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L. HILLIS-COLINVAUX 



capabilities of Halimedae, therefore, is necessary both for studies of 

 energy flux through tropical reef communities, and for studies of the 

 building of reefs themselves. 



The productivity of atolls, on which so many Halimedae grow, is a 

 particularly interesting subject since atolls are typically set in the 

 unproductive blue waters of the main ocean deserts. Atolls are without 

 the nutrients supplied by run-off from the land to coastal sites, nor are 

 they typically supplied with the nutrients of upwellings. This has made 

 their high productivity difficult to explain, and even now it is not 

 understood in detail, though the outlines are becoming clear. 



That the atoll marine system is not dependent on the nitrogen, 

 phosphorus and zooplankton transported into it from the impoverished 

 surrounding waters was demonstrated first by Sargent and Austin 

 (1949, 1954) for Rongelap Atoll in the northern Marshall Islands. 

 They established that this atoll was inwardly sustaining, using the 

 technique of flow-respirometry, a method subsequently employed by 

 Odum and Odum (1955) for their work on Enewetak Atoll which is 

 also in the Marshall Islands. This atoll too they found to be self- 

 supporting, and their figure for the gross productivity of a coral reef 

 community, together with a subsequent one obtained by Kohn and 

 Helfrich (1957), is a useful base-line calculation of the productivity of 

 tropical reefs since the technique measures community productivity 

 for the transect of measurement. These studies do not tell us, however, 

 how this high productivity is achieved, or the contribution of individual 

 reef components, like the populations of large algae. 



Answers to both these questions are essential to an understanding 

 of productivity of the whole reef and energy flow in the reef system, as 

 well as to reef modelling. Some appreciation of the first question, how 

 the high productivity is achieved, is provided by the realization that 

 nitrogen and possibly phosphorus are recycled within the system 

 (Johannes et al., 1972), the presence of some such pattern being expressed 

 earlier in the review of coral reefs by Yonge (1963). The 1972 report 

 is part of the recent work of the SYMBIOS team that studied essen- 

 tially the same part of the Enewetak reef as did the Odums, as well as a 

 more northerly transect. Some details of the results of this team effort 

 are still being published, but what is available adds strongly to the 

 evidence that atolls may be nearly closed systems. They fix their own 

 energy and cycle their own nutrients, are fertile islands in an infertile 

 sea. In this respect coral atolls have to the ecologist something of the 

 impact of lowland tropical rain forest, because this too supports itself 

 on nutrient-poor substrate through very efficient nutrient retrieval 

 systems. Eor the rain forests we have a faMy clear idea how nutrient 



