ECOLOGY AND TAXONOMY OF Holimeda 253 



its nutrient supplies and its role in maintaining the nutrient cycles of a 

 reef system. 



B. Carbonate production 



Halimeda is a producer, not only of reduced carbon, but also of 

 loose carbonate sediment, and it has become clear that unconsolidated 

 sediments are more important to the building of a reef than the car- 

 bonate incorporated into the reef framework (Milliman, 1974). Stoddart 

 (1969) estimates that four to five times more loose sediment is produced 

 than is incorporated as reef framework. This loose sediment is produced 

 by Foraminifera, by the large benthic animals of the reef system like 

 molluscs and some echinoderms, as accretionary deposits on the blades 

 of sea grasses, and by the green calcareous algae, the most abundant 

 and widespread of which are Halimedae. 



There seems some uncertainty in the minds of students of coral 

 reefs over the relative importance of Halimeda segments to the total 

 flux of loose sediments into a reef. The subject is naturally approached 

 from the point of view that coral reefs are just what they are called, 

 "reefs made of coral", which is to say that they are animal creations. 

 Furthermore, a visit to a reef reveals corals in abundance but, unless 

 you dive and hunt, no obvious green plants. As we noted earlier, 

 the transect at Enewetak studied by the Odums and the SYMBIOS 

 group (Johannes et al., 1972) contains few to no Halimedae or other 

 large green algae. Apart from the borers and the symbionts, the main 

 evidence of plant life is given by the encrusting red algae of the fore-reef 

 where the waves break. But these red algae are properly regarded as part 

 of that reef framework that contributes only a quarter of the reef mass 

 (Stoddart, 1969). 



1 . Halimeda as one of the principal contributors to atoll mass 



The classic borings at Funafuti (Hinde, 1904; Judd, 1904) provided 

 the first evidence that the actual mass of a coral atoll (and hence 

 perhaps of all coral reefs) might not be made of the visible corals and 

 red algae of the reef ridge itself, and these borings implicated Halimeda. 

 In the boring through the floor of the lagoon the first 20 m of sediment 

 was between 80% and 95% recognizable Halimeda segments (Fig. 82). 

 Similar conclusions were reached for the unconsolidated lagoon deposits 

 at Bikini, Enewetak and other atolls of the Marshall Islands by Emery 

 et al. (1954) (Fig. 83). Table XXIV shows similar results from a number 

 of lagoons collected by Milliman (1974). It seems clear that Halimeda 

 segments are a major constituent of many lagoon deposits. Table XXV 



