164 



MARSH 



waters flowing over the reef flat could be used as an indi- 

 cator of community metabolism. He further argued that 

 "recycling of phosphorus for a whole reef flat is not tight, 

 and that the system can depend primarily on exchange 

 with the water column for its nutrients." 



Role of Regenerative Spaces 



One aspect of ecosystem-level processes that has prob- 

 ably not received sufficient attention is the role of the 

 extensive internal spaces of the reef in nutrient regenera- 

 tion. DiSalvo (1969, 1971), in work conducted partly at 

 Enewetak and partly in Kaneohe Bay, Hawaii, recognized 

 the potential importance of these spaces and attempted to 

 assess some of their quantitative aspects. He obtained bac- 

 terial counts of 10'' to 10^ bacteria per g dry wt and found 

 that some isolated bacteria were capable of digesting chitin 

 in vitro, suggesting there might be bacterial degradation of 

 the organic matrix of coral skeletons (as opposed to the 

 CaCOa making up the bulk of the skeletal mass). DiSalvo 

 also obtained estimates of the oxygen demands of internal 

 sediments amounting to 0.06 to 0.50 mg O2 g sediment"' 

 h ; these rates were considerably lowered by antibiotics. 

 The O2 consumption of sediments in suspension was 10% 

 of the total consumption by two intact heads. Water 

 samples collected from within the regenerative spaces in 

 situ generally showed oxygen debts as compared with 

 ambient reef water. DiSalvo reported that the oxygen 

 debts of inshore (stressed) reefs in Kaneohe Bay were 

 greater than those of offshore reefs. It is curious that the 

 role of internal spaces, which are quite extensive in reef 

 systems, has not been the subject of more studies. Their 

 potential significance seems great, but the preliminary 

 work of DiSalvo has not received much follow-up. 



STUDIES OF INDIVIDUAL 

 POPULATIONS 



Studies of energy and materials flux in individual reef 

 populations have focused on the corals. There is a relative 

 paucity of information about other populations. Further- 

 more, there has been no attempt (other than that of Odum 

 and Odum) to investigate the role of individual populations 

 and to integrate these roles to arrive at an estimate of 

 whole-community function. There is thus a wide gap 

 between ecosystem- or community-level studies on the one 

 hand and individual-population studies on the other hand. 

 However, several studies have certainly resulted in the 

 implication, if not direct evidence, that the corals are a 

 relatively unimportant component in total energy and 

 materials flux; although they are visually impressive and 

 are generally taken as characterizing the very essence of 

 the reef, their role may be misinterpreted. 



Algal Productivity and Growth 



Marsh (1970) made one of the earliest attempts to 

 assess the primary productivity of an individual algal popu- 

 lation and evaluate its role in total reef productivity. He 



worked with encrusting forms of calcareous red algae, sam- 

 ples of which were removed from the reef and placed in 

 respirometers in the laboratory. He measured an average 

 gross photosynthesis of 0.048 mg O2 m~^ h"' under con- 

 ditions of light saturation; the light-saturation intensity was 

 reported to be 1000 ft-candles (less than 10% of full sun- 

 light), with no photoinhibition at higher light intensities. 

 Rates of photosynthesis and respiration in flowing water 

 showed no correlation with different water velocities but 

 were greater than rates in still water. Daily patterns of 

 photosynthesis calculated for populations living on the sub- 

 marine face of atolls suggested that light is probably not a 

 limiting factor for that process during most daylight hours. 

 Marsh calculated productivity for various calcareous algal 

 zones and concluded that these zones do not contribute 

 significantly to overall community productivity. Island reefs 

 on atolls, where such zones account for a larger percent- 

 age of the total reef area, were estimated to be less pro- 

 ductive than interisland reefs on the same atolls. Marsh 

 stated that the productivity of calcareous algae is of the 

 same order of magnitude as values reported in the litera- 

 ture for other benthic producers but is near the lower end 

 of the range His estimates, however, were based on the 

 actual surface area of individual samples rather than the 

 flat map area (for which 1 m^ is occupied by more than 1 

 m of irregular photosynthesizing surface). Marsh's values 

 were thus underestimates as compared with those based 

 on flat map area. 



A study by Hillis-Colinvaux (1977) focused primarily on 

 a field survey of natural distributions of various species of 

 Halimeda. However, the author combined this with infor- 

 mation from previous laboratory studies of photosynthesis 

 and respiration of Halimeda from the Caribbean (Hillis- 

 Colinvaux, 1974) to calculate productivity for some 

 Enewetak populations. In localized reef and lagoonal areas 

 where the coverage of these species approached 100%, 

 Hillis-Colinvaux calculated that their productivity could be 

 as high as 2.3 g C m"^ d~' and could contribute a signifi- 

 cant amount of energy to the total reef system. Hillis- 

 Colinvaux further concluded that Halimeda populations 

 contribute a significant proportion of the loose sediments 

 in the atoll system, with seven species being the main con- 

 tributors. 



Bakus (1967) used a different approach in evaluating 

 the primary productivity of "dense but thin" algal mats 

 growing on the hard substratum of intertidal reef flats. He 

 scraped off all the algae in small quadrats and weighed, 

 combusted, and reweighed the harvested material to get 

 an estimate of the initial organic standing crop. After plac- 

 ing exclosures over the scraped quadrats to keep out graz- 

 ing fishes, he then repeated the scraping after 15 to 17 

 days to get an estimate of the productivity by the harvest 

 method. The dominant species of the scraped algae were 

 the blue-greens Calothrix Crustacea and Schizothrix cal- 

 cicola. Bakus' estimates of net daily production measured 

 by this technique ranged between 0.6 and 2.15 g C m~^. 



One major conclusion of the Odums regarding the 

 importance of an individual population received widespread 



