INTRODUCTION 



Coral reefs have attracted interest over the last 30 yr in part because 

 they are among the most productive biological communities in the world. Early 

 studies of reef productivity (Sargent and Austin, 1949; Odum and Odum, 1955) 

 focused on determining the productivity of individual reefs. There is little 

 agreement in these and subsequent studies as to how productive reefs can be, 

 and there is even less agreement concerning why high levels of production are 

 attained or why reefs differ from each other so much (Lewis, 1977; Sournia, 

 1977; Mann, 1982). Smith (1981) considered some of these problems in an 

 examination of the Houtman Abrolhos Islands off western Australia. We address 

 these questions in some detail in our study of the reefs of St. Croix. 



In an examination of the "Potential Productivity of the Sea," Ryther 

 (1959) concluded that (net) "primary productivity of organic matter of some 

 10-20 g (dry)/m 2 /d may be expected," with a maximum of 25 g (dry)/m2/d under 

 ideal conditions of maximum radiation and no nutrient limitation. Expected 

 gross primary production (GPP) levels of up to 23-38 g (dry)/m2/d (50-80% higher 

 than the net values) also were cited. Ryther suggested that, while planktonic 

 open ocean production (particularly in tropical /subtropical areas) is limited 

 by nutrients, such limitation in benthic communities is not likely because 

 nutrients "are continually being replenished as the water moves over the plants 

 [and] probably prevents their ever being [nutrient] limited." 



Recent reviews of coral reef primary productivity by Lewis (1977) and 

 Sournia (1977), integrating 30 yr of research, reveal a range in GPP of nearly 

 an order of magnitude (3.4 - 20 g C/m 2 /d). The mean of these cited reef values 

 is about 10 g C/m^/d, but only a few points are above 13 g C/m^/d. The mode is 

 7 g C/m^/d, which, using a typical conversion ratio, would convert to a net biomass 

 production of about 15 g (dry)/m2/d. Overall, these values are much less than 

 the theoretical maximum suggested by Ryther (1959). Significantly, all but one 

 of the 17 coral reefs cited by these authors were located in the Indo-Pacif ic. 



Despite Ryther's (1959) suggestion to the contrary, nutrient limitation of 

 reef productivity has been the focus of numerous studies. Many of these have 

 focused on explanations of how highly productive reef ecosystems can thrive in 

 nutrient deserts (Odum and Odum, 1955; Stoddart, 1969; Johannes, et a! . , 1972; 

 D'Elia, et al . , 1981; Andrews and Muller, 1983). Recently, the role of major 

 nutrients was questioned due to the finding that benthic algae have lower 

 nutrient requirements than once thought (Atkinson and Smith, 1983). Also, some 

 reefs are known to have a surplus of nitrogen (Webb, et al . , 1975; Wiebe, et 

 al . , 1975) due to the in situ nitrogen fixation by cyanobacteri a (blue-green 

 algae) (Mague and Holm-Hansen, 1975; Capone, et al . , 1977; Hatcher and Hatcher, 

 1981; Wilkinson and Sammarco, 1983). The benthic algal turf of reef communities 

 usually is dominated by cyanobacteria (Sammarco, 1983; Van den Hoek, et al . , 

 1975). 



Phosphorus concentrations over reefs generally have been thought to change 

 little (Pilson and Betzer, 1973), and some authors have suggested that a "tight 

 recycling" or retention of phosphorus occurs (Pomeroy, et al . , 1974; Pomeroy 

 and Kuenzler, 1969). Atkinson (1982), on the other hand, has shown reactive 

 phosphate depletion over broad reef flats. 



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