1.4 A Study of the Benthic Communities of 



Caroline Atoll (Line Islands, Pacific Ocean) 



BORIS I. SIRENKO and VLADIMIR M. KOLTUN 



Zoological Institute, USSR Academy of Sciences, Leningrad, USSR 



Introduction 



The study of coral atolls, which constitute one of the most 

 highly productive biological systems in oligotrophic tropical 

 waters, is of considerable theoretical and practical value. On 

 one hand, rapid human population growth over the past few 

 decades, with the concomitant shortage of protein-rich foods, 

 has driven a never-ending search for new protein sources. The 

 study of coral reefs as highly productive biological systems 

 would be helpful for establishing marine farming facilities. On 

 the other hand, burgeoning industrialization and the increasingly 

 intensive use of all natural systems has caused considerable 

 damage to coral reefs (Gomez & Yap, 1985). There is, 

 therefore, an urgent need for coral reef monitoring. Uninhabited 

 Caroline Atoll, situated far from principal sea routes, would be 

 considered an excellent monitoring site. With this in mind, we 

 spent a week studying the status of the coral reefs and surveying 

 the benthic communities around South Island and the Southern 

 Leeward Islands ( Kepler e t al. , Subchapter 1 . 1 , this volume ) in 

 Caroline Atoll (Fig. 1 ). Regrettably, time constraints did not 

 permit a study of sufficient scope and depth. The present paper 

 is therefore limited to a general description of benthos 

 distribution in the accessible portions of the reef and to an 

 account of a uniquely interesting reef situated within the atoll 

 lagoon. 



At low tide, the Caroline Atoll Lagoon is linked to the 

 ocean by shallow passes of no greater than 0.5 m in depth. 

 Several narrow intralagoonal reefs subdivide the central lagoon 

 at low tide. The depth of the lagoon does not exceed 10 m. The 

 largely sandy bottom includes isolated patches of fragile coral 

 colonies, mostly Acropora. 



With few exceptions, the outer side of the reef near the 

 southern islands has the classical structure of most coral reefs 

 (Preobrazhenskiy, 1986). The narrow shallow-water lagoon 

 facing the islands gradually becomes a reef flat cemented by 

 encrusted calcareous algae. This is rather extensive, averaging 

 562 m: range 396-759, N = 100 sites (Kepler et al.. 

 Subchapter 1.1, this volume). The reef Oat is surmounted by 

 widely spread limestone coral knolls or "coral heads." Further 

 seaward, the reef flat, with the usual channels and overhanging 

 ledges, breaks off. At a depth of 5-6 m it becomes a buttress 

 zone (i.e., a radically crosscut sloping terrace consisting of 

 individual spurs or benches that become narrower in the 

 oceanward direction I. Ii is the latter zone that constitutes the 

 main portion of the reef, with its abundant growth of corals 

 (Acropora, Pocillipora, etc. ). It is this growth that accounts for 

 the origin and continued development of the reef and the entire 

 coral atoll. It was the status of the coral settlements in the 



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3 / ^ / 



4 / rX-* ' Acropora-Tridacna Reef 



5 (QA * 



Fig. 1. Location of the Acropora-Tridacna reef and of hydrobiological 

 section I-I of Caroline Atoll. 



buttress zone that afforded an indication of the "health" of the 

 reef. Our studies showed that not less than 50^ of the surface 

 area of the outer slope of the Caroline Atoll reef was covered 

 by living coral colonies. Assessment on this basis, using the 

 method of cross sections and areas (Gomez & Yap, 1985), 

 showed the condition of the reef to be sound. The notorious 

 "crown of thorns" starfish (Acanthaster planet), responsible 

 for the devastation of reefs in other parts of the Pacific Ocean, 

 was seen only once. 



The littoral zone of the lagoon facing South Island was 

 sandy. The sand was filled with holes made by burrowing 

 PolychaetaandBalanglossi. Also present, sometimes in clusters, 

 were the mollusks Cerithium columna and Ccrithiuni sp., a few 

 Calappa sp. crabs, and gastropod mollusksAfe/ar/iape undulata 

 and Nerita plicata, the latter settling by water's edge on trees 

 whose branches dip all the way down to the water. 



166 



