308 L. HILLIS-COLINVAUX 



shallows. As with the rock forms, there seems to be a geographical 

 separation of their ranges, at least to the extent that stuposa is mostly 

 restricted to the northern parts of the lagoon (Fig. 98). In shallow 

 parts in the north the two species together formed extensive meadows, 

 though these are not really comparable to the incrassata meadows of 

 Jamaica. Perhaps more significantly, these meadows in no way 

 compare to the Thalassia meadows of Jamaica, not even in the apparent 

 density of the Halimeda contribution to the Jamaican Thalassia 

 meadows. It was remarked in the study of Glory Be that a Thalassia- 

 dominated community seemed to be involved in stabilizing patches of 

 sand, and that Rhipsalian Halimedae were more abundant near or in 

 these communities than in the open. Part of the low density of 

 Rhipsalian Halimedae at Enewetak, therefore, may be a reflection of 

 the absence of sea grasses. And this absence of sea grasses may be 

 significant to the economy of the atoll. 



(g) The lagoon floor. If the flats of the lagoon floor were covered with 

 dense stands of Rhipsalian Halimedae, the source of the bulk of the 

 reef carbonate would be explainable in one observation. But the floor 

 is not so covered. This was first demonstrated by Gilmartin (1960) 

 and our observations confirm this (Fig. 102b). There are very low 

 densities oi cylindracea (called monile in Gilmartin's paper), particularly 

 near the bases of coral pinnacles. But for the most part the lagoon 

 floor has few populations of Halimeda, and the ones that are there 

 are rock-attached forms on coral blocks, particularly macrophysa. 

 Gilmartin (1960) noted changes in the form of cylindracea with depth 

 (Fig. 104), and postulated that the paucity of the species on the lagoon 

 floor was a function of two processes : low light intensity and distur- 

 bance by burrowing animals. The relative prominence of animal 

 mounds and castings on the lagoon floor near the base of pinnacles in 

 40 m of water certainly encourages acceptance of Gilmartin's hypothesis. 

 Mounds and diggings show that the bottom is constantly overturned 

 (Fig. 103). Plants growing at light intensities of 5% or 10% of that at 

 the surface may well not achieve sufficiently vigorous growth to 

 survive such constant digging. 



3. Halimeda at Enewetak: summary 



There are no obvious rivals to Halimedae as carbonate producers 

 among the green algal macrophytes at Enewetak. Tydemania expedi- 

 tionis is second, but a much lesser producer. 



