biomass. The intense grazing maintains the algal community at the turf stage, 

 wherein the productivity per unit biomass is very high. 



Facilitating this high productivity are nitrogen fixing cyanobacteria that 

 fix more nitrogen than is required by even the most productive reef zones. 

 Other nutrient requirements, particularly phosphorus, are met by the combination 

 of continuous and strongly flowing equatorial currents as well as by constant 

 wave-driven currents. This, with the lower phosphorus requirements of algal 

 turfs, results in a highly productive community that is not limited by nutrient 

 avai labi 1 ity. 



Thus, synergy is attained by the interaction of factors that optimize reef 

 productivity. If any factor is removed from this system (i.e., water depth, 

 SAR, wave oscillation, water flow, or algal community structure), the primary 

 productivity would be less than we found. 



The back reefs we stu 

 communities in the world, 

 mean annual gross producti 

 14 g C/m 2 /d exceed the the 

 Ryther (1959) of 17 g C/m 2 

 other hand, this is not un 

 production of 22-31 g (dry 

 found in both laboratory a 

 production due to desiccat 

 Whittaker, 1975) are nonex 

 Croix. Thus, productivity 

 limiting source: incoming 



died are among the most 

 Using standard convers 

 on of about 23 g C/m 2 /d 

 oretical maximum gross 

 /d and the net producti 

 reasonable, since maxim 

 )/m 2 /d (approximately 9 

 nd field environments, 

 ion, nutrient availabil 

 istent or of reduced im 

 of these reefs seems t 

 solar radiation. 



productive biological 

 ions, our measurements of a 



and a net production of about 

 productivity hypothesized by 

 vity of 12.3 g C/m 2 /d. On the 

 urn experimental rates of net 

 -12 g C/m 2 /d) have now been 



Limitations to primary 

 ity or gas exchange (Lieth and 

 portance on the reefs of St. 

 o be limited by the ultimate 



ACKNOWLEDGMENTS 



This study was supported by the Smithsonian Institution, the Virgin Islands 

 Government and the West Indies Laboratory of Fairleigh Dickinson University. 

 We were assisted for much of the study by Caroline Rogers and Norman Salesky. 

 Additional help came from Sara Armstrong, Susan Brawley, Robert Carpenter, and 

 members of the West Indies Laboratory staff. Caroline Rogers, James Norris, 

 Susan Brawley, Chris D'Elia, and John Ogden read early drafts of this manuscript 

 and offered suggestions for its improvement. Steve Smith offered some valuable 

 suggestions for improvement of the final manuscript. 



REFERENCES 



Adey, W. 1975. "The Algal Ridges and Coral Reefs of St. Croix: Their Structure 

 and Holocene Development." Atol 1 Res. Bull . 187:1-67. 



Adey, W. 1978a. "Coral Reef Morphogenesis: A Multidimensional Model." 

 Science 202:831-837. 



Adey, W. 1978b. "Algal Ridges of the Caribbean Sea and West Indies." Phycologia 

 17:361-375. 



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