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L. HILLIS-COLINVAUX 



1954; Odum and Odum, 1955; Odum, 1957; Wells, 1977), the most 

 elaborate being that of Wells. But none of these attempts have been 

 directed to Halimeda or, indeed, to other large tropical macrophytes. 

 They have measured gas exchange in encrusting or micro -communities 

 of sand only. Johnston (1969) cut fresh discs from the non-calcareous 

 siphonaceous alga Caulerpa in the field and measured the productivity 

 of these discs by the standard light and dark bottle technique. Both 



Fig. 81. Apparatus for measuring productivity of macrophytes growing in laboratory 

 aquaria, by monitoring changes in oxygen production occurring within the aquarium 

 over 36 hours or more. The aquarium with established, growing, clean plants is 

 filled with water, and sealed with a thick plexiglass lid machined to fit, which in 

 addition is held firmly in place by an 0-ring seal and turnbuckles. The lid is designed 

 with a number of entrance ports. A peristaltic pump, left front, mamtains a water 

 flow of approximately 0-3 m s"^ past the oxygen probe. Other equipment items of 

 the systein are an oxygen nieter (top right) and recorder (top left). The aquarium 

 in the photograph contains 40 Penicillus capitatus thalli, but production in Halimeda 

 was measured in the same way. 



Marsh (1970) and Hillis-Colinvaux (1974) established populations in 

 aquaria which could then be converted into closed circulating-water 

 systems (Fig. 81), with exchanges in oxygen production being monitored 

 for a number of hours. Both authors manipulated environmental 

 parameters to measure effects on photosynthesis. Marsh for calcareous 

 red algae, and Hillis-Colinvaux for Halimeda and other calcareous 



