160 



MARSH 



potential for an integrative understanding of reef 

 processes, and the greatest realized development of such 

 an understanding to date, has not seen this potential fully 

 developed. 



Most of the studies of Enewetak processes have 

 focused on reef flats rather than deep faces or lagoons. 

 Indeed, "reef" is synonymous with "reef flat" for many 

 researchers. There is some justification for this position 

 since this is the portion of the reef system that receives 

 the greatest inputs of solar energy required for primary 

 productivity and has been shown to be the most actively 

 calcifying portion of the system (Smith and Harrison, 

 1977). It also appears to be the major site of nitrogen fixa- 

 tion (Wiebe, 1976). Finally, at least as a first approxima- 

 tion, the reef flat appears to be the source of the major 

 nutrient and energy inputs to the lagoon, the most exten- 

 sive subsystem of the atoll. Indeed, the impression shared 

 by many reef ecologists is that the comparatively small, 

 intensely productive reef flats provide the major inputs 

 driving the whole system. 



DESCRIPTIVE STUDIES OF 

 REEF STRUCTURE 



The general picture that many f)eople have of biologi- 

 cal zonation on reefs, especially Pacific atoll reef flats, is 

 strongly influenced by the description provided by Odum 

 and Odum (1955). They described a series of zones 

 extending roughly parallel to the breaking surf on the 

 windward reef margin and perpendicular to the direction of 

 water flow across the reef flat. They also provided esti- 

 mates of the standing crops of dominant reef organisms. 

 This built upon extensive earlier descriptions by Tracey et 

 al. (1948) and Emery et al. (1954). The former paper pro- 

 vided a general system of classification for various reef 

 types and made the distinction between island reefs and 

 intcrisland reefs on atolls. The Odums' description applies 

 generally to at least some fringing reef flats (e.g.. Marsh, 

 1974) as well as to atolls. Their work still stands as a use- 

 ful general description of reef flats. 



The Odums distinguished six zones on their reef, 

 proceeding from seaward to lagoon. The windward buttress 

 zone constitutes the seaward face of the reef outside 

 breaking surf; it was inaccessible to the Odums but they 

 estimated that there is about 50% coral coverage on the 

 submarine buttresses in this zone. The coral-algal ridge 

 zone is dominated by calcareous red algae and fleshy algal 

 mats, with scattered encrusting forms of Acropora. Pocillo- 

 pora. and Millepora corals. Behind this the encrusting zone 

 likewise has sheets of yellow encrusting Acropora and 

 Millepora and low, rounded heads of Porites and several 

 species of favid corals. Filamentous algae of all four major 

 benthic algal divisions form heavy encrusting mats here. 

 Coral cover is far less than 50%. Next is the zone of 

 smaller heads with massive heads of Porites lobata and 

 favid corals; encrusting Acropora is not present but scat- 

 tered colonies of branching corzils of the genus can be 



found. The zone of larger heads is slightly deeper and has 

 massive heads up to a meter high and 2 m across, with 

 sand channels between the heads. The blue octocoral 

 Heliopora is common here, with a distinct narrow zone of 

 the stinging coral Millepora at the back edge of the zone of 

 larger heads. Parrotfishes and surgeonfishes commonly 

 browse and school here. Algal cover is much lower in the 

 zones of smaller and larger heads than in the two zones 

 immediately upstream. The zone of sand and shingle has 

 very low occurrences of either algae or corals and few of 

 the fishes found in the upstream zones; however, there arc 

 schools of sardine-like fishes that feed on "pseudoplank- 

 ton" (algal fragments) drifting downstream in the current. 

 To the Odums, the reef structure suggested a transition 

 from water-filtering as a source of nutrients in upstream 

 zones to subsurface decomposition as a source of plant 

 requirements in back reef zones. 



Odum and Odum (1955) attempted to get biomass 

 estimates of the different trophic levels for different reef 

 zones. This was an ambitious undertaking that has not 

 been repeated, presumably because of the large amount of 

 work involved and the uncertainties of assigning particular 

 organisms to specific trophic levels. An attempt to repeat 

 their estimates in the light of more recent information on 

 the basic biology of the organisms involved is obviously 

 called for in a variety of reef ecosystems. 



The Odums estimated the dry weights of primary pro- 

 ducers by chlorophyll extractions (based on Harvey pig- 

 ment units) calibrated by establishing a ratio of chlorophyll 

 to dry weight for the free-living macroalga Codium. This 

 was done for free-living algae of various growth forms, for 

 zooxanthellae contained in the living tissue of coral polyps, 

 and for filamentous green algae ("boring" algae) within the 

 skeletons of corals and other calcareous material. The 

 mean estimate for producers averaged over all reef zones 

 was 703 g dry biomass m~ , with little evident difference 

 in the producer component of live corals from different 

 reef zones. The white sand area of the back-reef zone was 

 found to be the only major reef zone with a definitely 

 lower biomass of producer organisms. From data on colo- 

 nization of glass sides left on the reef, the Odums calcu- 

 lated the growth of encrusting algae on the front reef to be 

 twice that on the back reef, consistent with the observed 

 predominance of encrusting forms in the former zone and 

 boring forms in the latter zone. 



Animal biomass at the primary consumer level (second 

 trophic level) was reported by the Odums to consist pri- 

 marily of fish, coral polyps, molluscs, echinoderms, 

 annelids, and crustaceans, depjending upon the particular 

 reef zone. The measured biomass averaged 132 g dry 

 weight m~^ for all reef zones combined. The third trophic 

 level was found to consist primarily of predatory fish, mol- 

 luscs, crabs, annelids, and starfish, averaging 11 g dry wt 

 m~^. The ratio of herbivores to producers was calculated 

 to be 18.9% and of carnivores to herbivores, 8.3%. This 

 resulted in a pyramid of biomass with a broad base and a 

 small peak, a result stressed by Odum and Odum as being 

 consistent with ecological theory. 



