ECOLOGY AND TAXONOMY OF Halimeda 247 



green algae. The extent of the measurements for the productivity of 

 Halimeda and the other calcareous green macrophytes is therefore small. 



1 . Productivity of Halimeda in laboratory culture 



Figure 81 illustrates the apparatus used for measuring the pro- 

 ductivity of separate populations of Halimeda, Thalassia and Penicillus 

 by gas exchange. Details of the method are given in Hillis-Colinvaux 

 (1974), but essentially the procedure is to select tanks of healthy- 

 looking plants that have been maintained in the laboratory for a period 

 of one or more months, to attach the fitted lid and apparatus to their 

 aquarium tank, to record oxygen tensions for the cycle of 12 hours 

 light and 12 hours dark to which they have been accustomed, and to 

 calibrate the oxygen probe with Winkler titrations at the beginning 

 and end of each run. Halimedae used were all the sand-dwelling incras- 

 sata in populations of about a dozen individuals, each approximately 

 7 cm tall (excluding the holdfast). This population density of about 220 

 plants m~^ is within the natural range for the species. Natural popula- 

 tions of 100 m~^ are common, and densities of nearly 500 m'"^ are 

 found in parts of the Glory Be reef (Section X). Energy provided was 

 white light at intensities between 320 and 600 ft-c. 



Under these conditions H. incrassata produced at the following 

 rates : 



net production 2-5 mg C per thallus per day, 



or 0-56 g C m-2 dr^ 



gross production 4-5 mg C per thallus per day, 



or 1-0 g C m~^ d~^. 



Several of the conditions of the laboratory measurements, however, 

 suggest these estimates are conservative. The thalli, at 7 cm, were small, 

 since many wild plants grow to 20 cm above the sand. This difference 

 may, however, be somewhat offset by the fact that the lower portions 

 of tall wild plants tend to be covered with epiphytes, or to have seg- 

 ments which are losing their green colour. Light intensities at noon in 

 the shallow areas where this species attains high populations are much 

 higher than those in our laboratory, and the plants are supplied with 

 flowing tidal water instead of recirculating water. 



Finally, it must be noted that laboratory measurements have not 

 been applied to the Halimedae that attain the highest cover on reefs, 

 the sprawling, rock-anchored forms like opuntia. On the Glory Be reef 

 opuntia populations may attain a cover of 90%, more than four times 



