310 L. HILLIS-COLINVAUX 



At least 14 species are present, of which macro'physa, opuntia, 

 cylindracea, gigas, distorta, copiosa and lacunalis f. lata appear to be the 

 commonest (Hillis-Colinvaux, 1977). These, then, are the species which 

 would contribute most to the atoll's primary productivity and calcium 

 carbonate budgets. 



Except for one collection, we have no information of the species, 

 their species richness or biomass, seaward of the algal ridge. The high 

 cover they provided at this one site, approximately 15% of a spur, 

 presents a marked contrast to the spurs of Jamaica where Halimedae 

 are rare. 



It is likely that the Halimeda populations of the sides of the 

 pinnacles are the prime producers of the Halimeda segments incorporated 

 into the reef mass and thus are prime builders of the atoll. There are 

 few Halimedae on the lagoon floor, and the lagoon floor, therefore, is 

 not a prime source of Halimeda sediment. 



There are no sea grasses at Enewetak, and this appears to have 

 important consequences for Halimeda populations in unconsolidated 

 sediments. Some of the densest sand-growing populations of Halimeda 

 elsewhere are in association with sea-grass stands, and these dense 

 stands are therefore not found at Enewetak. Communities of Halimeda 

 and other algae do not seem to be able to replace sea grasses on sand 

 substrates of the shallows. Whatever cause keeps sea grasses from 

 Enewetak probably lowers the contribution of the lagoon shallows to 

 the carbonate flux of the atoll. 



XI. Acknowledgements 



My work on this paper has taken more than a decade. Very many people 

 helped, not all of whom can be listed below. I am grateful to them all. Research 

 in the field has needed the large financial support inseparable from work in 

 remote places, and I list all grant support below. I particularly want to give 

 my thanks to the United States Ofl&ce of Naval Research, without whose 

 enlightened support of a female planning to dive on coral reefs more than a 

 dozen years ago, much of the framework of this research would not have 

 been possible. 



I began writing the paper at the British Museum (Natural History) where 

 I held the Founder's Fellowship of the American Association of University 

 Women. I am particularly grateful for this vital and timely support, and to 

 Mr R. Ross, then Keeper of Botany of the British Museum (Natural History), 

 and my colleagues there, for the privileges and pleasure associated with the 

 Fellowship year, and for courtesies and assistance extended to me on other 

 visits as well. 



For the opportunity to examine Halimeda with the Ellis Aquatic 

 Microscope (c. 1752) and other contemporary microscopes, I thank Mr F. W. 



