave results in close agreement. At Tran- 

 sect II (tr II; 340 m), the red crustose coralline alga 

 Porolithon onkodes and the brown alga Lobophora varie- 



