284 L. HILLIS-COLINVAUX 



plants of opuntia and goreauii. The census occupied about 30 hours of 

 bottom time. 



In order to compute carbon and carbonate fluxes for the various 

 portions of the reef it was necessary to convert our laboratory measure- 

 ments on incrassata populations from fluxes per thallus to fluxes per 

 unit of cover. To arrive at a conversion factor we estimated the per- 

 centage cover of incrassata populations of known density. A density of 

 500 incrassata m~^ covers 50%, so 100% cover is taken as the equivalent 

 of 1000 thalli m~2. The crudeness of this conversion for opuntia is self- 

 evident, and it may be that the close-packed branches of a 90% cover 

 stand of opuntia produce considerably more than would 900 incrassata 

 plants. Without direct measures of the productivities of sprawling 

 forms, however, there seems to be no way of making a more accurate 

 estimate. 



2. Productivities at Glory Be 



The productivity of the parts of this reef has been discussed in 

 Section IX, and the principal data are given in Tables XXI and 

 XXXIII, with some of the results of the census being included in the 

 latter. Conversion factors for computing productivities from Halimeda 

 census data were derived from the Glory Be survey and the associated 

 laboratory work and are given in Table XXXII. 



3, Narrative description of Glory Be reef 



At least ten species of Halimeda live in the reefs of the north shore 

 of Jamaica, six of which {opuntia, goreauii, gracilis, tuna, incrassata and 

 simulans) grow with varying success over much of the depth range of 

 approximately —0-3 m to — 60 m (Fig. 90; Section IV), or to the start 

 of the deep fore-reef. None is regularly exposed at low tide. Of the 

 remaining species, monile appears to be restricted to shallow water, and 

 discoidea is limited essentially to the fore-reef and fore-reef slope 

 (although it grows in shallow water elsewhere in the Caribbean). The 

 thalli of discoidea sometimes appear to be growing in sand, but on 

 closer examination they are invariably attached to some rock outcrop 

 concealed by a thinnish veneer of sand. The species copiosa and cryptica 

 are restricted mostly to the fore -reef slope and upper part of the deep 

 fore-reef, that is, to depths of approximately 25-100 m. Halimeda 

 copiosa, however, does occur in shallower water in the Pacific (see 

 below and Section IV). These two species are the only ones to establish 

 sizeable populations on the deep fore-reef (Goreau and Goreau, 1973), 

 with cryptica being the more abundant. 



