REEF PROCESSES 



173 



to maintain the steady-state system. With low net produc- 

 tivity and efficient recycling, whole reef ecosystems should 

 not be expected to require large inputs of new nutrients. 

 Hence, the apparent paradox of high-productivity reef sys- 

 tems in the midst of nutrient-poor waters, as perceived in 

 the earlier studies, can now be viewed as not too surpris- 

 ing. Smith further argued that, since reef production is 

 dominated by benthic plants with a C:N;P ratio of approxi- 

 mately 550:30:1 (Atkinson and Smith, 1983), they can 

 produce more net carbon per unit of nitrogen and phos- 

 phorus availability than can planktonic systems with a 

 C:N:P ratio of 106:16:1. 



Atkinson (1981, 1982) challenged earlier ideas regard- 

 ing the cycling of phosphorus in reef metabolism and 

 argued that there is not a tight cycling of that element for 

 a whole reef flat. However, because of a high advective 

 flux of phosphate over most reef flats, the system can 

 depend primarily on exchange with the water column for 

 its nutrients; and only 10% of the P04~'^ available to reef 

 producers might be recycled through the water column. 

 This challenge to earlier ideas has contributed to a continu- 

 ing interest in the comparative roles of nitrogen and phos- 

 phorus fluxes in reef systems and the question of which of 

 these elements is limiting to metabolism. For example, in a 

 study of the whole lagoonal system at Christmas Island 

 (Kiribati), Smith et al. (1984) argued that net metabolism 

 of the system is limited by the availability of phosphorus. 

 Ideas are developing rapidly, and additional work is likely 

 to be reported in the near future. Crossland (1983) 

 recently provided an overview of nutrients in coral reef 

 waters. 



Studies of individual populations of reef organisms 

 have taken place at a larger number of geographic loca- 

 tions than have ecosystem studies. Work at Enewetak has 

 not played the fundamental role for the former type of 

 study that it has for the latter. Literature on the biology 

 and ecology of individual populations is extensive and 

 diverse. The reader is referred to the recent paper by Lar- 

 kum (1983) for entry into the literature on productivity of 

 plant populations, to the paper by Chalker (1983) for 

 recent calcification studies of corals and other animals, and 

 to the papers by Muscatine (1983) and Chalker and Dun- 

 lap (1983) for work on metabolism and production of 

 corals. 



SUMMARY 



A number of significant single-investigator and team 

 studies have been conducted at Enewetak and have 

 contributed to a general understanding of reef ecosystems 

 and to the development of methodology for studying such 

 systems. Many of these studies have focused on the reef 

 flats as the metabolically dominant subsystem of the whole 

 atoll. The early study by Odum and Odum (1955), which 

 attempted to relate structure and function in the windward 

 reef-flat community, established a conceptual framework 

 that is influential even today. Other important studies have 

 focused on community metabolism, calcification processes 



at the ecosystem and organismal level, nitrogen and 

 phosphorus cycling on the reef flats and in individual 

 organisms, the trophic role of detritus, nutritional sources 

 for corals, and the ecological relationships of reef fishes. 



Table 1 summarizes most of the studies discussed in 

 this chapter. Some of the highlights are reiterated in the 

 following paragraphs. 



Biological zonation is an important aspect of the struc- 

 ture of reef flats. Rates of community metabolism on such 

 flats are high, with gross productivities of 6 to 10 g C m~^ 

 d~^ for coralgal communities and 12 g C m"^ d~' for 

 algal-dominated communities; 24-h ratios of gross produc- 

 tivity to respiration approximate or exceed 1.0. Rates of 

 calcium carbonate production on reef flats are also high, 

 approximately 4 kg m" yr~', with little apparent differ- 

 ence between day and night; corals are a relatively minor 

 contributor to the process at the level of the whole ecosys- 

 tem. Rates of nitrogen fixation (and nitrogen export) on 

 reef flats are again high, up to 1000 kg ha~^ y~\ and 

 help account for the high productivity. Important nitrogen- 

 fixing organisms include blue-green algae and bacteria. 

 Studies at Enewetak suggest that internal phosphorus 

 cycling within reef-flat communities is very efficient, with 

 little exchange between the benthic biota and the water 

 column; however, this conclusion has recently been chal- 

 lenged. The role of regenerative spaces has been examined 

 in an exploratory way; such internal reef spaces probably 

 deserve much more attention than they have received. 



Various aspects of the biology of individual populations 

 have also been examined at Enewetak. The primary pro- 

 ductivity of the few algal populations that have been stud- 

 ied on a preliminary basis has been estimated at up to 2.3 

 gC m^ d for various species of Halimeda, with some- 

 what lower values for intertidal algal mats and calcareous 

 red algae. The productivity of filamentous boring algae, 

 originally thought by the Odums to be of major impor- 

 tance, is insignificant. Work conducted at Enewetak was 

 the first to demonstrate that zooxanthellae from the tissues 

 of corals and giant clams could release significant amounts 

 of radioactively labeled photosynthate, which could 

 presumably contribute to host nutrition. Other studies 

 investigating the oxygen balance of corals and their 

 enclosed zooxanthellae have indicated that at least some 

 species have the capability of obtaining all their nutritional 

 requirements from their symbiotic algae and that there can 

 be sun and shade differences in the same species growing 

 at different depths. Studies of the physiology of coral 

 calcification have demonstrated light enhancement of this 

 process (mediated by zooxanthellae), suggested the impor- 

 tance of active transport of calcium ions rather than diffu- 

 sion, and found a marked temperature effect on the 

 uptake of Ca. Cyclic variations in the radial density (den- 

 sity banding) of coral skeletons were calibrated by examin- 

 ing the distributions of radionuclides in the skeletons, then 

 used to estimate growth rates of individual colonies and 

 later found to result from seasonal temperature differences. 

 Nitrogen uptake in corals has been found to fit the active- 

 transport model of enzyme kinetics for at least some forms 



