Chapter 7 



Subtidal Environments and Ecologx; of 

 Eneioetak Atoll 



PATRICK L. COLIN 



Motupore Island Research Station. Universittj of Papua 

 New Guinea. Port Moresbi>. Papua New Guinea 



INTRODUCTION 



The area of subtidal marine environments of Enewetak 

 far exceeds the intertidal and terrestrial habitats. Subtidal 

 environments are the lagoon and outer reefs and the pas- 

 sages between them which arc submerged at low tides. 

 The lagoon and outer reefs are separated, except at 

 passes, by the intertidal reef flat. Although closely con- 

 nected to the subtidal habitat, the intertidal habitat of 

 Enewetak is not discussed in this chapter except as it 

 relates to circulation, energetics, and processes in the sub- 

 tidal environment. With an area of about 930 km , the 

 lagoon is more than 15 times the area of the intertidal reef 

 flat and 140 times the area of the islands. 



The Enewetak Lagoon is relatively deep by atoll stan- 

 dards (Wiens, 1962), with a mean depth of 48 m and a 

 maximum depth of 64 m. Only about 10% of the lagoon is 

 shallower than 18 m, and only 20% is less than 32 m 

 (Emery et al., 1954). The open waters of the lagoon are a 

 voluminous habitat of about 4.2 X 10^° m^ of water, 

 most of which overlies the deep portions of the lagoon. In 

 both area and volume, the lagoon is the largest subtidal 

 unit at Enewetak. For this chapter, an arbitrary depth of 

 30 m has been selected to distinguish between the "deep" 

 and "shallow" portions of the lagoon. 



The area of the reefs seaward of the reef flat and 

 islands has never been accurately determined. Based on 

 reef widths observed from the air compared to adjacent 

 reef flat widths, the area of the seaward reefs is certainly 

 less than that of the reef flat, perhaps by a factor of up to 

 3 or 4, but an accurate determination is not presently pos- 

 sible. 



The outer slope of the atoll is quite different from the 

 lagoon. The present discussion will include descriptive 

 information on the outer slope to 300 to 400 m depth, 

 but below those depths there is little detailed information 

 concerning the biological communities or geological per- 

 spectives. 



The subtidal environment of Enewetak contains a 

 number of units, divisible on the basis of location, physical 

 factors, substrate types, dominant organisms, and other 

 factors. Biological communities can be similarly identified. 

 The generally high diversity of marine organisms at 

 Enewetak increases the complexity in describing individual 

 communities and their relationship with the others. 



MARINE CONDITIONS 



Mean surface oceanic water temperatures at Enewetak 

 range between 27° and 29°C (Atkinson et al., 1981; 

 Coles et al., 1976), with March the coolest month and 

 August the warmest. Temperature extremes during any 

 one month vary about ± 1°C from the mean (Coles et al., 

 1976). Local conditions can alter these, with isolated tide 

 pools in the midday sun reaching the low 40s C. 



Typical temperature-depth profiles for the seaward 

 reefs of Enewetak during summer are shown in Fig. 1. 

 From the surface to about 125 m in depth, the tempera- 

 ture gradually decreases from 29° to about 25°C. A slight 

 thermocline begins at about 125 to 150 m, and it changes 

 from 25° to 20°C over a 30 to 40 m increase in depth. 

 At 220 m it is about 13°C with the temperature gradually 

 decreasing to about 9°C at 380 m. 



Atkinson et al. (1981) documented the isothermal and 

 isohaline nature of the lagoon water column with no more 

 than a 0.5°C variation in temperature and a 0.20 ppt 

 range in salinity. Almost without exception the shallow 

 waters of the open lagoon and ocean are ideal for eury- 

 thermal tropical organisms. The salinity of the lagoon is 

 essentially the same as that of the open ocean. Only in 

 areas of restricted circulation and shallow depth are tem- 

 peratures elevated significantly above genera! lagoon 

 water. 



The atoll is located in the North Equatorial Current 

 with a general westward movement of water past the atoll. 

 Currents observed on the windward (east) ocean reefs 

 agree with this generalized picture, even at depths of 100 

 to 200 m, with the ocean current seeming to split north 

 and south near the easternmost extension of the atoll at 

 Ananij. On the ocean side of Enewetak Island, the 

 alongshore component of the current varies in speed but 



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