274 L. HILLIS-COLINVAUX 



Table XXXII. Conversion Factors Computing Productivities from 



Halimeda Census Data 



Carbonate production = 0-8 g carbonate thallus"^ yr~^ 

 Productivity (net) — 2-5 mg C thallus"^ d~^ 



1% cover (incrassata) = 10 thalli m~^ 

 Turnover time is ^3 generations yr~i 



area, opuntia segments are heavier (mean weight of CaCOg 7-75 mg) 

 than incrassata segments, and goreauii segments are lighter (mean 

 1-38 mg). A single specimen ot goreauii in the laboratory collection had 

 175 segments and a single specimen of opuntia 626. An opuntia plant, 

 however, covers more space than an incrassata plant. With these great 

 uncertainties, it seems reasonable to base a rough estimate on convert- 

 ing cover data to incrassata thalli and proceeding on the assumption 

 that each thallus contributes the computed 0-8 g CaCOg yr~i to the reef. 

 The estimated reef population of 1 110 300 Halimeda plants then 

 contributes 888 kg of carbonate per year, or approximately 90 g m~^ 

 yr~^ (area 9840 m^, Table XXI). The equivalent rate of sedimentation, 

 assuming 60% porosity as did Stockman et at. (1967), is ,4066 mm 30""^. 

 This was on a reef where the average Halimeda cover was 10%, 

 although opuntia populations of shallow rocks reached local densities 

 of 90% cover. 



(d) Comparison with other calculations and possibilities of error. 

 Neumann and Land (1975) made similar calculations for the whole 

 Bight of Abaco in the Bahamas, a shallow basin of 2750 km^. With an 

 average density of Halimeda plants of 25 m"^ they estimated the 

 standing crop of the whole Bight of Abaco to produce 25-47 x 10** g 

 carbonate. If we assume one crop a year, as was done above, this gives a 

 carbonate flux of 9-26 g m"^ yr~^, or one-tenth of the estimate for the 

 Glory Be reef. The discrepancy is undoubtedly due to the different 

 Halimeda densities. A density of Halimeda of only 25 plants m"^ is low 

 unless it is a species like macroloba. Sand-dwellers may grow at densities 

 20 times this, and higher densities still are represented by the sprawling 

 thalli oi opuntia plants growing on rocks at densities of 90% cover. The 

 figures actually used for the Bight of Abaco work are in fact several 

 times larger than the 9-26 g m~2 yi--i calculated from the Abaco data 

 here because it was assumed that there would be 6-12 crops oi Halimeda 

 per year. As is argued above, there is no evidence for such rapid turn- 

 over in Halimeda. 



