ECOLOGY AND TAXONOMY OF HaUmeda 251 



0-2% of the unproductive world ocean (calculation based on reef area 

 information in Smith (1978)), or about twice the space Ryther (1969) 

 assigned to regions of up welling. Since Ryther was able to show that 

 upwelled areas produce about half the world's fish supply (he did not 

 specifically include coral reefs in his calculations), the primary pro- 

 ductivity of tropical reefs, to which HaUmeda contributes significantly, 

 is an important world resource. 



3. Halimeda productivity compared with other reef production 



In Table XXIII the Halimeda estimates are compared with other 

 reef productivity data. Halimeda productivity appears to be of the 

 same order of magnitude as that of Thalassia and species of intertidal 

 Cyanophyta. The discrepancy with Caulerpa may be accounted for, at 

 least partly, by differences in technique, particularly the cutting of 

 discs from the siphonaceous Caulerpa thallus which would inevitably 

 produce some loss and disruption of filament contents. More extensive 

 comparisons must be made with caution since the data have been 

 obtained by a variety of procedures, including different light inten- 

 sities, and the impact of light intensity has not been fully evaluated. 

 They have not been extrapolated to an annual basis because the per- 

 tinent standing crop data are sparse. 



Except where there are extensive beds of Thalassia, Halimeda is 

 usually one of the most obvious plants in a coral reef or atoll. In spite of 

 the roughness of the data in Table XXIII, if seems apparent that 

 Halimeda may well be as productive as other primary producers in the 

 reef. Some of the estimates for whole-reef metabolism of parts of the 

 reef where there are few to no Halimeda, like those of Odum and Odum 

 (1955), appear larger than the estimate for Halimeda based on labora- 

 tory culture, but there seems good reason to believe that the Halimeda 

 estimates are conservative. It must be of interest to obtain direct 

 measures of the productivity of Halimedae as they grow on reefs. 



It remains true that the Glory Be reef is the only reef for which we 

 have a Halimeda census, and that on a solitary occasion. For no reef 

 have we recurrent census descriptions of the standing crops of Halimeda 

 at different times of the year. And there are apparently no measure- 

 ments at all on the production of wild populations of Halimeda by gas 

 exchange, cropping or incremental harvest. Until these measurements 

 are made it will not be possible to state accurately the contribution of 

 Halimeda to the remarkably high productivity of coral reefs. If the 

 productivity of Halimeda in the wild is as high as these provisional data 

 suggest, however, there must be interesting questions to be asked about 



