ECOLOGY AND TAXONOMY OF Holimeda 245 



The Halimedae of an atoll live in the deep water of the outer slopes, 

 in the shallows of the lagoonal coasts of the islets, in the reef passages, 

 on pinnacles and on the lagoon floor. It is at these sites that much of the 

 productivity of the reef system, both of carbon and of carbonate, must 

 occur, and at these sites we have very few measurements indeed. 

 Halimedae and other large algae occupy rather similar sites in fringing 

 and barrier reefs, and there again the measurements are few. 



Apart from work on coralline red algae like those of the reef front 

 (Marsh, 1970; Littler, 1973; Smith and Marsh, 1973; Connor and Adey, 

 1977), our measurements of the productivity of macrophytes of reefs 

 seem to be confined to a few on sea grasses (Odum, 1957; Westlake, 

 1963; Qasim and Bhattathiri, 1971; Patriquin, 1973, and a few on 

 Holimeda and other Caulerpales (Gessner and Hammer, 1960; Drew, 

 1966; Drew and Larkum, 1968; Johnston and Cook, 1968; Johnston, 

 1969; Hillis-Colinvaux, 1974). Yet it obviously is necessary that we 

 master the contribution of the calcareous green plants to both the 

 energy and carbonate fluxes of reef systems. 



A. Production of organic carbon 



Primary productivity in aquatic communities is measured by a 

 number of methods of which two of the principal ones are the light : dark 

 bottle technique and the in situ ^^C uptake method (Steemann Nielsen, 

 1952; Strickland, 1960, 1966; Strickland and Parsons, 1968; National 

 Research Council, 1969; Vollenweider, 1969). Their use has led to 

 numerous papers on primary productivity, and the topic may appear at 

 times to be well worked. For macrophytes, however, this is deceptive. 

 These two techniques were developed for phytoplankton, and are hard 

 to apply directly to macrophytes, although Drew and Larkum (1968) 

 chose ^*C methodology for Udotea in the field, and Borowitzka and 

 Larkum (1977) used it in a laboratory measure of photosynthesis with 

 detached branches of Halimeda. The coenocytic nature of the algae 

 involved could present special problems which are not discussed in 

 these papers. The technique of flow-respirometry, which is also used, as 

 mentioned earlier, generally provides a measure of community rather 

 than population productivity, and is restricted to regions where there 

 is significant water flow in one direction for a meaningful length of time. 

 It is of limited use for most macrophytes except for calcareous Rhodo- 

 phyta of the reef algal ridge. 



A few attempts have been made to apply gas exchange techniques 

 to individual plants, or to bits of reef or sand surface, by enclosing 

 these in plastic bags, jars or acrylic hemispheres (Sargent and Austin, 



9 



