biomass of algae in the refuge tanks would have no measurable 

 effect on the chemistry of water being turned over at this rate. 

 Since the changes in timing of light-dark cycles were sometimes 

 complex, and since remembering the methodology of each experiment 

 is crucial for evaluating the results, the specific methodologies 

 and results of these experiments are given together in the 

 results section. 



RESULTS 



Diel patterns of Halimeda growth 



Production of new segments. The developmental sequence of 

 newly forming Halimeda segments is shown in Figures 1 and 3. 

 Very small (<1 mm wide) segment buds begin to form in the 

 afternoon (1200-1600 hrs ) with initiation of buds decreasing 

 rapidly after sunset (Fig. 3). Developing tips expand rapidly 

 over the course of only a few hours but remain unpigmented (Fig. 

 1) until just before sunrise when the segments rapidly change 

 from white to green. This pattern occurs for both Halimeda 

 incrassata and H^ simulans (Fig. 3). We did not quantify the 

 pattern for other Halimeda species but field observations on 

 numerous reefs in the Caribbean Sea and Indian and Pacific Oceans 

 suggest that most Halimeda species, and perhaps related genera 

 like Caulerpa , Udotea , Penicillus , and Rhipocephalus , follow this 

 basic pattern of nocturnal production of new growth (M. Hay and 

 V. Paul, personal observation). We have, however, noted two 

 exceptions to this pattern. Firstly, in St. Croix, Halimeda 

 copiosa appeared to initiate segment production at the same time 

 as other Halimeda species but it was common for some of the newly 

 formed apical segments to remain unpigmented for much of the next 

 day. Secondly, on Pacific reefs around Guam, newly produced 

 Halimeda macroloba segments are commonly white during the first 

 day after production and some plants occasionally produce new 

 segments during the day (V. Paul personal observation). 



If segments of H^ incrassata at our study site were being 

 produced throughout the 24 hr cycle, heavy fish grazing on 

 segments produced during the day could cause the appearance of 

 nocturnal production. We tested this hypothesis by separately 

 caging 56 Halimeda incrassata plants occurring at 21 m depth on 

 the sand plain and periodically noting their pattern of segment 

 production. These caged plants followed the same pattern shown 

 in Figure 3; new segments were never produced during the day. To 

 assess the effects of small day-active grazers that would not 

 have been excluded by our cages, we conducted experiments in the 

 coral-reef microcosm at the Smithsonian Institutions ' s National 

 Museum of Natural History (see Fig. 2). Segment production of 

 Halimeda opuntia in the microcosm follows the basic temporal 

 pattern shown in Figure 3 (M. Hay, personal observation). When 

 10 portions of H_^ opuntia were moved from the large microcosm 

 (7000 1) to two small refuge tanks (150 1) that had been cleaned 



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