248 L. HILLIS-COLrNrVAUX 



the cover of sand- dwelling forms like incrassata. The highest estimate 

 for productivity of Halimeda populations given by Hillis- Colin vaux 

 (1974) is arrived at by extrapolating the incrassata measurements to 

 populations growing at the density of opuntia populations. By this 

 procedure maximum productivities of 4-lgCm"~2d~^ (gross) and 

 2-3 g C m^^ d~^ (net) are attained. These estimates for opuntia popula- 

 tions cannot as yet be confirmed in the laboratory because of the 

 difficulty of maintaining the rock-attached forms of Halimeda in 

 culture. 



2. Contribution of Halimeda to carbon production in reefs 



A rough estimate of the contribution of Halimeda to the carbon flux 

 of an entire reef can be obtained by applying the above laboratory 

 results on the productivity of individual Halimeda thalli to population 

 data of the only reef for which we have a Halimeda census, the Glory Be 

 reef on the north shore of Jamaica. Glory Be is the name given to a 

 short section of the Jamaican fringing reef occupying a well-defined bay 

 separated by headlands (Hillis-Colinvaux, 1972, 1974). The reef is 

 described in Section X.A and illustrated in Figs 89-96. Percentage 

 cover of Halimeda, by species, was estimated for each section of the 

 reef from the shoreline to the region of the Acropora palmata corals 

 and the buttresses. By using the cover data (Table XXI) and assuming 

 that all species of Halimeda produce equally well regardless of depth or 

 microhabitat, we can compute the contributions of the Halimeda 

 populations to the total carbon flux of the reef. 



Table XXI gives the results of applying the laboratory productivity 

 data to the census of Glory Be reef. The total daily net production, from 

 the region of the shallowest-growing Halimedae to the outer channel, 

 an area of 9840 m^, which supports 1 110 300 Halimeda plants (calcu- 

 lated from Tables XXI and XXXIII) is 



-1 



2779 g C d-i or 1014 kg C yr 



yielding a range of net productivities for the reef, where estimated 

 Halimeda cover was 10% or higher, of 



0-31-2-25 g C m-2 d-i or 113-821 g C m-2 yr-i (Table XXI). 



The lower figures of this range were at sites where plants other than 

 Halimeda predominated, or where there was grazing by urchins or fish, 

 with one of the lowest figures in dense stands of Thalassia. Total 

 macrophyte productivity in these stands is, of course, very large. The 

 vertical sides of coral rocks are excluded from the range given, because 



