ECOLOGY AND TAXONOMY OF Halimeda 275 



Stockman et al. (1967) made a similar budget for the production of 

 lime muds in Florida by Penicillus, concluding that Penicillus con- 

 tributed between 3 g and 25 g carbonate m.~^ yr^^. These results 

 depended on very low population densities of between two and eight 

 plants per square metre, but the passage of six generations a year. This 

 generation time is in accord with my experience of growing Penicillus 

 in laboratory culture, it having a shorter life-cycle than Halimeda. But 

 it is possible for Penicillus to exist at much higher densities than were 

 found in the Florida study, when the total Penicillus carbonate produc- 

 tion could equal that of the Halimedae of Glory Be reef. 



It is evident that calculation of the carbonate flux produced by 

 calcareous green algae is sensitive both to population density and turn- 

 over times. Population densities vary by two orders of magnitude, 

 with commensurate effects on calculated carbon flux. Errors in estimat- 

 ing turnover time ought to be reduced by the realization that Halimeda 

 is predominantly a long-lived alga. We need field data for the typical 

 length of life of the various species, but it seems likely that more than 

 one to three generations a year, depending somewhat on species, will 

 be found to be unusual. Some additional data are provided by the 

 observations of R. Spies, P. Lamberson and myself on an experimental 

 plot of cylindraceae in Enewatak Lagoon, in which over 70% of the 

 original thalli were present at the end of 4 months. Granted the errors 

 that can be introduced by population densities and length of life, the 

 agreement of the calculations of Chave et al. (100 g carbonate m~2 yr~i) 

 with the results from Glory Be is possibly fortuitous. 



(e) Carbonate production in whole reefs: contribution of Halimeda. In 

 the analysis of the mass of an atoll given above and in Fig. 84, it was 

 suggested that the growth of a reef should be considered in two parts : 

 the reef ridges and the lagoonal areas. In the former there is very active 

 growth of the coral assemblage and encrusting algae, in the latter there 

 is a large contribution of Halimeda and other green macrophytes. Smith 

 and Kinsey (1976) likewise divided reef systems into two components, 

 and further suggest that each has a characteristic modal rate of car- 

 bonate production. 



Smith and Kinsey (1976) give calculations of carbonate production 

 for the Enewetak reef ridge based on measures of the changing 

 alkalinity of the waters flowing over the reef. Their measures are thus 

 comparable to the measures of carbon fixation by fiow-respirometry, 

 and give a direct measure of carbonate accumulation on the section of 

 the reef accessible to the technique. Accumulation on the reef ridge is 

 given as 4 kg m"^ yr~^. This is contrasted with estimates for lagoons 



