REEF PROCESSES 



161 



COMMUNITY STUDIES 



Oxygen Metabolism and 

 Primary Productivity 



The seminal nature of the Odum and Odum (1955) 

 study has been noted. Theirs was not the first upstream- 

 downstream study of reef metabolism, being preceded by 

 that of Sargent and Austin (1949, 1954). It succeeded, 

 however, in generating widespread interest and is probably 

 responsible for the frequently made statements that reef 

 systems are oases of high productivity surrounded by 

 nutrient-poor deserts, that they are among the most highly 

 productive systems on earth, and that they are highly effi- 

 cient transformers of solar energy into biological energy. 

 The Odums' work was also responsible for promoting the 

 knowledge that it is the benthic community rather than the 

 phytoplankton in the overlying water which is responsible 

 for this high productivity. These are all key elements in 

 our present understanding of reef ecosystems. 



More precise and thorough measurements of commu- 

 nity metabolism were carried out by the Project SYMBIOS 

 team (Johannes et al., 1972; Smith, 1973; Smith and 

 Marsh, 1973), but the basic conclusions reinforced those of 

 the Odums regarding the high productivity of the wind- 

 ward reef flat. The team further provided at least a partial 

 answer to the commonly asked question of how reefs 

 could be areas of such high productivity surrounded by 

 nutrient-poor and unproductive oceanic waters. It con- 

 cluded that high rates of nitrogen fixation and extremely 

 efficient internal recycling of phosphorus were major fac- 

 tors. This conclusion was later disputed by Atkinson 

 (1981), whose work is discussed in the section on com- 

 munity phosphorus cycling. 



Smith and Marsh (1973) made simultaneous measure- 

 ments of oxygen and carbon dioxide changes in water as it 

 flowed across the windward reef flat. The two independent 

 estimates gave strongly correlated results for daytime pro- 

 ductivity values but a weaker correlation for nighttime 

 respiration values. These results allowed them to make the 

 first estimate of the metabolic ratio (i.e., the molar change 

 in CO2 per molar change in oxygen) for a reef community. 

 They found this ratio to be —1, uncorrected for diffusion, 

 and suggested that corrections for diffusion without some 

 indication of the metabolic ratio did not increase precision 

 of productivity measurements. 



Smith and Marsh comjjared two reef-flat transects, one 

 similar to that described by Odum and Odum and contain- 

 ing coral zones as well as algal zones, and one transect of 

 similar length but crossing no coral zones. The algal tran- 

 sect had a much higher gross P, net P, and gross P:R ratio 

 (based on a 24-h period) but had similar rates of respira- 

 tion, perhaps reaching an upper limit imposed by diffusion 

 rates of oxygen to the large benthic infaunal community. 

 Their coralgal transect had an overall gross P;R ratio near 

 1 and was apparently just self-sufficient with resF>ect to 

 energy demands. Assuming, however, that the algal por- 

 tion of the coralgal transect was metabolically similar to 



the algal transect. Smith and Marsh calculated that the 

 coral portion of the former transect was heterotrophic. 

 Thus, there appeared to be an upstream autotrophic por- 

 tion and a downstream heterotrophic portion for that tran- 

 sect. They postulated that the large schools of herbivorous 

 fishes migrating between reef zones could be significant in 

 transferring energy and materials downstream. Perhaps 

 more attention should be directed to the question of 

 whether this distinction of an upstream autotrophic and a 

 downstream heterotrophic zone is a general feature of 

 reef-flat ecosystems, as originally suggested by Odum and 

 Odum. Much more comprehensive studies of community 

 metabolism, especially of reef flats, have been made at 

 other study sites by Kinsey (1977, 1979). 



Wells (1974) and Wells et al. (1973) described a 

 method for making in-situ measurements of benthic 

 metabolism in reef communities and presented some pre- 

 liminary results. Their basic technique was to place a trans- 

 parent plastic dome over a suitable portion of the substra- 

 tum and to monitor oxygen changes in the enclosed water 

 mass in the light and in the dark. Preliminary measure- 

 ments were made at Enewetak on the SYMBIOS transect 

 and at sites in the Virgin Islands. At Enewetak, enclosed 

 water masses were reported to show a rather constant 

 oxygen concentration of 125% saturation while gas bub- 

 bles were being produced in the light; the oxygen content 

 of the evolved gas was 28 to 32%. The oxygen evolution 

 of algae-covered pavement reached a maximum of 5.5 X 

 10~ ml cm~^ h"~^ with a maximum P:R ratio (24-h basis) 

 of 1.6. Coral rubble was observed to produce at about half 

 the rates of the algae-covered pavement. These prelim- 

 inary attempts to measure metabolic activity of the algal 

 pavement thus focused on a neglected, but probably 

 major, component of the reef-flat ecosystem. Marsh like- 

 wise has made a few unpublished measurements of small 

 sections of such pavement removed from the reef and 

 placed in respirometers. His preliminary values for gross P 

 ranged up to 0.087 mg O2 cm~^ h"^ and suggested that 

 this might be one of the most metabolically active seg- 

 ments of the reef-flat ecosystem; this should be followed 

 up. 



Calcium Carbonate Production and 

 Reef Growth 



While there were several earlier attempts to estimate 

 the growth rates of individual reef components, especially 

 corals (e.g., Mayor, 1924), one of the first attempts to 

 assess calcium carbonate deposition directly for the reef 

 community as a whole was made by Smith (1973). Along 

 with concurrent work by Kinsey in Australia (1972), this 

 research pioneered the technique of utilizing changes in pH 

 and alkalinity as water flowed across a reef flat not only to 

 assess organic productivity but also to evaluate the dynam- 

 ics of calcium carbonate deposition and dissolution at an 

 ecosystem level. Smith found that both a coralgal transect 

 and a transect dominated by an algal turf calcified at an 



