SUBTIDAL ENVIRONMENTS AND ECOLOGY 



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ceans, mysids, euphausiids, amphipods, siphonophores, 

 pteropods, dinoflagellates, medusae, other planktonic Crus- 

 tacea and larval forms were found. One station, near the 

 Enewetak-Medren reef flat had lower abundance, fewer 

 species of typically planktonic organisms, and more mero- 

 planktonic and benthic forms than the mid-lagoon station. 

 There was considerable variation in densities of zoo- 

 plankters among samples taken during the same study 

 periods of a few to several weeks at the mid-lagoon sta- 

 tion. 



Gerber and Marshall (1982) reported that lagoon con- 

 centrations of copepods, pteropods, and larvaceans were 

 higher during their summer sampling period. Phytoplank- 

 ton biomass in mid-lagoon in summer was also about twice 

 that of the winter. Individual components of the zooplank- 

 ton changed their densities considerably during periods of 

 several weeks during the summer. Copepods and larva- 

 ceans increased 1.5 to 3 times. Pteropods increased 20 

 times in 4 weeks, then declined rapidly. 



Coles and Strathmann (1973) collected mucus floes 

 from the water column at Enewetak and other areas and 

 found them to represent substantial quantities of organic 

 matter when compared to particulate organic material in 

 the water. They noted that under calm conditions few 

 mucus floes were seen in the water at Enewetak, but after 

 a storm abundant large floes were seen passing into the 

 lagoon. 



ASPECTS OF MARINE COMMUNITIES 

 AT ENEWETAK 



Reef Growth and Destruction 



Coral reef growth is a balance of factors: the accretion 

 of calcium carbonate by stony corals and other calcifying 

 organisms in addition to the consolidation of these materi- 

 als into a cohesive structure versus the erosive effects of 

 grazing pressure, physical weakening, and destruction of 

 the reef structure. Much work at Enewetak has focused 

 on questions related to the growth and maintenance of 

 reefs. Not all is summarized here but some environmental 

 factors concerning reef growth are. 



Calcification of corals and other organisms can be 

 affected by environmental conditions, such as light, tem- 

 perature, and water movement over the range of condi- 

 tions under which the organism ca.i survive. Stony corals 

 are also known to "compete," albeit in a relatively slow 

 manner. Methods include overgrowth, reducing the light 

 necessary for calcification and growth of competitors, and 

 by "extracoelentric digestion" in which mesentarial fila- 

 ments are extended to "attack" and kill tissue of other 

 species growing close by. 



The range of conditions inhabi*3d by a single species 

 or genera of corals is often broad. The genus Pocillopora is 

 illustrative. Stimson (1978) reports that the Pocillopora 

 species at Enewetak occur over a broad range of depths 

 but are most abundant on reef flats and in water <5 m 

 deep with currents. He reported P. verrucosa to reach 



15 m depth on pinnacles and windward and leeward reef 

 slopes and to occur in the "small head zone" (Odum and 

 Odum, 1955) north of Japtan. 



Pocillopora uerrucosa is also common in spur and 

 groove areas of the windward reefs. In eastern Australia 

 the species is found in areas of regular water movement 

 and good illumination, and its growth variations are less 

 diverse than those of P. damicornis (Veron and Pichon, 

 1976). Pocillopora damicornis occupies potentially a 

 greater range of habitats than any other coral at Enewetak. 

 Veron and Pichon (1976) have figured the wide variation 

 in corallum morphology and documented the broad range 

 of conditions this species inhabits in eastern Australia. 

 Pocillopora eudoxt^i occurs deeper than any other branch- 

 ing coral at Enewetak, to approximately 60 m on the sea- 

 ward slope. 



Members of Acropora are similar. Some are limited to 

 very shallow water. Stimson (1978) found A. aspera and 

 A. humilis only in water less than 2 m deep. Acropora digi- 

 tifera and A. aspera are sometimes exposed and killed by 

 extreme low tides. Others, such as A. s^ringoides, are re- 

 stricted to water deeper than 5 m. Acropora s\;ringoides is 

 abundant on the flanks of patch reefs and pinnacles near 

 Enewetak and Medren. Other species have broad depth 

 distributions. Acropora hyacinthus and A. nasuta occur 

 from 1 to 20 m depth. 



Coral growth rates have been examined for a number 

 of species of stony corals at Enewetak. The technique of 

 x radiography of slabbed coral specimens was first applied 

 to Enewetak coral specimens and used to verify the annual 

 nature of the density banding observed (Knutson et al., 

 1972). Autoradiographic exposures of coral slabs show dis- 

 tinct bands of activity from atomic test series and, there- 

 fore, serve as bench marks in coral growth chronology. 

 Knutson et al. (1972) also presented evidence that the 

 high density bands seen were formed during the rainy 

 season at Enewetak. Buddemeier et al. (1974) examined 

 skeletal growth rates of 15 species of corals, including the 

 same species from various locations at Enewetak. They 

 reported growth rates of generally 4 to 12 mm per year 

 with some exceptions above and below these figures. Not 

 all coral species examined showed variation in growth 

 rates with depth. Porites lutea did show a negative correla- 

 tion between growth rate and increasing depth, with about 

 one-half the rate at 25 to 30 m as was at 4 to 10 m 

 depth. However, colonies of Goniastrea sp. collected in 

 deep water grew as fast as those from shallow water. 



Although Buddemeier et al. (1974) focused attention 

 on obtaining large symmetric head corals, no specimens 

 examined indicated ages before 1952 and 1953. Whether 

 the nuclear tests of 1952, particularly the "Mike" test, had 

 any effect on this is uncertain. 



Stimson and Polacheck (MPRL, 1977) reported that 

 growth rates of Acropora and Pocillopora at four different 

 depths from 1 to 15 m on lagoon pinnacles and patch 

 reefs were statistically indistinguishable. Three species of 

 common shallow water Acropora had annual increments in 



